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Turtle: Innovative Software for the Learning of

1. Expression Factor 91 Figure 19 Syntax diagrams for parsing by recursive descent 92 7 6 Parsing and Code Generation for Individual Commands and Expressions The lowest of the three levels of parsing and code generation deals with individual Turtle Graphics or assignment commands and procedure calls and also the processing of arithmetical or boolean expressions We shall deal with the latter first for as we have seen such expressions not only occur within individual commands but also play a crucial role within the conditional and looping structures discussed in 7 5 To minimise complexity this level of the compiler uses a fairly straightforward implementation of the method of recursive descent combining code generation with parsing in a manner that directly reflects the relevant syntactical structures These structures are standardly represented in syntax diagrams such as those on the previous page note that they are mutually recursive since an expression always includes a simple expression which in turn always includes a term which in turn always includes a factor which in turn can include either an expression or a factor However a factor can also be an individual identifier i e a variable name an integer or a hexadecimal integer all of which have previously been identified and tokenised as described in 7 3 above and this is where the recursion in any particular case ultimately stops To illustra
2. blot markers 3 The third lecture is focused on procedures emphasising the importance of modularity to enhance comprehensibility and going on to show the amazing power of recursion though a simple triangle example which is explained using the metaphor of the Cat in the Hat 32 with recursive calls being introduced one at a time until the picture shown in Figure 2 p 20 is produced The built in Recursion Factory program invites the students to experiment for themselves with recursive patterns of real complexity and aesthetic interest 4 The fourth lecture introduces repeat loops in the context of producing animated graphics such as the bouncing balls effect in two of the illustrative programs ballsteps and multibounce The remaining illustrative programs are also briefly reviewed to give the students ideas that they can follow up for themselves in their coursework The coursework assigned at the end of the fourth lecture is designed primarily as a learning rather than an assessment exercise to give confidence in elementary programming through pleasurable exploration and creativity Accordingly students are given free rein to write a graphical program of their choice and are encouraged to make it entertaining To ensure appropriate coverage they are required to use a wide range of Turtle commands and program constructs but the work is not difficult and even timid students can safely g
3. 222 6 6 10 Baldwin Doug 2003 A compiler for teaching about compilers ACM SIGCSE Bulletin 35 1 pp 220 3 99 11 Bell D A and Wichmann B A 1971 An ALGOL like assembly language for a small computer Software Practice and Experience 1 pp 61 72 12 12 Ben Ari Mordechai 1998 Constructivism in computer science education ACM SIGCSE Bulletin 30 1 pp 257 61 3 96 13 Bonar J and Soloway E 1989 Preprogramming knowledge a major source of misconceptions in novice programmers in Soloway and Spohrer 126 pp 325 53 27 14 Bornat Richard 1979 Understanding and Writing Compilers Macmillan 76 77 15 Boyle Tom 2000 Constructivism a suitable pedagogy for information and computer science 1 Annual LTSN ICS Conference Edinburgh 3 4 16 Bridges Susan M 1993 Graphics assignments in discrete mathematics ACM SIGCSE Bulletin 25 1 pp 83 6 119 17 Brilliant Susan S and Wiseman Timothy R 1996 The first programming paradigm and language dilemma ACM SIGCSE Bulletin 28 1 pp 338 42 7 102 18 Bruce Kim B Andrea Danyluk and Thomas Murtagh 2001 A library to support a graphics based object first approach to CS1 ACM SIGCSE Bulletin 33 1 pp 6 10 9 19 Brusilovsky P E Calabrese J Hvorecky A Kouchnirenko and P Miller 1994 Mini languages a way to learn programming principles Educatio
4. belonging to the LISP family but due in part to its childish associations and the fact that in schools it is seldom exploited to anything like its potential it suffers from low esteem and credibility This lack of credibility has presumably both contributed to and been exacerbated by the language s virtual invisibility in the commercial world Although the choice of language in a programming module designed for elective students perhaps need not be so influenced by these sorts of concerns as in a module for prospective computing professionals there is little doubt that an introduction to programming based on LOGO whatever its educational merits would have difficulty attracting sufficient recruits to be viable An elective programming module must balance a number of factors some of which tend to pull in opposite directions a Ease of learning by students who cannot be presumed to have a strong commitment to mastering programming for career or academic progression b Quick results early positive feedback so that right from the start a student can get the satisfaction of visible progress again a need which is greatly amplified by the lack of presumed career commitment from these students c Minimisation of debug frustration the common experience of beginning students struggling with multiple syntax errors or manifestly incorrect output d Suitability for appropriate learning of both fundamental
5. forward and right this approach is also shared by Karel the Robot 99 The current system departs from this philosophy in order to provide students with more immediately satisfying feedback from their programming efforts e g by simplifying the construction of complex patterns and increasing the efficiency of circle and blot drawing to facilitate reasonably fast real time movement See 3 4 1 below for more on this 19 The Edit menu again largely follows the typical Windows pattern providing an undo redo facility which remembers up to 100 steps and four simple operations involving the Windows Clipboard which can therefore be used for transferring text into and out of the program editor e g from the Help system or into a text file Program line indentation controls are also provided including most notably an auto formatter which can if desired impose standard capitalisation and appropriate indentation on any legal program and is thus particularly useful for a teaching context to encourage students to keep their program code neatly structured The Layout menu provides two possible choices for the Canvas dimensions especially useful for beginning students who find their program drawing beyond the standard boundaries but have not yet encountered the canvas instruction It also gives various options for screen layout and font choice which can be particularly helpful if the system is b
6. 5 1 2 Coming back now to code line 23 this simply loads the various actual procedure parameters in order onto the Program Stack before performing PROC 1 to call the procedure which starts at code line 1 The first and third actual parameters are numerical constants yellow being FFFF in hexadecimal 65535 in decimal so these are loaded onto the Stack using LDIN The second is a global variable serving as a value parameter to the procedure so its current value is also simply loaded onto the Stack using LDVG But the last three actual parameters by contrast are global variables corresponding to reference VAR parameters within the procedure so it is not their values that get loaded onto the Stack but rather their Heap addresses this requires use of the instruction LDAG rather than LDVG Once all these Stack 67 operations have been completed the PROC 1 command at the end of code line 23 transfers control to line 1 but before doing so as explained in 6 1 it first saves the appropriate return location in this case 23 on the Procedure Return Stack so that when the procedure terminates with ENDP the Turtle Machine will have a record of where execution should continue in this case from the end of code line 23 hence code line 24 will be the next to be executed after the procedure terminates By the time procedure doball is called the condition of the Stack is as follow
7. Soloway 124 Urban Lurain and Weinshank 132 133 2 But I will add some additional considerations that bear particularly on the context of teaching programming concepts to elective students in a university Piaget s theory of human intellectual development as presented in works such as The Origin of Intelligence in the Child 103 and The Child s Construction of Reality 104 has had a huge impact on modern educational practice and also carries a number of potentially significant implications for the learning of programming in particular His work has given rise to the educational theory of constructivism whose central theme is that knowledge must be constructed by the learner rather than taught by authorities or read passively from the world This theme has sometimes been taken to epistemological extremes as in idealist or postmodern rejections of the very possibility of objective knowledge but its dominant educational manifestation has been an emphasis on student centred learning Boyle 15 usefully summarises the implications under five headings in the context of a discussion of learning environments for computing The first three headings are particularly relevant here 5 2 But as remarked earlier programming can have wider educational value quite independently of these concerns where it is used as a vehicle for facilitating learning about other domains 3 For a wide range of essays on constructivism and educatio
8. TURTT TURTC Identifier Scope Index Type value ref Line Range PCode Range TURTX PROGRAM 1 integer value 38 to 45 31 to 36 TURTY PROGRAM 2 integer value 38 to 45 31 to 36 TURTD PROGRAM 3 integer value 38 to 45 31 to 36 TURTT PROGRAM 4 integer value 38 to 45 31 to 36 TURTC PROGRAM 5 integer value 38 to 45 31 to 36 S STEPS 1 integer value 9to 19 1to 15 COUNT STEPS 2 integer value 9to 19 1to 15 XVEL THROWBALL 1 integer value 22 to 36 16 to 30 YVEL THROWBALL 2 integer value 22 to 36 16 to 30 GRAVITY THROWBALL 3 integer value 22 to 36 16 to 30 FLOOR THROWBALL 4 integer value 22 to 36 16 to 30 Procedures _Declaration PROGRAM procedure procedure identifier ballsteps 5 STEPS PROGRAM 1 2 THROWBALL PROGRAM 4 4 Parent Line Range PCode Range 381045 31 to 36 1 to 15 9to 19 22 to 36 16 to 30 Params Heap 0 5 Canvas Syntax Expressions Declarations Figure 6 Declarations Display 22 the variables and the procedures that have been defined within the program together with their scope and the corresponding ranges of Pascal and PCode instructions For each variable the type and calling method are also shown and for each procedure the parent routine parameter count and heap storage requirements 7 For a discussion of the design of the virtual Turtle Machine and its PCode see Chapters 5 and 6 below For details on the operation of the compiler see Chapter 7 We no
9. it is not enough just to store the relevant PCode integers as a continuous undifferentiated sequence An additional code is needed within each line either at the beginning to specify the length of each line or at the end to act as a terminator The former method is used for the actual storage of Turtle Graphics Compiled PCode files extension tgc when they are saved from the Compile menu and this enables the line structure to be restored very easily when it is loaded into the Turtle Run Time Standalone System which is designed to run compiled PCode directly In addition to these individual line length codes three other numeric codes are stored within each PCode file at the very beginning representing in turn the total number of compiled code lines the index of the code line from which execution is to start and the number of global variables these too are needed to facilitate the standalone execution of the compiled PCode The organisation of PCode into code lines naturally suggests their use for determining the sequencing and flow of control when the compiled code is executed though this involves some departure from the behaviour of a typical real life processor whose code locations jumps and branches are all likely to be specified by absolute or relative memory addresses It also brings some inefficiency because the compiled PCode has to be set up in a two dimensional array with consequent processing and space overh
10. test the counting variable against the second expression s value and branch with an IFNO command accordingly cf 5 3 4 gt 8 e repeat and begin each simply pushes the corresponding pending operation poRepeat and poBegin respectively onto the stack before looping back 55 Or a semicolon which just loops back without any further action and so is effectively ignored The treatment of semicolon statement separators is not discussed here because they have little relevance to the PDA operation but the compiler of course checks that they occur appropriately It also combines with the auto formatter to remove redundant semicolons 56 This is the meaning of the READ lt expr gt ItThen box in the PDA diagram Here and elsewhere the diagram shows only the syntactically legal options if the syntactic constraints are not respected the compiler will generate an appropriate error message and terminate 57 Since the extent of the then part is not yet known however this IFNO instruction is given a temporary dummy argument which will in due course be backpatched as in c below 58 This IFNO instruction is given a dummy argument which will be backpatched as in b below 89 until pops the top pending operation from the stack and checks that this is poRepeat before going on to read an expression which specifies the until condition then a PCode IFNO command is generated to branch back to the pcode
11. would be an argument for reserving also such words as case function or while that are both reserved words and keywords within Delphi and other full implementations of Pascal 80 ltSemicolon ItMinus ambiguous ItDot after final end ItComma ItSubt binary ItInt decimal integer ItLbkt ItNeg unary ItHex hexadecimal integer ItRbkt ItEqual ltIdK identifier or keyword ltColon ItNotEq lt gt ItId identifier ItAsgn ItLess lt ItNull no type yet assigned ItPlus f ItMore gt ItError illegal character ItMult i ItLessEq lt comments are completely ignored ItDivide ItMoreEq gt illegal characters generate an error The process that assigns these lexical types is very straightforward reading through the source text one character at a time and sending each character through a modified finite state machine structure which either makes an immediate decision about the lexical type e g in the case of semicolons and brackets or else moves to an appropriate pending state to await the next character s and process accordingly When a lt is encountered for example the FSM moves to state stLess in which e if the next character is either gt or it is linked together with the lt as a single lexeme of type tNotEg lt gt or ItLessEq lt respectively e otherwise the lt is recorded as of type tL
12. 1 2 above and which like Turtle s visual compiler faithfully displays its inner workings consistently with that metaphor cf Pane and Myers 95 section 5 2 8 1 3 Automata Early Although formal automata theory is generally considered far too abstract to be given a place in the first year studies of Computing students Chua and Winton 29 argue that automata are best introduced early in a way that emphasises their practical applicability in order to prepare the ground for later work by developing maturity in thinking about them Again this approach would give Turtle an obvious role enabling the explanation of machine execution to be combined with a discussion of some of the stages of compilation aided by the Visual Compiler display showing the ongoing state of the FSM and PDA that it uses Neither the FSM nor the PDA are overly complex and both have a very clear practical role in the compilation process so this can be used to motivate an appreciation of their more general practical value It is interesting to speculate for example what effect such a background might have had on students faced with the kind of arithmetic expression evaluation tasks that led to such disappointing results in McCracken et al 83 where the lack of a confident understanding of stack structures was seriously prejudicial 99 8 1 4 Familiar Compilation Courses on compilers are notoriously difficult and time consuming so that typically very
13. 28 29 31 31 33 36 40 Chapter 5 The Virtual Turtle Machine and its PCode Object Language 5 1 Turtle Graphics Commands the Program Stack and Arithmetical Boolean Operators 5 2 Command Parameters Global Variables and the Heap 5 3 PCode Sequential Structure and Flow Control 5 3 1 PCode Line Structure Storage and Sequencing 5 3 2 The Conditional if then 5 3 3 The Iterative repeat until 5 3 4 The Counting Loop for do Chapter 6 Turtle Machine Procedures 6 1 Procedure Calls and the Return Stack 6 2 Local Variable Storage the Heap Top Pointer and Procedure Heap Pointers 6 3 Claiming and Releasing Heap Space the Heap Control Stack 6 4 Dealing with Parameters 6 4 1 Value Parameters 6 4 2 Reference Parameters 6 5 The Procedure Register Stack 6 6 Putting Procedures Together 6 6 1 Note on the Trace Facility 6 7 Stack Variations on the Turtle Machine 6 7 1 Doing Without the Procedure Return Stack 6 7 2 Doing Without the Heap Control Stack 6 7 3 Doing Without the Procedure Register Stack Chapter 7 The Turtle Compiler 7 1 Standard Compilation Subtasks 7 2 The Structure of the Turtle Compiler 7 3 Lexical Analysis and Screening 7 4 Parsing the Block Structure of the Program 7 4 1 Generation of Procedure Code 7 5 Parsing and Code Generation for Program Control Structures 7 6 Parsing and Code Generation for Individual Commands and Expressions 7 6 1 Code Generation for Commands and Proce
14. 6 LDIN 200 BLOT end colour red 7 LDIN 255 COLR blot 100 8 LDIN 100 BLOT END 9 HALT The crucial code line here is the second where the conditional test is performed First the current value of turtc is pushed onto the Stack LDVG 5 then the integer 1 LDIN 1 The command EQUL then tests the top two Stack values for equality leaving a true result i e 1 in their place if they are indeed equal and a false result i e 0 if they are different If the result was true then the following command 52 IFNO 5 is simply ignored and processing continues through code line 3 before meeting the unconditional JUMP 7 command at line 4 which jumps over code lines 5 and 6 directly to 7 If on the other hand the result was false then IFNO 5 branches the processing to code line 5 and continues from there In either case the program halts at code line 9 5 3 3 The Iterative repeat until The repeat until structure requires only a single conditional backward branch and so can be straightforwardly compiled using the IFNO instruction for example PROGRAM concentric VAR radius integer BEGIN radius 500 1 LDIN 500 STVG 6 repeat randcol 8 DIN 8 RNDC DVG 6 BLOT blot radius Nn Ees SON EEO vio Ee UO lt Q OY s radius radius 12 4 DIN 12 SUBT STVG until radius lt 20
15. Cat in the Hat who offers to help However so far from helping he simply manages to turn most of the snow red so now the children have the extra job of cleaning the snow It s at this point that the Cat in the Hat introduces his helpers Little Cats A to Z each of which lives inside the hat of the previous cat The Cat in the Hat Powerpoint show gives four double page spreads from the book covering the unveiling of Little Cats A to G read these through in an appropriate rhythm to bring out the humour 112 f g h i G Now copy and run the simple non recursive triangles program in the Recursion sub section and explain that the procedure gives the instructions that a cat needs to follow for drawing a triangle What s special about recursion though is that at any point in drawing its own triangle a cat can pop another cat out of its hat and instruct it to go and do something and then jump back into the hat before the initial cat continues on its way Illustrate this first by having the triangle procedure include just one recursive call then all three as in the illustrative program When working through all this it helps a lot if you walk out the steps of the various cats and mime the hat operations etc Show the students The Recursion Factory from the Help menu and invite them to play with it to produce their own recursive designs Explain how such designs can be saved
16. How students attempt to reduce abstraction in the learning of mathematics and in the learning of computer science Computer Science Education 13 pp 95 122 12 56 Hoc J M 1989 Do we really have conditional statements in our brains in Soloway and Spohrer 126 pp 179 90 41 57 Hoc J M T R G Green R Samurcay and D J Gillmore eds 1990 Psychology of Programming Academic Press contains 58 84 112 58 Hoc J M and Nguyen Xuan A 1990 Language semantics mental models and analogy in Hoc et al 57 pp 139 56 96 59 Holmes Geoffrey and Smith Tony C 1997 Adding Some Spice to CS1 Curricula ACM SIGCSE Bulletin 29 1 pp 204 8 100 60 Jarc Duane J 1992 Ada Pascal s replacement for introductory courses in computer science Proceedings of the ACM Conference on TRI Ada Orlando Florida pp 126 34 7 61 Jenkins Tony 2001 Teaching programming a journey from teacher to motivator 2 Annual LTSN ICS Conference London 41 42 62 Jenkins Tony 2002 On the difficulty of learning to program 3 Annual LTSN ICS Conference Loughborough 41 63 Jenkins Tony 2003 The first language a case for Python 4 Annual LTSN ICS Conference Galway 7 64 Jenkins Tony and Davy John 2001 Diversity and motivation in introductory programming Italics 1 41 65 Jensen Kathleen and Wirth Niklau
17. R G R K E Bellamy and J M Parker 1987 Parsing and gnisrap a model of device use in Olson Sheppard and Soloway 93 pp 132 46 8 47 Grune Dick Henri E Bal Ceriel J H Jacobs and Koen G Langendoen 2000 Modern Compiler Design Wiley 78 48 Guimaraes M C de Lucena and M Cavalcanti 1994 Experience using the ASA algorithm teaching system ACM SIGCSE Bulletin 26 4 pp 45 50 117 49 Guzdial Mark 1993 Software realized scaffolding to facilitate programming for science learning Interactive Learning Environments 4 pp 1 44 2 41 50 Hadjerroult Said 1998 Java as first programming language a critical evaluation ACM SIGCSE Bulletin 30 2 pp 43 7 7 51 Hagan Dianne and Markham Selby 2000 Does it help to have some programming experience before beginning a Computing degree program ACM SIGCSE Bulletin 32 3 pp 25 8 43 52 Harel I and Papert S 1990 Software design as a learning environment Interactive Learning Environments 1 pp 1 32 2 53 Hartley James and Greggs Michael A 1997 Divergent thinking in Arts and Science students Contrary Imaginations at Keele revisited Studies in Higher Education 22 pp 93 7 6 54 Haynes William R Jr Charles E Hughes and Charles P Pfleeger 1977 Oracle a tool for learning compiler writing ACM SIGCSE Bulletin 9 7 pp 37 51 99 104 55 Hazzan Orit 2003
18. SUPPORT system Zelkowitz et al 140 which also uses Pascal or automated translation from pseudocode as provided by SCHEMACODE Robillard 110 which supports Pascal as well as other syntaxes However such options would require integration with the standard text editor if the system were to continue to provide familiarisation with conventional code editing as required for it to serve as an appropriate bridge to Delphi or Kylix 65 For the same reason I do not consider here the use of graphics based programming tools along the lines of Pict Glinert E and Tanimoto 45 Amethyst Myers et al 87 DSP Olsen 92 ASA Guimaraes et al 48 or BACCII Calloni and Bagert 23 118 The Environment Another worthwhile addition might be a library of templates and examples as provided by the Example Based Programming System EBPS of Neal 89 Sloman 122 suggests a large number of other features worthy of consideration within a programming environment for novices including interactive stepping through a program incremental compilation and various intelligent aids Similar themes are also explored in Atwood et al 8 while Pane et al 96 though primarily focused on the design of a visual environment for children draw attention to a number of usability criteria that would also be worth bearing in mind The Language For the teaching of loops it is useful to have a break command like that of Delphi l
19. The lecturer was always clear and audible 52 29 4 1 0 88 4 0 14 2 The lecturer made good use of examples etc 47 33 4 0 1 86 8 include ali time spent at relevant lect The lecturer made things easier to understand 36 38 11 1 0 81 7 ures reading or preparing as well 8 The lecturer helped to make things interesting 46 27 10 2 1 83 4 time spent working with the computer a either in practicais or by yourself The lecturer seemed responsive and approachable 50 26 8 1 1 85 8 Meximumnoure ta The paure meon was well designed one appropriate 40 32 8 0 2 82 9 include in statistics 14 E I feel this work has taught me useful skills 34 32 10 4 2 78 0 Not Fairly Average Very Helpful Little Hel Useles Hours Spent on Coursework Used os ee Mei Care rae AVAD The LECTURE HANDOUTS were 84 13 9 2 0 81 9 The relevant ACOM BOOKLET was ia 28 14 4 0 84 1 The ACOM WEB RESOURCES were 64 16 27 1 0 78 0 The relevant ACOM NEWSGROUP was My 4 5 5 2 56 3 NoEnquiry Dealt withOK Not yet sorted Ignored OK 100 peas My enquiry to ACOMPOST LEEDS was Mom 4 0 0 100 0 Not Approachable Approachable Unapproachable Average Bh Ue al ca en eee Consuited and Helpful but Unhelpful and Unhelpful 484 7087 Staff at the ACOM HELPDESK were ism 13 0 0 100 0 The ACOM DEMONSTRATORS were ERT 20 1 0 97 6 Other Comments ACOM teaching staff assisting in office hours etc 1 It was REALLY useful to have small exercises set each week It hel
20. a square by visiting its corners in turn is no less natural but is significantly more generalisable than specifying its coordinates within a dedicated routine 3 4 2 Colour Handling Turtle allows 24 bit colours to be specified following the Delphi convention of giving the three components in BGR order within a hexadecimal number prefixed by Thus colour 0080FF will create an orange with zero intensity of blue 128 of green and 255 of red However for compatibility with the widespread HTML convention Turtle also allows these colours to be specified in RGB order prefixed by e g colour FF8000 24 bit colouration allows very fine variation which can be used to produce 3D shadowing effects as in the built in balls3D illustrative program For introductory teaching purposes it is important to be able to access the basic colours more simply and therefore eight predefined colour constants have also been 26 Again without redundancy since if movexy were to be defined as penup followed by drawxy then the state of the pen would be affected whereas movexy leaves it unchanged 30 provided namely blue green cyan red magenta yellow white and black Each of these constants is precisely equivalent to the corresponding BGR integer hence colour red for example has the same effect as colour FF or colour 255 Fur
21. an engaging and non threatening manner preparing the ground for more serious programming in follow on courses For this reason student perception is the most important single criterion by which the course must be judged and accordingly anonymous feedback was sought systematically through a specially written program which invited students to express their views immediately after having submitted their coursework Similar feedback was also solicited for all other components in the ACOM teaching programme enabling comparisons to be drawn To maximise the chance that all students would indeed express their views as fully as possible the feedback interface was divided in two the first simply asking them to click on radio buttons to give an overall appraisal and the second inviting more detailed opinions As a result the feedback response rate for the first part was extremely high typically 75 to nearly 100 depending on the question but only around one in six of the students apparently disproportionately those with the stronger opinions also ACOM Feedback Reporter c PJR Millican 2001 SBE Select Coursework Option poe e kOe ket Responses from 1 10 2001 until 30 9 2002 Change Dates Number of Student Peter Millican Responses Strongly Unsue No p Strongly Average P 108 Agree Agree View Disagree Disagree SA 100 The lectures were well structured and organised 39 42 4 0 1 84 3 Average Time Spent k in Hea P
22. as follows 4 Informal description of Assembler Decimal Hex command behaviour mnemonic code code load variable or value parameter LDVV store variable or value parameter STVV 22 Suppose for example that a program has the five procedures proc to proc5 of which the first is called by the third which is in turn called by the fifth 4 Suppose also that the first procedure proc1 currently running has three local variables named one two and three Then assuming that the only other currently active procedures are proc5 and proc3 the context in which LDVV and STVV reference these variables can be pictured as shown in Figure 11 below To load the second local variable of procl i e two onto the Program Stack the appropriate command is LDVV 1 2 where 1 indicates the first procedure in the program and 2 the second local variable of that procedure To identify the required address on the Heap we simply take the Procedure Heap Pointer for the procedure in question i e Procedure Heap Pointer 1 and add the index of the particular variable i e 2 As Figure 11 shows this indeed generates the correct address and the same applies to the corresponding inverse command STVV 1 2 42 40 In these mnemonics the first V stands for variable and the second for value because these are the commands pertaining to local variabl
23. better that they should achieve genuine competence and confidence with a small number of types structures commands etc than that they should run any risk of being confused at this early stage by the niceties of distinctions in meaning and usage between a variety of syntactic options The same point applies to some extent to the system s role as a vehicle for introducing more advanced concepts of compilation There would be little point from this perspective in providing a variety of control structures that are compiled in much the same way e g repeat and while since this would add complexity without widening the conceptual repertoire More potential benefit would come from adding a greater range of types and data structures e g reals strings enumerations sets arrays records objects methods of access e g pointers and subroutines e g functions methods units since these would involve significant extensions to the machine 18 Tn addition to the references in the text see for example du Boulay 41 du Boulay et al 42 and Motil and Epstein 86 19 Lewis and Olson 72 identify abundance of low level primitives as a major cognitive barrier to programming while Eisenberg et al 43 found confusions arising particularly where novices were presented with a variety of syntactic structures to achieve a similar effect 24 processes and compiler techniques that the system could be used to illust
24. bf 50 13 LDHT LDRJ LDIN 50 PROC 1 STHT The only complication here is that the Heap pointers must be loaded onto the Program Stack using LDHB and LDHT before the procedure return jump location is loaded with LDRJ enabling this location to be pulled at the end of the procedure by PLRJ without affecting the Heap pointers 6 7 3 Doing Without the Procedure Register Stack Since the Procedure Register and the stack that maintains it are used by the Turtle Machine only to enable procedures to be traced the PSPR 1 and PLPR commands can simply be removed from the compiled PCode without significantly affecting it When it is then run the only apparent difference apart from a very small increase in speed will be within the trace display where the Proc column will continuously show 0 0 instead of for example 1 1 1 2 1 3 etc depending on how many copies of the procedure are active In Chapter 7 we move on to the Turtle system s compiler examining the mechanisms that it uses to translate the Pascal source language presented in Chapter 3 into the Turtle Machine code structures explained in Chapters 5 and 6 76 Chapter 7 The Turtle Compiler Having examined the virtual Turtle Machine we must now turn to the Turtle compiler whose job is to turn source Pascal code as described in Chapter 3 into Turtle machine code or PCode as described
25. can be consulted if further details of the syntax or the relevant instructions are required 2 2 The System Menus Again the Help file contains a full description of the various system menus in the six sections entitled File menu Edit menu Layout menu Compile menu Options menu and Help menu and there is no need to discuss them in detail here However the following summary may prove helpful for drawing attention to some of the system s more distinctive aspects e The File menu provides typical Windows style facilities for clearing loading and saving the program standardly as a text file with extension tgp and for transferring images drawn on the Canvas as bitmaps either to the Windows Clipboard or to a disk file 15 Pascal commands are formally called statements a convention mostly followed here The words instruction and command will generally be reserved for referring to PCode where the distinction matters instruction is used to mean a PCode or Turtle Pascal primitive and command a particular instance of an instruction together with its arguments 16 The original Turtle Graphics philosophy as exemplified by Papert s discussion 97 mentioned earlier in 1 2 is to include as few primitives as possible encouraging students to build up for themselves more complex instructions e g circle as procedures based on the simple primitives such as
26. compiler users in a learning environment ACM SIGCSE Bulletin 30 3 pp 157 61 119 74 Liffick Blaise W and Aiken Robert 1996 A novice programmer s support environment ACM SIGCSE Bulletin 28 SI pp 49 51 6 75 Linn M C and Dalbey J 1989 Cognitive consequences of programming instruction in Soloway and Spohrer 126 pp 57 81 42 76 Lippert R C 1980 Teaching problem solving in mathematics and science with expert systems Journal of Artificial Intelligence in Education 1 pp 27 40 2 77 Martin Peter 1998 Java the good the bad and the ugly ACM SIGPLAN Notices 33 4 pp 34 9 9 78 Marton F and S lj R 1976 On qualitative differences in learning British Journal of Educational Psychology 46 pp 4 11 98 79 Mayer R E 1989 The psychology of how novices learn computer programming in Soloway and Spohrer 126 pp 129 59 5 96 80 Mayer R E J L Dyck and W Vilberg 1986 Learning to program and learning to think what s the conmnection Communications of the ACM 29 7 pp 605 10 3 81 Mayo E 1933 The Human Problems of an Industrial Civilization Macmillan 39 82 McConnell Jeffrey J 1996 Active learning and its use in Computer Science ACM SIGCSE Bulletin 28 SI pp 52 4 5 83 McCracken Michael Vicki Almstrum Danny Diaz Mark Guzdial Dianne Hagan Yifat Ben David Kolikant Cary L
27. few students take them and those that do are given the task of compiling an unfamiliar toy language rather than the real thing such languages include Haynes et al s ORACLE 54 Appelbe s MINIPASCAL 6 Aiken s Cool 4 Baldwin s MinimL 10 and numerous others with and without names In this context there may be a real virtue in both using the Turtle system to introduce programming to novices and also later returning to it when they come to consider the study of compilation At least one hurdle of unfamiliarity is thus removed probably making the subject seem both more approachable and more relevant 8 2 Conclusion The Achievements of this Work The last few sections have been to some extent speculative and the potential value of the Turtle system in teaching about machine code automata and compilation is largely independent of whether this can indeed also be closely related to the teaching of introductory programming My more fundamental claims about the Turtle Machine and its workings are A4 That it is simple enough for more advanced students to understand and to learn from and is indeed simpler in relevant respects than any machine of comparable power that I know of A5 Likewise its compiler though in a sense relatively complex because it mixes parsing techniques rather than starting from a comprehensive language grammar presents the parsing process in a way that is easier to grasp and from which to l
28. followed by the assignment operator lexical type tAsgn and an expression e The identifier is a declared and in scope procedure name in which case it should be followed by an appropriate number of parameters in brackets e The identifier is a Turtle Graphics command name in which case it should be followed by an appropriate number of parameters in brackets In the first case the expression is processed by the method described in 7 6 which generates the necessary PCode for leaving the result of the expression on the Program Stack Then depending on the nature of the variable an appropriate store command STVG STVV or STVR is generated to perform the assignment The second and third cases are initially treated in the same way the opening bracket is checked and then the parameter expressions separated by commas are repeatedly processed as in 7 6 until the final bracket is encountered any deviation from this pattern producing an error message The PCode thus generated will leave one item on the Program Stack for each expression encountered and these will be passed as the parameters for the procedure cf 6 6 or command cf 5 1 2 Hence a test is made to ensure that the number of parameters is correct and if so the necessary PCode is added either to call the recognised procedure as in 6 1 or to execute the relevant Turtle Graphics command see 5 1 This concludes our det
29. having to handle seriously complex programs However this point should be taken in the context of 4 4 below which highlights evidence that fundamental student difficulties occur at a very early stage of understanding so an introductory component arguably faces a task which is harder rather than easier compared to later components Moreover the programs produced on the Turtle component were not in any case particularly simple many extended to several hundred lines and numerous procedures though admittedly they were simple in terms of data structures and techniques e Turtle was taught by two members of staff who were individually very committed to the system including its developer the other was Dr Sarah Kattau who has contributed teaching materials for Turtle and been its most assiduous tester Moreover both teachers have scored well in other teaching implying that the good results and feedback in the programming component might have been due at least in part to their general teaching abilities and their specific enthusiasm for Turtle 2 This concern probably has an element of truth but it risks putting the cart before the horse because part of what makes an effective teacher is the ability to identify and develop effective tools and methods If Turtle is seen by teachers elsewhere as a useful tool there is no evident reason why it should not inspire them and their students with comparable enthusiasm to that which in Leeds has led to s
30. in Chapters 5 and 6 The compiler involves nearly 2 000 lines of Delphi Pascal so obviously it would be inappropriate here to attempt to consider it all in detail Hence this chapter will focus only on the compiler s overall structure and processes and on points of sufficient generality to be of educational interest independently of the specific environment and programming language used It aims to provide not only an explanation of the compiler s processes but also an illustration of how the system can provide a vehicle for presenting compiling techniques to students in a relatively accessible manner without any ascent to the level of abstraction typically demanded by textbooks on compilation theory 7 1 Standard Compilation Subtasks Standard accounts of compilation typically divide the process conceptually into some combination of the following set of subtasks gt a Lexical analysis handled by a scanner which decomposes the source text into individual lexical units e g treating gt and whole words as single such units b Screening which refines the output of the lexical analysis by processes such as the elimination of units that are not part of the execution code e g separators and comments and distinguishing between source words that are mere identifiers and those that are keywords or reserved words of the language e g begin 50 Out of a system total of around 6 500 lines of Delph
31. line where the repeat occurred depending on the expression s value cf 5 3 3 5 end likewise pops the stack checking that either it is empty in which case the end signifies the finish of the current program block or else that the popped pending operation is poBegin These checks ensure that the relevant syntactic structures have been properly nested so that each until matches with a previous repeat and each end with a previous begin An identifier indicates an assignment statement a Turtle Graphics command or a procedure call the main processing of these is dealt with in 7 6 1 below As the diagram shows the last two cases here encompassing until end assignment statements Turtle Graphics commands and procedure calls do not immediately loop back to read the next tokenised lexeme because before doing this a check needs to be made to see whether the current statement which may be a statement sequence bracketed by repeat until or begin end is within the immediate scope of a conditional or a for loop This check is carried out by popping the top pending operation from the stack and what happens next depends on the nature of this pending operation a If the pending operation is poElse then the conditional structure is terminated by b backpatching the argument to the PCode JUMP instruction which jumps around the else
32. novices to establish such a mental model 42 an extremely difficult topic e g Dicheva and Close 38 Kahney 66 Kessler and Anderson 68 Levy 71 Yet the Turtle system for all its relative simplicity can very effectively convey an understanding of recursion sufficient at least to persuade most of a novice class that they grasp it after only three lectures 4 Even if the level of understanding conveyed by experimenting with Turtle were relatively modest the very fact that it can give students confidence and enjoyment is itself of potentially major significance Robins et al 111 p 158 summarise research by Linn and Dalby 75 and Kurland et al 70 in words that could serve as a manifesto for the type of graphics based self learning system that Turtle exemplifies The reinforcement and encouragement derived from creating a working program can be very powerful In this context students can work and learn on their own and at their own pace and programming can be a rich source of problem solving experience Working on easily accessible tasks especially programs with graphical and animated output can be stimulating and motivating for students They also report a survey by Rountree et al 113 as indicating that for novice students the most reliable indicator of success was the grade that the student expected to achieve p 155 Obviously it can be debated here which factor was cause and which effect but Jenkins
33. of program syntax semicolons etc tell them not to worry too much about the details of this yet just to note and of course how the turtle s activity is determined by the commands When showing them the Help section on The Program draw attention to the links it makes to the reference sections give them a quick glimpse of these and mention again the illustrative programs and exercises Work through Exercises 1 and 2 in the lecture illustrating as you do so how you are making use of the Help system to find details on new commands etc Don t bother to do the face particularly carefully but it s a good idea to show them how to do the mouth illustrating how the order of commands can be important e g so a white masking blot doesn t obliterate a nose you ve already drawn Exercises 1 and 2 give the opportunity to cover some of the menu features especially in the File and Edit menus take advantage of this by drawing attention to them for example by using the Clipboard commands to take stuff from the Help file End by inviting them to do Exercises 1 and 2 for themselves before the next lecture and to have a go at Exercises 3 and 4 Draw their attention to the illustrative programs on FOR loops and take the opportunity to show them the Layout menu because the second illustrative program requires the larger Canvas Advise them to use the smaller Canvas in general because then if they find that their program move
34. of simplicity therefore and to motivate the use and hence learning of these parameter distinctions Turtle does not incorporate literal functions as such 4 23 For example a function dist that calculates the distance between coordinates permits the elegant syntax if dist x1 y1 x2 y2 gt 100 then whereas the same condition requires two statements if the distance is instead returned as the VAR parameter to a procedure 24 Arguably the importance of functions is such that they ought to be included nonetheless an argument that is likely to become decisive if Turtle is extended to other source languages 28 3 4 Special Turtle Graphics Facilities 3 4 1 Turtle Graphics Instructions In accordance with the purposes of the system Turtle s graphics primitives all of which are explained in the online Help file have been selected with the aim of making it easy to produce interesting and satisfying patterns while at the same time avoiding unnecessary complexity Complexity here is partly a psychological matter for example the provision of both forward and back makes it psychologically simpler to produce movement in two opposite directions though logically it would be more economical to provide only the single instruction forward which can be used with either a positive or a negative parameter a similar point applies to left and right The same principle of psycholog
35. part cf 5 3 2 If the pending operation is poTo or poDownTo then the counting loop structure is terminated by generating PCode commands to load and increment or respectively decrement the counting variable before the final JUMP the argument to the IFNO instruction at the beginning of the structure is also backpatched cf 5 3 4 59 This implies that the relevant code line number was recorded within the poRepeat pending operation when it was pushed onto the stack Every pending operation is in fact stored as a record structure which includes both an item of type TPendOp and also a code line reference 90 c If the pending operation is poThen the complier looks ahead to see whether the next lexeme is else if so that else is read poElse is pushed onto the pending operations stack and a PCode JUMP command is generated to jump around the else part of the conditional structure 6 Finally whether or not the next lexeme was else the argument to the IFNO instruction at the beginning of the conditional structure is backpatched to branch around the then part cf 5 3 2 d If the pending operation was anything other than poElse poTo poDownTo or polfThen it is pushed back onto the stack of pending operations In cases a and b and also case c where there is no else the process of popping the top pending operation from the stack continues as shown in Figure 17 Thus at
36. r variable parameter r The process of releasing this space is more complex and must be done outside the procedure itself because it depends on whether the procedure is called from a procedure as at code line 8 or from the main program as at code line 13 In the latter case the only thing needed to release the space is to reset the Heap Top Pointer to the value it had prior to the procedure call this is easily done by pushing that value onto the Program Stack before the procedure is called the LDHT in code line 13 and then pulling it from the Stack to reset the Heap Top Pointer after the procedure has terminated the STHT in code line 13 In the former case however where the procedure call is itself within a procedure it is essential to save and restore both the Heap Top Pointer and also the Procedure Heap Base Pointer of the calling procedure hence the sequence LDHB 1 LDHT STHT STHB 1 in code line 8 bracketing the procedure call 75 The options discussed in these last two sections can be combined so that both the procedure return location and the Heap pointers are handled through the Program Stack rather than using Procedure Return and Heap Control Stacks In this case the relevant lines from the previous example would be modified as follows bf r 1 8 LDHB 1 LDHT LDRJ LDVV 1 1 LDIN 1 PLUS PROC 1 STHT STHB 1 end END 9 PLPR PLRJ
37. say the keyword else to match with a previous if then does not need to look for the four characters e P s e with no other contiguous letters or digits etc but need only check for a single lexeme of type tElse Because the keywords are recognised at the beginning of compilation and henceforth treated as tokenised lexemes rather than as strings they cannot be used in any other capacity e g from cannot be an identifier and hence cannot be the name of a variable indeed it is this fixedness of interpretation that characterises a Pascal reserved word 53 Each keyword has its own lexical type thus and is of type tAnd begin is of type tBegin and so on through do downto else end for if mod not ce 39 66 39 66 39 66 39 66 39 66 or procedure program repeat then to until var while and xor The other lexical types are listed in the following table 53 Although the terms keyword and reserved word are often as here used interchangeably there is a subtle difference between them in the case of a language which prevents certain words from being used as identifiers i e they are reserved but which does not use them itself so they are not strictly keywords For simplicity the Turtle system reserves only its own keywords though there
38. syntax Emphasise use of the auto format and how this can reveal problems e g if they put a semicolon immediately after do g h g h 111 Advice when working with loops i Start with the loop working once only e g for count 1 to 1 do ii Draw a black blot at the end of the loop so you can see clearly where the turtle s ending up iii When that s OK move on to the loop working twice only for count 1 to 2 do iv Only when that s OK move on to more It helps to motivate this advice if you fix it that the first time you run Exercise 3 you get five faces in different orientations and on top of each other Then you can illustrate how to fix the problem systematically in this way Don t spend long on the nested loops since it s important not to confuse Show it briefly and make reference to the relevant illustrative program drawing attention again to the value of indentation If there s time briefly introduce the notion of a procedure putting all the face commands into a procedure and showing how this makes the loop particularly easy to understand point out that this is in Exercise 6 Finally ask them to complete Exercises 1 to 4 before the next lecture and invite them to try Exercises 5 to 8 but again emphasise that they shouldn t worry if they get stuck Plan for Third Lecture on Turtle a b c d e Remind students about
39. system around a virtual Turtle Machine with its own machine code into which the user s Pascal programs would be compiled and giving visual access to the results of that compilation both static and dynamic This decision made Pascal a particularly good choice of language in yet another respect since Pascal was designed in part with ease of compilation in mind e g Jensen and Wirth 65 p 8 making it feasible to develop a compiler whose operations would be accessible even to non specialist students Unfortunately however the original type of Pascal P code developed by Niklaus Wirth the designer of the Pascal language was too complex to provide a plausible basis for such teaching see for example Wichmann 136 Bell and Wichmann 11 and Pemberton and Daniels 101 ch 10 12 So it was necessary to design and develop a new type of Turtle Machine with as simple a design as possible yet adequate to support a wide enough range of programming constructs to meet the various educational aims outlined above 1 5 An Introduction to Computing Concepts The upshot of all this was that the Turtle system ended up as something rather broader than originally envisaged Having begun life as a vehicle for introducing only basic programming concepts it became also a potential aid to exploration of the broader context of programming languages within Computer Science and a means of developing a practical understanding of a
40. that its overall behaviour can be summarised in a single one page diagram and which serve respectively to introduce finite state machines pushdown automata and recursive descent To incorporate into the programming environment a visual interface to the operations of the virtual machine and compiler to enhance the system s value as an introduction to Computing concepts Visual Compiler displays introduced in 2 3 and also illustrated in 7 3 5 overall value of the system in this capacity discussed in Chapter 8 17 Chapter 2 External Design The external design of the Turtle Graphics Programming System can most efficiently be conveyed by means of a quick guided tour through its main facilities so this chapter will accordingly be devoted to such a tour For simplicity it is written as addressed to a user who has a computer running with the Turtle system installed 2 1 Getting Started To start up Turtle navigate to the appropriate directory here presumed to be C Turtle and run the program turtle exe To load an illustrative program go to the Help menu and move the mouse down to the Illustrative programs item then click on the first program in the list named Simple drawing with pauses When the program appears click on the RUN button just to the right of the menu Turtle Graphics Programming System c PJR Millican 2000 2003 File Edit Layout Compile Options Help PROGRAM d
41. the multiple faces program and introduce the notion of a procedure using this as covered in Exercise 6 pointing out how it makes the loop s easier to understand especially when there are nested loops Again show the FOR loop illustrative programs and remind them to look at these Work through Exercise 5 systematically drawing attention to all the points mentioned in the Help file in relation to it Move on as in the Help for Exercise 5 to the material in the section on Procedures and Parameters up to and including Introducing Simple Value Parameters again drawing attention to the fact that everything you are covering can be read there too and also to the two relevant illustrative programs the second one of these introduces DOWNTO which is worth mentioning for the sake of completeness but don t waste time doing any more on it Now move on to recursion again following the material in the Procedures and Parameters section and again pointing this out take the opportunity to emphasise at appropriate points how a recursive procedure has to have its own local copy of the relevant variable s as explained in the section on Scope Start by introducing the Cat in the Hat from the eponymous children s book by Dr Seuss a mischievous cat who tries to be helpful but invariably causes trouble A boy and a girl left home by their mother with the job of clearing snow from the path are visited by the
42. these various checks fails then an appropriate error message is given to report precisely where the failure occurred The error location is usually given by the value of exnum at the point where it is located but sometimes this needs to be adjusted as in the final else clause where dec lexnum ensures that the error message for a missing type specification will be linked to the source line containing the variable name itself rather than whatever lexeme generated the error Simple mechanisms of the sort just discussed tied into the FSM state transitions deal with most of the parsing of block headers but various processes need to go on behind the scenes bookkeeping and keeping track of the overall program structure For example the addvtype routine mentioned above presupposes the operation of a prior addvar lexnum integer isparam boolean routine called whenever a new parameter 85 or variable name is declared i e any transition to state ParName ProcVName or VarName and which adds that name to a table into which the corresponding type can later be inserted by addvtype Recording of variable identifiers also brings the complication of checking for non duplication which requires that issues of scope be taken into account This is done by means of a procedure stack which monitors the nesting of procedures with each procedure s index being pushed when it starts through a transition to state ProcName and popped whe
43. turtc 4 setxy xp yp colour bgc blot size Xp XPtxv yp yptymove setxy xp yp colour col blot size update noupdate if xp lt size or xp gt 1000 size then XV XV END procedure doball BEGIN main program bgcol 5000 blank bgcol xpos 30 xvel 1 ypos 500 repeat until ypos lt 0 or ypos gt 1000 END main program PROCEDURE doball size bgc col integer VAR xp xv yp integer doball 30 bgcol yellow xpos xvel ypos N 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 po ONO P Ww PSPR1 HPCL17 ZERO17 STAR16 STAR15 STAR14 STW13 STW12 STW11 LDIN7 RNDC LDVG5 LDIN4 SUBT STW17 LDVR14 LDVR16 TOXY LDW12 COLR LDW11 BLOT LDVR14 LDVR15 PLUS STVR14 LDVR16 LDW17 PLUS STVR16 LDVR14 LDVR16 TOXY LDW13 COLR LDW11 BLOT UDAT NDAT LDVR14 LDW11 LESS LDVR14 LDIN1000 LDW11 SUBT MORE OR IFNO17 LDVR15 NEG STVR15 HPRE 1 PLPR ENDP LDIN 20480 STVG6 LDVG6 BLNK LDIN30 STVG7 LDIN1T STVG8 LDIN 500 STVG9 LDIN30 LDVG6 LDIN 5535 LDAG7 LDAG8 LDAG9 PROC1 LDVG9 LDINO LESS LIWGY LDIN1000 MORE OR IFNO23 HALT 66 This program is adapted from one of the built in illustrative programs which uses reference parameters to enable a single doball procedure to handle the motion of two or more balls that are bouncing around the screen each with its own stored set of position and velocity x and
44. which a procedure calls 11 3 4 LDIN 2 Pd U 6 1 1 2 256 29 itself Note the importance 12 3 6 MREQ Pad U 6 1 1 1 w Canvas Syntax Expressions Declarations PCode Figure 3 PCode and Trace displays as the triangles program executes 21 The PCode table here shows the assembler code into which the triangles program has been compiled the instructions can also be shown as purely numeric machine code by clicking on one of the left hand radio buttons above the table The lower table is the trace display which will appear only if the trace facility is operative This shows the assembler commands that have actually been executed while the program was running together with their parameters It also shows the momentary state just prior to each command s execution of the various program flags the Heap Top Pointer showing the extent of variable storage and Procedure Register showing which procedure is in progress and how many procedures are active and also the resulting state immediately after the command has been performed of the top three locations on the Program Stack which stores the parameters of the various instructions and operators as these are executed cf 5 1 and note 47 in 6 7 Three other displays are also provided through the bottom tab controls A syntax analysis including details of lexical divisions and also the finite state machine and pu
45. wide range of relevant concepts such as e high and low level languages machine and assembler code e the concept ofa stack and its value in computer architecture e subroutine calling return addresses stack frames etc e heap structures and dynamic variable management e parameter passing mechanisms call by reference and call by value e finite state machines and pushdown automata and their value in parsing e recursion and its application in recursive descent parsing Many of these are particular hard to grasp in the abstract but perhaps their concrete and transparent use within the Turtle Machine may prove of value in helping students to learn them 3 The system has not yet been used for teaching at this level but I hope before long to develop materials to support such use 13 Tt is a common observation reinforced by Piaget that concrete understanding develops far earlier in life than abstract and is typically far easier to acquire at any stage of development Hazzan 55 for example drawing on parallels with research in mathematics education gives a range of examples where students are assisted in learning computer science concepts by reducing abstraction Hence it is not unrealistic to expect that studying a concrete realisation of such concepts would make them easier to learn even taking into account the overhead of understanding the concrete model 13 1 6 Outline of the Thesis The
46. y components Only one ball is included here but because the procedure is designed in this way it would be easy to extend the program to include more balls perhaps to make a simple betting game Which ball will reach the top or bottom of the Canvas first Note also the ymove local variable which is set randomly via randcol to a value between plus and minus 3 to change the ball s y coordinate accordingly thus generating the ball s vertical random walk When run the PCode starts executing at code line 18 which sets the global variable bgcol to an appropriate background colour here the hexadecimal colour code 005000 20480 in decimal specifies 50 FF 80 255 in decimal intensity of green with zero of blue and red see 3 4 2 Then code line 19 uses this global variable to blank out the Canvas accordingly after which lines 20 to 22 set up the starting values of the other three global variables 5 2 The body of the repeat loop is just code line 23 which executes the procedure call to doball Code line 24 then performs a conditional branch back to line 23 unless the condition ypos lt 0 or ypos gt 1000 evaluates as true the mechanism by which a repeat loop achieves the required conditional branching is explained in 5 3 3 Most of the PCode in line 24 is devoted to evaluating the relevant condition operating in standard reverse Polish fashion using the Program Stack
47. 17 Shafto Sylvia A S 1986 Programming for learning in mathematics and science ACM SIGCSE Bulletin 18 1 pp 296 302 2 118 Sime M E T R G Green and D J Guest 1977 Scope marking in computer conditionals a psychological evaluation International Journal of Man Machine Studies 9 pp 107 18 8 119 Sims Knight Judith E and Upchurch Richard L 1993 Teaching software design a new approach to high school computer science American Educational Research Association Atlanta Georgia 41 120 Slack James 2000 Introduction to Programming and Problem Solving in Java Brooks Cole 9 121 Sleeman D R T Putnam J Baxter and L Kuspa 1988 An introductory Pascal class a case study of students errors in R E Mayer ed Teaching and learning computer programming multiple research perspectives Hillsdale New Jersey Lawrence Erlbaum Associates pp 237 57 27 41 122 Sloman Aaron 1984 Beginners need powerful systems in M Yazdani ed New Horizons in Educational Computing Ellis Horwood pp 220 35 118 123 Smyth Graham 2001 Survey on programming language preferences ACSEnt Association of Computer Studies Educators 2 pp 1 3 7 124 Soloway E 1993 Should we teach students to program Communications of the ACM 36 10 pp 21 4 3 125 Soloway E J Bonar and K Ehrlich 1989 Cognitive strategies and looping constru
48. 5 DVG 6 LDIN 20 LESS IFNO END 6 HALT Here code line 5 makes the test for radius lt 20 if this is false then processing branches back to code line 2 repeating the loop again 5 3 4 The Counting Loop for do The for do loop or for downto if the counting is downwards like if then involves one jump and one conditional branch though it is significantly more complex because of the handling of the loop variable Here is the complete forloops illustrative program together with its compilation 53 PROGRAM forloops VAR count integer BEGIN 1 LDIN 1 for count 1 to 200 do 2 STVG 6 LDIN 200 begin 3 LDVG 6 MREQ IFNO 11 forward count 3 4 LDVG 6 LDIN 3 DIV FWRD right 5 5 LDIN 5 RGHT colour red 6 LDIN 255 COLR blot 200 7 LDIN 200 BLOT colour black 8 LDIN 0 COLR circle 200 9 LDIN 200 CIRC end 10 LDVG 6 LDIN 1 PLUS JUMP 2 GI zZ jw p n HALT The program starts by loading 1 onto the Stack and at code line 2 this value is moved into the variable count and replaced on the Stack by the loop termination value of 200 Now the variable is reloaded onto the Stack in order to compare it against that 200 38 with the operator MREQ leaving the result true i e 1 on the Stack if and only if 200 is more than or equal to the variable s current value Hence if the result
49. 5 5 9 63 65 12 7 predecessor involving additional 66 69 7 8 66 69 13 7 advanced concepts such as recursion le hoe oe ere o o The crucial difference between the two 76 79 2 0 76 79 3 6 80 82 5 4 80 82 2 3 was apparently that Turtle made it far 83 85 24 83 85 1 0 easier for students to get started 86 90 5 9 86 90 0 0 Again it must be emphasised that this teaching did not take place in the context of a controlled study or systematic comparisons with other courses so conclusions regarding 27 This is why the grouping in the 30 s takes the form 30 32 33 36 and 37 39 Within the 40 s and likewise for the other decades it more appropriate to use the grouping 40 42 43 45 and 46 49 because of all these grades 49 is by far the least used 39 the effectiveness of the Turtle system must be to that extent tentative Potential complicating factors include the following e Those taking the introductory programming component were self selecting elective students free to drop out or switch options had they so chosen and this might be expected to reduce the tail of failures characteristic of compulsory programming courses However this does nothing to impugn the favourable comparison with the 1996 99 Borland Pascal course which was also entirely optional 8 e Turtle covered only the first four weeks of programming enabling students to get by without
50. 6 5 that Turtle contains a trace facility which can display in detail the execution steps of any program including all transfer of data via the Stack This trace can be used with the program above to inspect the mechanisms described but before doing so it is helpful to modify the until condition to until turtx gt 33 to restrict the repeat loop to three procedure calls Each procedure call involves around 50 program cycles i e individual PCode commands so allowing the unmodified program to run will quickly build the trace display to an unmanageable length with very evident effects on program speed 6 7 Stack Variations on the Turtle Machine As described so far in this chapter and the last the Turtle Machine involves no fewer than four stack structures e The main Program Stack 5 1 used for holding command parameters calculating intermediate values and parameter passing e The Procedure Return Stack 6 1 used for storing return code line locations for program continuation after each procedure terminates e The Heap Control Stack 6 3 used for managing Heap pointers that indicate where each procedure s dynamic storage is to be found e The Procedure Register Stack 6 5 used to keep track of the active procedures in order to service the trace display facility These stacks are made independent in the default version of the software to enable each mechanism to be introduced sep
51. 61 argues that a student s expectation of success is crucial to its achievement since such expectation impacts so directly on student motivation drawing on Keller 67 he suggests that motivation can be thought of as the multiplicative product of expectancy and the perceived value of success Such a view would strongly support the use of any system however modest that can increase students enjoyment and confidence and introduce programming in a positive manner A related virtue of the Turtle style of learning is that students as their confidence grows are strongly motivated to enhance their own designs adding features to graphically interesting programs that evince evident pride and satisfaction Hence from 34 At the end of each year s third Turtle lecture most students have raised their hands when asked to indicate whether they understand the triangles example 2 3 Obviously this does not prove that they have a deep and secure understanding but it is surely significant that this much can be achieved with novices less than an hour after they have first acquired the concept of a procedure 43 simple beginnings students characteristically end up developing programs of significant complexity and thus encounter at an early stage the typical problems of design and plan composition that seem to underlie so many of the difficulties that beset inexperienced programmers including many bugs that are standardly t
52. Command DownTo ae poBegin READ LOOK lt expr gt AHEAD 1tDo ltElse PUSH PUSH READ poDownTo poElse 1tElse Figure 17 Pushdown Automaton to analyse program control structures L8 88 The main loop of the automaton begins with the reading of a tokenised lexeme which if the program is syntactically correct must be either an identifier or one of the 39 66 keywords if for repeat begin until or end 55 e if must be followed immediately by an expression cf 7 6 and the keyword then 5 assuming these are found the PDA loops back to the beginning but first pushes polfThen one of the constants of enumerated type TPendOp onto the stack of pending operations Immediately following the processing of the expression a PCode IFNO command is generated to jump around the then part of the conditional structure cf 5 3 2 gt 7 e for must be followed immediately by an identifier which must be a declared integer variable name then the assignment operator then an expression and then one of the keywords to or downto followed by another expression and the keyword do Again assuming all these are found the PDA loops back having pushed poTo or poDownTo respectively onto the stack Meanwhile PCode commands are generated to store the first expression s value in the counting variable and then after the processing of the second expression to
53. DHB and STHB which load and store the Procedure Heap Base Pointer for a specified procedure Informal description of Assembler Decimal Hex command behaviour mnemonic code code load Heap Top Pointer LDHT 226 E2 store Heap Top Pointer STHT 227 E3 load Heap Base Pointer LDHB 228 E4 store Heap Base Pointer STHB 229 E5 This Heap maintenance is done as follows Some code line numbers are changed but for convenient comparison the previous numbering is shown in brackets 49 For the sake of simplicity the procedure call still involves PROC which adjusts the Procedure Return Stack so that stack is in fact maintained However it is not used PLRJ removes the return location from this stack to prevent overflow but just discards it using the Program Stack value instead 74 PROCEDURE bf r integer 1 PSPR 1 LDHT STHB 1 LDHT LDIN 1 PLUS STHT BEGIN 2 STVV 1 1 bf r 1 8 LDHB 1 LDHT LDVV 11 LDIN 1 PLUS PROC 1 STHT STHB 1 end END 9 PLPR ENDP BEGIN bf 50 13 LDHT LDIN 50 PROC 1 STHT END 14 HALT At the start of the procedure HPCL 1 1 is replaced by a command sequence which explicitly sets Procedure Heap Pointer 1 to the current value of the Heap Top Pointer LDHT STHB 1 and then adds 1 to the value of the Heap Top Pointer itself LDHT LDIN 1 PLUS STHT this claims space on the Heap for the one local 664 99
54. End C Figure 16 Finite State Machine to analyse the block structure of Pascal source code procedure id procedure id ParType id id procedure id ES 84 case lexitems lexnum lextype of 1ltComma nextlex ltId ssProcVName Comma must be followed by a variable name 1tColon begin if nextlex ltId ssProcVName ms must be followed by a type name then if nextlex 1ltSemicolon ssProcVSemi VAR declaration must be followed by then addvtype lexnum 1 end else begin result Variable must be followed by a type specification dec lexnum end end case Here exnum is the index of the next lexical item lexeme to be processed within the array lexitems and hence lexitems lexnum lextype gives its identified lexical type as discussed in 7 3 above If that type is tColon then the nextlex function is called twice first checking that the following lexeme is of type t d i e an identifier and then that the next after that is of type tSemicolon to facilitate such iteration nextlex automatically increments exnum as it goes If both checks succeed then a transition is made to syntax state ProcVSemi and the addvtype lexnum integer routine is called this checks that the specified variable type i e the identifier in question is either integer or boolean and if so assigns it to the variable or list of variables concerned If any of
55. Heap Control Stack We must now deal with the details of how the Heap reference mechanism outlined above is to be maintained while a program is running Clearly at least two more PCode instructions are needed one HPCL to claim space on the Heap for the local 58 variables of each new procedure gt and another HPRE to release that space once the procedure has terminated Informal description of Assembler Decimal Hex command behaviour mnemonic code code claim space on Heap HPCL 34 release space on Heap 35 The intended behaviour of HPCL and HPRE can be deduced if we compare the situation before and after the first procedure call made within the hypothetical five procedure program described in 6 2 above Situation prior to initial call of procs Heap Top Pointer lt global variables gt Situation following call of procs Heap Top Pointer _ lt local variables of proc5 gt Procedure Heap Pointer 5 lt global variables gt Figure 12 Top of Heap before and after a procedure call Moving from the first situation to the second i e claiming Heap space for local variables is straightforward when the procedure starts its Procedure Heap Base Pointer should be given the current value of the Heap Top Pointer then the Heap Top Pointer itself should be incremented by the number of local variables in the procedure 43 Strictly
56. J LDVV 1 1 LDIN 1 PLUS PROC 1 end END 9 HPRE 1 PLPR PLRJ BEGIN bf 50 13 LDRJ LDIN 50 PROC 1 END 14 HALT 48 In the same spirit as note 45 above it is perhaps worth noting that the PLRJ instruction could be used to replace JUMP if the aim were to minimise the instruction set since JUMP 10 for example which jumps to the beginning of code line 10 is equivalent to LDIN 9 PLRJ which jumps to the end of code line 9 73 The only other change is that the procedure here ends with PLRJ rather than ENDP procedure termination has to involve a different command if the machine is to continue execution from the return location stored on the Program Stack the effect of PLRJ rather than looking for the return location on the Procedure Return Stack 6 7 2 Doing Without the Heap Control Stack Managing Heap space without the Heap Control Stack is a relatively complex business requiring that the relevant Heap pointers be passed and calculated on the Program Stack rather than being controlled through a specially tailored mechanism The Heap Top Pointer is retained as a single register rather than as the top of a stack and each procedure still has its own Procedure Heap Pointer keeping track of the relevant Heap Base address But now these must be maintained explicitly using the instructions LDHT and STHT which respectively load and store the current Heap Top Pointer and L
57. OMO OMnROMDAHAMHDAH HOMO OHM O ANAA TY THO HOH OO GOKK KR DO BO WO 38 The peak in the Borland Pascal 37 39 column actually represents pass grades because prior to 2000 the Leeds pass threshold was 37 27 However the peak itself occurs because students are limited to a bare pass on resitting so the fact remains that many of these students initially failed The detailed figures are shown in the table below Particularly notable is that on the earlier course 27 8 of students scored below 40 and of these 17 6 below 37 whereas on the Turtle Graphics Borland Pascal Turtle Graphics for Windows Programming and 4 7 respectively But any 1996 1999 2000 2003 course these figures reduced to 6 0 suspicion that the contrast between the Number Average Number Average grade profiles might have resulted from a sacle 307 e193 a crude dumbing down is easily 20 22 2 9 20 22 1 0 l l 23 25 2 9 23 25 0 3 dispelled by looking at the other end of 26 29 3 9 26 29 0 7 30 32 1 5 30 32 0 7 33 36 6 3 33 36 2 0 the scale where on the Borland Pascal course 13 7 scored over 80 while the 37 39 10 2 37 39 1 3 40 42 2 9 40 42 2 0 corresponding figure for Turtle 43 45 3 9 43 45 2 9 i 46 4 39 46 4 29 Graphics was only 3 3 As _ this ape ee a aay indicates the later course was in some 53 55 5 9 53 55 1 6 56 59 4 9 56 59 13 4 ways rather more demanding than its 60 62 6 3 60 62 13 0 ws 63 6
58. Peter J 2002 Subsetting language elements in novice programming environments Proceedings of RESOLVE 2002 Columbus Ohio pp 108 11 23 37 DeRemer Franklin L 1976 Lexical analysis in Friedrich L Bauer and J rgen Eickel eds Compiler Construction An Advanced Course second edition Springer Verlag first edition published in 1974 pp 109 120 76 38 Dicheva D and Close J 1996 Mental models of recursion in Journal of Educational Computing Research 14 pp 1 23 42 39 Dijkstra Edsger W 1989 On the cruelty of really teaching Computing Science Communications of the ACM 32 12 pp 1398 404 2 40 Donovan Steve 2001 C By Example Que 9 41 du Boulay B 1989 Some difficulties of learning to program in Soloway and Spohrer 126 pp 283 99 23 41 96 7 42 du Boulay B T O Shea and J Monk 1989 The black box inside the glass box presenting computing concepts to novices in Soloway and Spohrer 126 pp 431 46 23 96 43 Eisenberg M M Resnick and F Turbak 1987 Understanding procedures as objects in Olson Sheppard and Soloway 93 pp 14 32 23 44 Gibbs G 1994 Improving Student Learning Theory and Practice Oxford Oxford Centre for Staff Development 98 45 Glinert E and Tanimoto S 1984 Pict an interactive graphical programming environment JEEE Computer 17 pp 7 25 117 46 Green T
59. Turtle Innovative Software for the Learning of Computing Concepts Peter J R Millican The University of Leeds School of Computing Submitted in accordance with the requirements for the degree of MSc by Research February 2004 The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the work of others il Contents Figures and Illustrative Programs Acknowledgements Abstract Chapter 1 Background Aims and Achievements 1 1 The Electives Market and its Needs 1 2 Why Turtle Graphics 1 3 Why Turtle Graphics Pascal 1 4 Why an Integrated Visual Compiler 1 5 An Introduction to Computing Concepts 1 6 Outline of the Thesis 1 7 Achievements and Novel Contributions Chapter 2 External Design 2 1 Getting Started 2 2 The System Menus 2 3 The Visual Compiler Displays Chapter 3 The Turtle Graphics Pascal Source Language 3 1 Data Types 3 1 1 Numbers 3 1 2 Other Data Types 3 2 Arithmetical and Boolean Operators 3 3 Program Structures 3 4 Special Turtle Graphics Facilities 3 4 1 Turtle Graphics Instructions 3 4 2 Colour Handling Chapter 4 Using Turtle to Teach Introductory Programming 4 1 Brief Outline of the Course 4 2 Students Perceptions of the Course 4 3 How Well Did the Students Learn 4 4 Conclusion The Value of Turtle as a Vehicle for Introducing Programming iii vi viii 20 23 24 24 25 26 26 28
60. ailed discussion of the Turtle Machine and its compiler which has been intended to show by example how relatively straightforward the system is enabling it to be used effectively as a vehicle for understanding at a relatively early stage such fundamental concepts of Computer Science as machine code stack operations compilation recursive descent and various automata In Chapter we conclude by bringing the two main themes of the thesis together and briefly examining whether they may be more complementary than contrasting 96 Chapter 8 Conclusion 8 1 The Value of Turtle as a Vehicle for Introducing Computing Concepts We have now seen two apparently distinct aspects of the Turtle system On the one hand as discussed in Chapters 3 and 4 and summed up in 4 4 it can be used as a teaching environment for novice programmers On the other hand as illustrated in Chapters 5 to 7 it can be used as a vehicle for explaining the concepts of machine code and compilation presumably to more advanced students So far it might seem that these two teaching tasks are largely unrelated but I shall now conclude by suggesting that in fact they are complementary and that there may be unexpected benefits in having a system that combines them 8 1 1 Notional Machines As discussed in 4 4 many students of programming and not by any means only novices experience considerable difficulty in understanding flow of control within a program B
61. al sight of it somewhat daunting despite all my prior programming experience Even provision of a Turtle Graphics unit within Delphi would not help much to overcome the problem that attempting to initiate novice students both into the language and the environment at the same time might well prove overwhelming So the ideal approach seemed to be to separate their initial learning of the Pascal language from their learning of the Delphi environment Hence the need for a self standing environment that could support an introduction to Pascal with an emphasis on Turtle Graphics but with sufficient sophistication that it could prepare the students for a I was confident however that using Turtle Graphics as a bridge into the Pascal language itself would work smoothly since I had personal experience of using a simple custom written unit in this way when teaching Turbo Pascal to elective students in 1994 95 relatively easy initiation into Delphi and also preferably enable the more proficient students to stretch themselves even prior to this initiation 1 4 Why an Integrated Visual Compiler The idea of using Turtle Graphics to introduce algorithmic thinking to novice programmers has proved extremely popular over recent years and there are any number of such systems to be found on the Web and referenced in the Computer Science teaching literature Dedicated environments however tend to be almost exclusively based aro
62. ample will work by first identifying the required indirection register on the Heap calculating this by taking the Procedure Heap Pointer for the procedure in question i e Procedure Heap Pointer 1 and adding the index of the particular variable i e 3 But then instead of simply loading onto the Program Stack whatever value 5 say it finds in that address as would happen in the case of LDVV 1 3 it instead proceeds to interpret this value as itself an indirected Heap address and accordingly loads the value which is to be found in the corresponding Heap location in this case at address 5 which is the address of the turtc value so this is the situation that will arise where turtc is the actual parameter corresponding to the formal parameter three STVR acts very much like LDVR except that it stores into the indirected Heap address rather than loading from tt The other PCode instructions that handle reference parameters are used for setting up the appropriate aliases when a relevant procedure is called Informal description of Assembler Decimal Hex command behaviour mnemonic code code load address of global variable LDAG 14 load address of local variable or value param LDAV 15 load indirected address of reference parameter 16 store indirected address of reference parameter 26 The first three of these are used to load the Heap address of the relevant actual parameter variable onto
63. any global variables declared within the currently running program gt index 5 index 4 index 3 index 2 index 1 Figure 10 Overall heap structure showing predefined global variables Since turty has an index number of 2 to compile the statement forward turty we need an appropriate PCode command for loading the global variable with index 2 onto the Program Stack This command is LDVG 2 giving the compiled result 49 forward turty LDVG 2 FWRD The inverse PCode instruction to LDVG is STVG which pulls or pops the top value from the Program Stack and stores it in the appropriately indexed global variable The codes of these two instructions are as follows Informal description of Assembler Decimal Hex command behaviour mnemonic code code load global variable LDVG 11 store global variable STVG 21 Together with the PCode commands already introduced these enable the compilation of assignments and other operations involving global variables The following fragment for example shows the start of one of the built in illustrative programs PROGRAM cyclecolours VAR length integer cinc integer nextc integer BEGIN 5 RNDC randcol 5 5 DVG 5 LDIN 1 SUBT STVG 7 6 0 cinc turtc l RNDC STVG 6 randcol 6 Et EA Ee Ee Os sy so length 0 When this is compiled t
64. arately in an educational context but only the first of them is genuinely necessary and the Compile menu provides three options which respectively enable each of the others to be dispensed with involving some new PCode instructions These options are intended to facilitate teaching of the concept of a stack frame since they illustrate how a variety of data relevant to a procedure call including not only parameters but also return location and heap pointers can all be combined into a frame on the Stack and viewed as such through the trace display facility which 71 shows the contents of the top three Program Stack locations at every cycle of the program 47 To explain the differences between the options we can see how they affect the compilation of a short program with one recursive procedure The procedure draws a coloured blot then moves forward hence the name bf and turns before calling itself with an incremented parameter The visual effect is of nine pillars PROGRAM pillars PROCEDURE bf r integer 1 PSPR 1 HPCL 1 1 BEGIN 2 STVV J if r lt 350 then 3 LDVV 1 1 LDIN 350 LESS IFNO 9 begin randcol 6 4 LDIN 6 RNDC blot r 5 5 LDVV 1 1 LDIN 5 SUBT BLOT forward r 2 6 LDVV 1 1 LDIN 2 MULT FWRD right 40 7 LDIN 40 RGHT bf r 1 8 LDVV 1 1 LDIN 1 PLUS PROC 1 end END 9 HPRE 1 PLPR ENDP BEGIN blan
65. awing repeated patterns the repeat until construct in Pascal is syntactically far easier than while do begin end since repeat and until already provide the necessary bracketing For all these reasons repeat until has been preferred to while Given the provision of if then else and repeat until there is no strict 29 need for the counting iterative structure for do However this is so useful in situations where an exact number of iterations is required and so helpful and relatively natural for introducing the concept of a counting variable that there is very good reason to include it Finally any system that aspires to teach good programming practice must make provision for modular divisions of the program and given the intention of using Turtle as a bridge to Delphi procedures are absolutely essential since Delphi methods take this form Procedures if properly implemented should also permit recursion for which a graphics system can provide a wonderful introduction and in combination with Pascal s distinction between call by value and call by reference parameters enable results to be returned and hence the effective definition of functions Such quasi functions lack the syntactic convenience of literal functions but contexts in which this is a major issue seem unlikely to arise within introductory Turtle Graphics For the sake
66. axer Lynda Thomas Jan Utting and Tadeusz Wilusz 2001 A multi national multi institutional study of assessment of programming skills of first year CS students ACM SIGCSE Bulletin 33 4 pp 125 80 41 98 84 Mendelsohn P T R G Green and P Brna 1990 Programming languages in education the search for an easy start in Hoc et al 57 pp 175 99 96 85 Moser Robert 1997 A fantasy adventure game as a learning environment Why learning to program is so difficult and what can be done about it ACM SIGCSE Bulletin 29 3 pp 114 6 6 41 86 Motil John and Epstein David JJ a language designed for beginners less is more available from www publicstaticvoidmain com 23 87 Myers B A R Chandhok and A Sareen 1988 Automatic data visualisation for novice Pascal programmers in Proceedings of IEEE Workshop on Visual Languages Pittsburgh pp 192 8 117 88 Naps Thomas L Guido R6 ling Vicki Almstrum Wanda Dann Rudolf Fleischer Chris Hundhausen Ari Korhonen Lauri Malmi Myles McNally Susan Rodger and J Angel Velazquez Iturbide 2003 Exploring the role of visualisation and engagement in Computer Science education ACM SIGCSE Bulletin 35 2 pp 131 52 5 89 Neal L R 1989 A system for example based learning in Proceedings of CHI Conference on Human Factors in Computer Systems Boston pp 63 8 118 106 90 Newton D P New
67. be useful to them in the future and giving students the possibility of exploration into the Turtle Machine which may be particularly appreciated by those who could otherwise find the exercise trivial This concludes our detailed discussion of Turtle as a vehicle for introductory programming In Chapter 5 we move on to the second half of the thesis concerned with the virtual Turtle Machine and its operations and thus having a teaching orientation towards Computer Science rather than introductory programming 44 Chapter 5 The Virtual Turtle Machine and its PCode Object Language By far the most difficult aspect of the system design was the development of a virtual Turtle Machine capable of sustaining full recursive and versatile parameter handling behaviour whilst remaining simple enough to be broadly comprehensible after only a moderate amount of study to a non specialist Perhaps the acid test here is whether the Help file sections on An Introduction to PCode Technical Note on Variables Procedures and Parameters and PCode Reference Guide are in fact potentially accessible to interested students who have not studied the relevant computer science in detail Experience here is positive both by reference to the students themselves and to a peer observer Dr John Davy no doubt in large part because this aspect of the system has been confined to purely optional final lectures designed to appeal to self select
68. ck analysis program 33 Figure 8 Grade profile for the 1996 99 course teaching Borland Pascal 37 Figure 9 Grade profile for the 2000 03 course teaching Turtle Graphics Pascal 37 Figure 10 Overall heap structure showing predefined global variables 48 Figure 11 Illustrative Heap structure while three procedures are active 57 Figure 12 Top of Heap before and after a procedure call 58 Figure 13 Top of Heap where procedure involves reference parameters 62 Figure 14 Part of the Syntax table 81 Figure 15 FSM states in the Syntax table 82 Figure 16 Finite State Machine to analyse the block structure of Pascal source code 83 Figure 17 Pushdown Automaton to analyse program control structures 87 Figure 18 PDA stack and indents in the Syntax table 90 Figure 19 Syntax diagrams for parsing by recursive descent 91 References to Turtle s Built In Illustrative Programs excluding Appendix A drawpause Simple drawing with pauses 17 47 forloops FOR counting loop 52 3 triangles Recursion 20 1 42 ballsteps Combining structures 21 2 26 32 multibounce Multiple bouncing balls 32 66 cyclecolours Cycling colours 26 30 49 flashlights Using Booleans 25 30 balls3D 3 D effects with colour 30 recursion Recursion factory 25 References to Other Illustrative Programs randblots 5I concentric 52 randomdrift 65 pillars 71 5 vii Acknowledgements I am grateful to the School of Computing at th
69. concepts and software disciplines to pave the way for any students who ultimately decide to continue with Computing through say a major minor programme e Perceived usefulness so that even those students who do no more programming beyond this introductory module will feel that they have learned something of practical value to them Obviously it is very arguable which programming language and environment best satisfy these sorts of criteria see for example Jarc 60 Brilliant and Wiseman 17 Cantu 25 Trott 130 Hadjerroult 50 Warford 135 Callear 22 Smyth 123 Burton and Bruhn 21 and Jenkins 63 but at the time of developing the module I took the view that the optimal combination was to teach Pascal with a view to Borland s Delphi development environment for Windows 8 There would now be good reason to consider an alternative solution based on Java syntax given that language s commercial value and general prominence in an Internet aware culture its capacity to provide immediate perceived usefulness through Web applets and the free availability of highly regarded environments such as BlueJ which themselves exploit graphics and visualisation to facilitate further learning See Appendix B for some discussion of possible future developments in this direction Briefly the reasons for this decision were as follows First it is widely accepted that Pascal s syntax is significantly easi
70. could be engaging and friendly enough to interest a high proportion of such students whilst at the same time providing scope for those few who wish to dig deeper to learn some more advanced concepts of computer science such as recursion machine code compilation and memory models in a straightforward and approachable but nevertheless suitably rigorous context 1 2 Why Turtle Graphics For students like those described above whose other subjects demand the lion s share of their commitment and time and with little academic motivation for programming except in so far as they find the work interesting and enjoyable it would be hard to find a better starting point than Seymour Papert s idea of Turtle Graphics Papert s own 1 Of course the extent to which such models are appropriate is debatable in some of these cases perhaps most strikingly in Stephen Wolfram s recent attempt to provoke an algorithmic revolution within Physics through his book A New Type of Science 139 famous and seminal discussion 97 provides a detailed and powerful case for this conclusion on general educational grounds based in part on Jean Piaget s insights into the theory of learning here I shall be much briefer and will not endorse Papert s more ambitious claims about the value of programming for the general development of transferable problem solving skills cf Pea 100 Mayer et al 80 Kurland et al 70 Palumbo 94
71. cting appropriate command sequences and use of internal registers etc h Machine Dependent Code Improvement focused mainly on local optimisation rather than the sort of global considerations typically involved at stage e For reasons of efficiency and convenience however these processes are not usually separated into independent sequential modules and traditionally a common approach has been to aim for a two pass compiler e g Bornat 14 p 17 in which the first pass through the program performs the bulk of the analysis phases a to d if not e also and then the second pass performs the synthesis that remains to be done However what mainly drives the separation of the passes here is not so much the conceptual distinction between analysis and synthesis but rather the separation of machine independent from machine dependent considerations to avoid the runaway complexity that would result from attempting to deal with both of these together where they interact notably in the optimisation stages e and h Where no code optimisation is required it is often simplest to perform the synthetic code generation together with the analysis resulting in a single pass compiler 52 See for example Wilhelm and Maurer 137 p 232 Even a single pass compiler is unlikely to process the program purely sequentially since there is often a need for backpatching e g Aho and Ullman 3 p 9 Typical exampl
72. cts in Soloway and Spohrer 126 pp 191 207 27 118 108 126 Soloway E and Spohrer J C eds 1989 Studying the novice programmer Hillsdale New Jersey Lawrence Erlbaum Associates contains 13 41 42 56 66 68 70 75 79 102 107 114 125 127 128 127 Spohrer J C and Soloway E 1989 Novice mistakes are the folk wisdoms correct in Soloway and Spohrer 126 pp 401 16 43 128 Spohrer J C E Soloway and E Pope 1989 A goal plan analysis of buggy Pascal programs in Soloway and Spohrer 126 pp 355 99 41 129 Stein Lynn Andrea 1998 What we swept under the carpet radically rethinking CS1 Computer Science Education 8 pp 118 29 119 130 Trott Peter 1997 Programming languages past present and future ACM SIGPLAN Notices 32 1 pp 14 57 7 131 Tucker Allen B Keith Barker Andrew P Bernat Robert D Cupper Charles F Kelemen and Ruth Ungar 1998 Developing the breadth first curriculum results of a three year experiment Computer Science Education 8 pp 27 55 97 132 Urban Lurain Mark and Weinshank Donald J 2000 Is there a role for programming in non major Computer Science courses Frontiers in Education Conference Kansas City 1 3 133 Urban Lurain Mark and Weinshank Donald J 2001 Do non computer science students need to program Journal of Engineering Education 90 p
73. ctual parameter Turtle initialises other local variables and all global variables to zero see note 46 in 6 6 below So the only further issue that needs to be discussed here is how those initial values get passed into the procedure 61 Given the structure of the virtual Turtle Machine the obvious and natural way to pass parameters is via the Program Stack this also avoids any need to limit artificially the number of parameters that can be passed which would apply if a fixed number of registers were to be used So the PCode sequence prior to calling a procedure with one parameter will be exactly as when calling a Turtle Graphics command with one parameter cf 5 2 above namely pushing the actual parameter value onto the Program Stack then the procedure is called by transferring control to the appropriate codeline with PROC The same mechanism applies if there are more parameters with these being pushed onto the Stack in their natural syntactic order i e the first parameter gets pushed onto the Stack first To match up with this the PCode within the body of the procedure will start with one STVV command for each value parameter except that now they must be dealt with in reverse order because when the PROC command is performed the last parameter will be at the top of the Stack see 6 6 below for a detailed example of this last in first out unpacking 6 4 2 Reference Parameters Reference parameter
74. d the Students Learn As explained above in 4 1 the coursework required students to produce three programs the longest of which was expected to use each of a fair range of commands and structures The required range together with a tally for the last compiled program can be seen in the Expressions table in the Visual Compiler display shown on p 21 this was available to students to check their work and proved invaluable for markers To do even moderately well students had to exhibit reasonable competence with a number of programming constructs and to put these together within a coherent program of their own design Moreover because their programs were all so visibly distinctive and were often evidently objects of pride and enjoyment as manifest from the coloured plates there was little temptation or opportunity for students to copy from others except in the unlikely event that one of them effectively did the coursework twice over Hence one can be fairly confident that those students who passed the module had at least mastered some of the basic skills of programming such as linking sequences of commands together dividing code into procedures iterating with counting and repeat loops and using simple conditionals Given this background it is gratifying how few students failed to reach a pass standard in their coursework over the sessions 2000 2003 only 5 9 were graded less than 40 To put this in perspective the nea
75. d to specialist Computing degrees and who can therefore be expected to put in serious effort for considerable amounts of time if necessary then it is obviously even more of a difficulty albeit less grave in its implications when the students concerned are on elective modules with no guaranteed formal background limited time at their disposal and no need to pass the module in order to proceed with their degree programme One possible reaction to this difficulty would be to give up the attempt to teach programming to elective students but such a reaction would be very regrettable e Most non Computing students will not have encountered programming before entering university and so may be interested in a taster course to see if it appeals sufficiently to merit a change of degree programme e g to include minor Computing or consideration as a possible future career e A basic mastery of programming concepts is genuinely useful even for those who have no intention of pursuing a computing career since the powerful modern software systems that they are likely to encounter in the workplace notably the ubiquitous Office packages provide considerable scope for customisation and automation using macros which can be made vastly more powerful if the user understands for example the concept of a loop The 1999 report from the National Academy of Sciences Being Fluent with Information Technology 30 goes e
76. dicates its relative address within the appropriate area of the Heap The major difference is that this area is not fixed but determined dynamically when the procedure concerned begins To achieve this the system continuously maintains a Heap Top Pointer which keeps track of the top of the Heap i e the top limit of that part of the Heap being used for active variable storage Then whenever a procedure is called the system takes note of the current value of this Heap Top Pointer and stores it in association with that procedure this storage address is called the Procedure Heap Pointer or Procedure Heap Base Pointer for the procedure concerned The local variables of the new procedure instance are then stored immediately above the Heap Base address indicated by its Procedure Heap Pointer in the order of their index codes which correspond to the order of their 39 Hence a program that contains three procedures will involve three Procedure Heap Pointers It is not possible to make do with a single Procedure Heap Pointer dynamically adjusted to whichever is the current procedure because two or more procedures can be nested in which case when the inner procedure is running the local variables of both must be simultaneously accessible 56 declaration within the procedure The two instructions used to load these local variables onto the Program Stack and to store them from the Program Stack are
77. dure Calls 44 44 47 50 50 51 52 52 54 54 55 57 60 60 61 64 65 69 70 72 73 75 76 76 78 79 81 85 86 92 94 Chapter 8 Conclusion 96 8 1 The Value of Turtle as a Vehicle for Introducing Computing Concepts 96 8 1 1 Notional Machines 96 8 1 2 Deep Understanding 98 8 1 3 Automata Early 98 8 1 4 Familiar Compilation 99 8 2 Conclusion The Achievements of this Work 99 References 101 Appendix A Lecture Plans and Example Coursework 109 Plan for First Lecture on Turtle 109 Plan for Second Lecture on Turtle 110 Plan for Third Lecture on Turtle 111 Plan for Fourth Lecture on Turtle 112 Illustrative Programming Concepts Coursework 113 Appendix B Possible Future Developments 117 The Editor 117 The Environment 118 The Language 118 The Compiler 119 The Illustrative Programs 119 Appendix C Printout of Online Help File numbered independently i iv 1 61 vi Figures and Illustrative Programs Figures Figure 1 Turtle running the drawpause illustrative program 17 Figure 2 Recursive triangles 20 Figure 3 PCode and Trace displays as the triangles program executes 20 Figure 4 Syntax Analysis Display for triangles program 21 Figure 5 Expressions Display for ballsteps program 21 Figure 6 Declarations Display for ballsteps program 22 Coloured plates illustrating student work are inserted between pages 32 and 33 Figure 7 The automated student feedba
78. e University of Leeds for encouraging me in this project and in particular to Professors Graham Birtwistle now retired and Roger Boyle for acting as my supervisor and providing useful feedback on the draft of this thesis as a result of which I believe it is substantially improved I am also very grateful to Dr Nick Efford for helpful discussions on the teaching of compiler design and would like to express my special appreciation of Dr Willi Riha now retired who taught me so much about programming at both a high and low level and inspired my interest in learning the fundamental computer science that informs this thesis Many people have also given helpful comments on the system as it evolved including students and staff at the University of Leeds but most especially Dr Sarah Kattau who produced additional teaching materials to accompany the course based on this software and in 2003 took over its teaching entirely Her carefulness and attention to detail picked up a number of issues both in the software and its documentation resulting in significant improvements I would like to end by expressing my deepest gratitude to my family for their moral support especially my wife Pauline who has generously tolerated when necessary my spending far more time at the computer than is compatible with giving a fair contribution to family life Our children David Katie and Jonathan have also helped by playing with the system bringing to light va
79. eads Conceptually however the departure is not huge because it is obvious that the code line organisation can easily be translated into a linear form and the sequencing of control flow along each code line and then from one code line to the next is closely analogous to the familiar process of following the text of a book 51 5 3 2 The Conditional if then To compile if then structures within this context we need one PCode instruction that will jump to the beginning of a specified code line unconditionally and another that will branch conditionally depending on the evaluation of some condition For simplicity this condition is always that the value on top of the Program Stack is 0 i e false so the branch is followed if and only if the previously tested condition is not satisfied The HALT instruction is also included here for completeness Informal description of Assembler Decimal Hex command behaviour mnemonic code code jump to code line JUMP 30 halt execution 38 To see how all this fits together consider the following simple program and its compiled PCode equivalent in which the code lines are numbered PROGRAM randblots BEGIN DIN 2 RNDC DVG 5 IDIN 1 EQUL IFNO 5 randcol 2 if turtc 1 then ah U N Cau oe ee a oO zZ blot 300 300 BLOT else UMP 7 begin circle 300 5 LDIN 300 CIRC blot 200
80. earn widely applicable practical lessons than the more abstract treatments which dominate the teaching literature A4 and A5 here refer to the intended achievements listed in 1 7 the others of which have already been presented earlier in the thesis To recapitulate briefly Chapter 1 argued the case for developing an integrated Turtle Graphics environment for novice programming Al and this case was later strongly corroborated by the teaching 100 outcomes reported in Chapter 4 Chapter 2 gave a preview of the finished product while Chapter 3 justified the choice of source language A2 The program itself and the feedback from its extensive practical use as presented in Chapter 4 testify to its power robustness and attractiveness to students A3 Chapters 5 and 6 then discussed the Turtle virtual machine A4 and Chapter 7 the compiler A5 The visual interface to these illustrated both in 2 3 and 7 3 5 significantly adds to their teaching value A6 Overall therefore I am very satisfied with the outcome of this project and feel that the Turtle system is a worthwhile contribution to the teaching of Computing I hope moreover that it has gone at least some way to meeting the challenge laid down by Holmes and Smith 59 to present both introductory and more advanced topics of Computer Science in a way that is accessible to relative novices in a manner that will capture the imagination of the learners and c
81. ed enthusiasts within the student body However even these enthusiasts would have had little chance of understanding standard textbook presentations of the relevant theory so this success is gratifying In these next two chapters I shall approach the virtual Turtle Machine from the opposite direction to the Help file focusing more on explaining why it has the structure it does rather than on the detail of how that structure operates in practice At the same time I shall endeavour to explain these things in a way that presupposes very little technical background so that the chapters can serve at the same time as an illustration of how the Turtle system can provide an appropriate vehicle for Computer Science education at the non specialist level 5 1 Turtle Graphics Commands the Program Stack and Arithmetical Boolean Operators Since the educational aim of the Turtle Machine is to give an insight into the general behaviour of a compiler and dynamic memory management rather than the specifics of graphical processing it is clearly appropriate to keep the handling of Turtle Graphics commands as simple as possible which is done by straightforwardly defining primitive machine code instructions corresponding to all of the Turtle Graphics primitives These are shown in the following table where each instruction is followed by the 45 relevant assembler mnemonic and machine code value in decimal and hexadecimal using the De
82. ed by simply counting how many procedure calls have been made and recording the last such call For as the middle line of the following sequence illustrates the current procedure is not always the last to have started nor the next to finish proc2 starts proc starts and finishes proc2 running procl starts and finishes proc2 finishes The simplest solution to this bookkeeping problem is to maintain yet another stack the Procedure Register Stack whose topmost value the Procedure Register is set according to the procedure which is currently running and whose height indicates how many procedures are active The operation of this stack is very straightforward when a procedure begins its number is pushed onto the stack and whenever a procedure ends the top value is pulled from the stack using the following two instructions Informal description of Assembler Decimal Hex command behaviour mnemonic code code push procedure index onto Procedure Register Stack PSPR 36 pull procedure index from Procedure Register Stack PLPR 37 65 6 6 Putting Procedures Together We can now see how all this fits together in compiling a program with a fairly complex procedure interface This example produces the effect of a ball bouncing back and forth horizontally while performing a slow random walk vertically PROGRAMrandomarift VAR bgcol xpos xvel ypos integer VAR ymove integer BEGIN randcol 7 ymove
83. een to be believed it ought to be filmed and distributed to all programming students everywhere Being able to download Turtle Graphics to use on my computer was very useful It was REALLY useful to have small exercises set each week It helped to consolidate the material covered in the lectures and it was good to be able to try out specific tasks to make sure that you weren t having any problems with the work Overall the lectures were brilliant the best lectures I ve ever been to The only difficult aspect of this coursework was formulating an idea for a program for which we could apply the relevant programming Apart from this the course was excellent It was a pleasant and not very daunting introduction to programming I found this part of the module very enjoyable and very useful I really enjoyed doing this but I sat down and worked it all out for myself rather than reading the notes The lecturer was very helpful and seemed really enthusiastic about the work himself Enjoyed it Turtle Graphics could be easier if it had a grid on the page so you could always see where you are I thought the course was very interesting and quite challenging I thought the turtle graphics program was very helpful as the introduction to programming as there was visible results so you felt like you were actually achieving something I enjoyed the cat in the hat analogy to recursion Though it did seem somewhat silly it did help to clarify the idea
84. eing run on the minimum 800x600 resolution screen The Compile menu gives access to the various machine code and memory analysis facilities described below in 2 3 and also provides a method of running or halting the program independently of the RUN button Most of the settings in the Options menu concern various automatic operations enabling the system to be configured so that programs are processed immediately on loading by compiling running and or auto formatting the Canvas either cleared or preserved when a new program is run and the auto formatter s behaviour controlled The various configuration settings which prove particularly valuable when processing large quantities of student assessments can also be saved for future use or set as defaults The Help menu provides direct access to various sections of the Help file and also to a number of illustrative programs that are built into the system 20 2 3 The Visual Compiler Displays To see the Visual Compiler at work first use the Help menu to select the built in illustrative triangles program labelled Recursion in the menu and click on RUN The program will generate the recursive pattern shown here with eight levels of triangles of successively halved dimensions but note that while it executes the RUN button changes to HALT you can click on this at any time to terminate it manually Figure 2 Recursive tria
85. ents for learning software engineering Computer Science Education 10 pp 39 56 118 28 Chalk Peter Tom Boyle Poppy Pickard Claire Bradley Ray Jones and Ken Fisher 2003 Improving pass rates in introductory programming 4 Annual LTSN ICS Conference Galway pp 6 10 41 29 Chua Y S and Winton C N 1987 Teaching theory of computation at the junior level ACM SIGAPL APL Quote Quad 17 4 pp 69 78 98 30 Committee on Information Technology Literacy 1999 Being Fluent with Information Technology Washington DC National Academy Press 1 31 Cooper Stephen Wanda Dann and Randy Pausch 2003 Using animated 3D graphics to prepare novices for CS1 Computer Science Education 13 pp 3 30 5 43 32 Dann Wanda Stephen Cooper and Randy Pausch 2000 Making the connection programming with animated small world ACM SIGCSE Bulletin 32 3 pp 41 4 5 33 Decker Rick and Hirshfield Stuart 2001 The PIPPIN machine simulations of language processing Journal of Educational Resources in Computing 1 pp 4 17 97 34 Deek Fadi P 1999 The software process a parallel approach through problem solving and program development Computer Science Education 9 pp 43 70 41 35 Deek Fadi P and McHugh James A 1998 A survey and critical analysis of tools for learning programming Computer Science Education 8 pp 130 78 117 103 36 DePasquale
86. er for beginners to read and understand than that of C or Java probably in part because its control structures are more verbose cf Sime et al 118 moreover I had nearly a decade s experience of teaching Turbo Borland Pascal which gave me a high opinion of the system s learnability and robustness notably error messages that are relatively unconfusing criteria a and c Secondly as regards rigour and appropriateness for software engineering criterion d Pascal seemed preferable to Visual Basic which was another obvious contender on grounds of initial simplicity and credibility moreover Green et al 46 found that syntactically Pascal is easier than Basic for beginners to parse in what they term the parsing gnisrap cycle where new chunks are inserted into existing code Visual Basic would also have implied commitment to a specific proprietary platform notably Microsoft Windows and its variants and although Windows was indeed the obvious default platform to use given the target audience of the module on grounds of familiarity availability ease and perceived usefulness to them criteria a and e it was a significant virtue of Delphi that plans were afoot to port it to Unix soon to result in the Kylix system since 2001 Choice of Delphi however left the problem of initial learning and the need for quick results criterion b Delphi is a complex environment and I remember myself finding the initi
87. er the system s secondary role of providing a vehicle for the learning of fundamental concepts such as machine code compilation dynamic memory management automata and stacks Chapter 5 introduces the specially designed virtual Turtle Machine and its basic operation Chapter 6 takes this further into procedure calls and dynamic memory management Then Chapter 7 explains the detailed operation of the Turtle compiler designed in a modular fashion to provide a relatively accessible way in to compiling and associated techniques FSM PDA recursive descent In the absence of specific classroom experience to validate the effectiveness of these aspects of the system Chapters 5 to 7 have all been written in an expository style intended to demonstrate by example the Turtle Machine s simplicity of operation and its appropriateness as a teaching tool Finally Chapter 8 reflects on the system s overall value suggesting surprisingly strong links between its two main roles before ending with a retrospective summary of the project s achievements Chapter 1 Background Aims and Achievements 1 1 The Electives Market and its Needs Beginning students find it notoriously hard to learn basic programming concepts as testified both by the Computing Education literature see 4 4 below and by the many intense and long running discussions of the issue in schools of Computing But if this is a major problem with students who are already committe
88. er values in the case of the three value parameters Variable addresses are unpacked using the PCode instruction STAR cf 6 4 2 while integer values are unpacked using STVV By this process the six items from the Program Stack shown in the table above are all stored appropriately for the procedure to make use of them 46 The Turtle Machine initialises all variables to 0 except for the five predefined globals Here it differs from Delphi which initialises global variables to 0 but leaves local variables initially undefined Although students should be taught that it is good practice to initialise all variables explicitly the confusion that can result from undefined variables and resulting inconsistent program behaviour seems to make automatic initialisation most appropriate in a system designed for novice programmers 69 The main body of the procedure can now get under way working through the following sequence the numbers at the left refer to code lines of the compiled PCode 3 4 ymove is set to a random value between 3 and 3 5 7 a background coloured blot is drawn using the x and y coordinates xp and yp which are aliases for the globals xpos and ypos 8 9 xp and yp are adjusted by the required horizontal xv and vertical ymove movement but since xp and yp are aliases for xpos and 66 ypos it is really these two globa
89. es and value parameters of procedures Reference parameters need different commands whose mnemonics are LDVR and STVR see 6 4 2 below 41 In Turtle Graphics Pascal the called procedure must precede the calling procedure except when two procedures are nested in which case mutual references are possible cf note 44 in 6 3 below 42 This handling of local variables can be seen as a natural extension of the method for global variables explained in 5 2 In effect all global variables are indexed relative to a single Program Heap Pointer which points just below the entire Heap whereas local variables are indexed relative to a Procedure Heap Pointer which is specific to the particular procedure within which they occur 57 Heap Top Pointer gt index 3 index 2 index 1 Procedure Heap Pointer 1 ____ points to Heap Base address for first procedure in program Procedure Heap Pointer 3 gt points to Heap Base address for third procedure in program Procedure Heap Pointer 5 z Note that the Heap Base for each procedure is the last Heap address before the relevant new storage area begins Hence in this example Procedure Heap Pointer 1 points to the last local variable address of proc3 Program Heap Pointer Figure 11 Illustrative Heap structure while three procedures are active 6 3 Claiming and Releasing Heap Space the
90. es of such backpatching occur within the Turtle compiler when control structures are terminated described in 7 5 below and also when a compiling sub procedure 78 7 2 The Structure of the Turtle Compiler The Turtle compiler like the Turtle virtual machine aims as its first priority for easy comprehensibility rather than efficiency so for example no attempt is made to optimise the code at any stage and all source language constructs are consistently translated in the same manner as outlined in Chapters 5 and 6 Moreover since it handles only integer data there is no need for type checking cf 3 1 2 above or any other kind of semantic processing intermediate between syntactic analysis and code synthesis All this enables code generation to be integrated very easily into the syntax analysis so that a one pass compiler would be fairly straightforward to implement However in order to facilitate the creation of lexeme indexed analysis tables that are transparently accessible see 7 3 4 below the Turtle compiler operates in two passes with the first combining lexical analysis and screening subtasks a and b after which the initial analysis tables can be created while the second pass integrates syntax analysis address assignment and code generation subtasks c f and g in a manner that avoids any need for an explicitly stored syntax tree Again partly for educational reasons this second pass is itself modu
91. ess In the former case the next character to be sent through the FSM will be the one following gt or in the latter case the character just tested as not being either gt or will itself be resent through the FSM But either way the FSM reverts back to the default initial state stNew before continuing 81 As shown on the right here the lexical Lexeme String Type analysis results can be inspected after 14 PROGRAM program 1 2 triangles identifier 3 compilation has taken place in the String and 4 A PROCEDURE procedure i T 35 7 5 2 z 3 5 triangle identifier Type columns of the Syntax table which is 3 6 3 7 size identifier 1 one of the Visual Compiler displays introduced 3 8 I 3 9 integer identifier 3 10 in 2 3 above In this table the types are 3 m T 7 4 12 BEGIN begin usualy shown as the literal expressions 5 13 i if l 5 14 size identifier g gt rather than ItMoreEq S 5 themselves e g er tha oreEq e h Da 5 17 then then but where different expressions can be of the 6 leek ee i 7 19 forward identifier same type that type is indicated using a standard word such as identifier or integer Figure 14 Part of the Syntax table Some lexical types involve various complications for example t nt and tHex must be sequences of valid decimal or hexadecimal digits and generate er
92. essing invokes as necessary the middle 7 5 and bottom 7 6 82 levels which together analyse and translate the source code lying between the begin and end that bracket each block s body But this interplay between the levels is almost entirely one way enabling the block structure parsing to be treated as an independent task which is sufficiently straightforward to be handled almost completely by a finite state machine FSM Moreover where the task goes beyond the capacities of an FSM this can be remedied by the addition of a single counter to track the depth of nested procedures resulting in the simplest possible form of counter machine e g Krishnamurthy 69 p 81 Handling the parsing in this way illustrates nicely how abstract machines can reduce a potentially confusing problem to a transparent relative simplicity and thus provides an excellent opportunity to motivate students interest in and appreciation of such machines just as the use of stacks within the Turtle Machine can motivate interest in and appreciation of them cf note 36 in 5 1 Turtle s FSM for analysing the block structure of Pascal source code is represented by Figure 16 on page 83 labelled with the same state names that are used in the Syntax table of the Visual Compiler display pictured below In the Delphi code that implements the compiler these syntax state names are prefixed with ss to ensure uniqueness a
93. et through it with support though the main focus of such support is enabling them to help themselves by making use of the Turtle system s built in resources notably the illustrative programs and the Help file In addition to their main program students are also asked to produce two short illustrative programs of their own each designed to highlight the features of a different language construct The intention of this exercise is to give them a relatively concrete task to start on while working out their ideas to encourage them to reflect on the use of program structures and to extend their coursework without requiring them to put all their eggs in the one basket of their main program It also brings the benefit of giving insight into students perception of the structures in question and can provide useful additional illustrative resources to benefit future cohorts The next four pages show examples of the students work labelled and arranged to correspond with their organisation on the Turtle website though virtually every one of these programs involved animation and the pictures show only a snapshot or in some cases two of what were in many cases amusing programs It is very clear that the Turtle system both entertained these students and inspired them creatively 33 4 2 Students Perceptions of the Course As discussed in 1 2 above the primary aim of the course is to introduce novice students to programming in
94. fic Jams Explorations in Massively Parallel Microworlds MIT Press 119 109 Roberts Eric and Picard Antoine 1998 Designing a Java graphics library for CS1 ACM SIGCSE Bulletin 30 3 pp 213 18 9 110 Robillard P N 1986 Schematic pseudocode for program constructs and its computer automation by SCHEMACODE Communications of the ACM 29 11 pp 1072 89 117 111 Robins Anthony Janet Rountree and Nathan Rountree 2003 Learning and teaching programming a review and discussion Computer Science Education 13 pp 137 72 41 42 43 88 96 112 Rogalski J and Samurcay R 1990 Acquisition of programming knowledge and skills in Hoc et al 57 pp 157 74 27 41 113 Rountree N Rountree J and Robins A 2002 Identifying the danger zones predictors of success and failure in a CS1 course Technical Report OUCS 2001 07 Otago University New Zealand available from www cs otago ac nz trseries 42 114 Samurgay R 1989 The concept of a variable in programming its meaning and use in problem solving by novice programmers in Soloway and Spohrer 126 pp 161 78 41 115 Shackelford R and Badre A 1993 Why can t smart students solve simple programming problems International Journal of Man Machine Studies 38 pp 985 97 41 116 Schaub Stephen 2000 Teaching Java with graphics in CS1 SIGCSE Bulletin 32 2 pp 71 3 9 1
95. hat Arts students in particular benefit educationally from the opportunity of learning in a creative divergent manner rather than in the linear manner which tends to characterise traditional data or mathematically oriented teaching of programming Hartley and Greggs 53 Nulty and Barrett 91 e Partly because Turtle Graphics lends itself so well to divergent creative development it is relatively easy to structure a course around it that enables learners to work at their own speed cf Liffick and Aiken 74 Moser 85 and which stresses discovery learning cf Baldwin 9 e Assessment of Turtle Graphics programs can take advantage of the marker s intuitive familiarity with visual patterns and consequent ability for easy recognition of the program s behaviour and results One consequence of this is that creative graphics coursework not only greatly reduces the frequency of plagiarism it can also make plagiarism relatively simple to detect For all these reasons I decided that my intended introductory programming module should start with Turtle Graphics But this still left open the difficult and in other contexts notoriously controversial choice of programming language and target development environment 1 3 Why Turtle Graphics Pascal Turtle Graphics was developed by Papert in tandem with LOGO his programming language for children This is in many ways an excellent and versatile language
96. he program that you produce should contain at least two procedures each of which should have a clear purpose The specification of each procedure s purpose and a brief description of the program s intended behaviour and of any issues you have encountered in its development should be written in a text file and submitted with your programs You are also invited but not expected to submit your work on the built in Turtle Graphics exercises which may act as a safety net to help you pass if for some reason you have a disaster with your main program e g if having done a lot of work on that program you can t get it to run you will NOT pass if you haven t at least made a substantial effort Detailed Requirements and Assessment Criteria Part I Having chosen your two features of Turtle Graphics Pascal from the six options of POLYLINE or POLYGON FOR DO loops REPEAT UNTIL loops PROCEDUREs with parameters Recursion and Colour codes and colour handling you should try to produce a short illustrative program for each of them satisfying the following criteria e Programs named appropriately see details of items to be submitted below e At most 20 lines in each illustrative program e Program clearly written and formatted e g using the auto format facility and easy to understand e Producing a visually interesting result e A relevant visual result in that someone looking at it together with the program could see fairl
97. he three newly declared global variables length cinc and nextc are assigned addresses 6 7 and 8 on the Heap directly above the five predefined globals Hence the last compiled line above which has the effect of loading the integer 0 onto the Stack and then storing this in global variable index 6 is indeed equivalent to length 0 The second compiled line is more complex involving a calculation prior to the global assignment It begins by loading variable 5 i e turtc onto the Stack then the integer 1 is also loaded and SUBT is performed which leaves the result of the subtraction turtc 1 on the Stack Finally this result is stored in global variable index 7 i e cinc yielding the desired cinc turtc 1 50 5 3 PCode Sequential Structure and Flow Control 5 3 1 PCode Line Structure Storage and Sequencing Given the educational purpose of the Turtle virtual machine easy comprehensibility of its PCode is far more important than economic storage and processing Hence it makes sense to divide it for both logical and display purposes into small sections henceforward code lines corresponding to individual Turtle Graphics commands or other relatively self contained units as illustrated by the four code lines LDIN 5 RNDC etc shown in the short compilation table on the previous page This implies a storage overhead because to preserve the line structure of the code
98. her weak because the arguments to these instructions e g representing lengths and angles anyway have to be integers and the overall aim of simplicity therefore implies a preference for treating the basic machine code unit as being an integer in general even if the instructions themselves are limited to a single byte The case for real numbers is somewhat stronger but by no means decisive because limitations of fractional co ordinate accuracy are unlikely to be much of an issue within 20 As implemented Turtle uses four byte integers following the Delphi standard Moreover the PCode display allows integers to be shown in hexadecimal so that byte values can be distinguished 25 the programs produced by beginners while more advanced programmers can straightforwardly mitigate them if necessary by adjusting the Canvas dimensions and by saving and restoring coordinates at appropriate places as illustrated by the built in Recursion factory program Given also the decision to use integers as the machine code unit the complications of introducing real numbers seem to outweigh any benefits Hence only integer numbers are accepted within the system 3 1 2 Other Data Types Turtle implicitly uses Boolean values in conditional and iterative structures e g after f and until but in common with many other systems represents these as integers thus avoiding the need for type distinctions within the Program S
99. here it will be indexed under the formal parameter which is serving as its alias This register cannot be hard wired because one and the same procedure could be called from many different places in the program with a wide variety of different variables providing the actual input parameter implying that the same formal parameter would be acting as an alias for different actual variables on these different occasions So the obvious way of dealing with this is to assign a local variable storage address for each reference parameter just as for value parameters except that when these addresses are accessed using LDVR and STVR as opposed to LDVV and STVV they will be interpreted as registers containing indirection addresses rather than as storage locations for variables themselves So if in the situation we were considering earlier at page 57 parameters one and three of the first procedure were reference rather than value parameters then the top of the Heap would be as follows Heap Heap Top Pointer gt index 3 lt address of variable given as actual parameter to formal parameter three gt index 2 two index 1 lt address of variable given as actual parameter to formal parameter one gt Procedure Heap Pointer 1 points to Heap Base address for first procedure in program Figure 13 Top of Heap where procedure involves reference parameters 63 Then LDVR 1 3 for ex
100. hough often perhaps mistakenly attributed to syntactic difficulties cf Spohrer and Soloway 127 Winslow 138 Pane and Myers 95 p 30 This gives an excellent context for novice students to encounter and overcome such problems where they have willingly taken them on have a strong personal motivation also a fairly clear conception of what they are trying to achieve and are working within a system that gives straightforward visible and immediate feedback Having developed the strategies to become effective Robins et al 111 pp 165 6 in this context the hope is that they can then take these strategies forward to other more advanced systems The value of developing this sort of initial competence and confidence on simple systems is emphasised by the research of Hagan and Markham 51 who found that prior programming experience brings a major continuing benefit to those embarking on Computing degree programmes cf also Cooper et al 31 This opens another possible role for Turtle as a pre university taster taking advantage of the system s engaging character and built in independent learning resources to provide advance preparation prior to formal courses In a somewhat similar vein Turtle could be used to assess student s varied programming backgrounds and aptitudes playing the same sort of role that Poulton 105 suggests for LOGO but with the advantage of introducing a more standard syntax that might well
101. i Pascal plus another 1 000 lines or so of interface i e Windows form specifications 5 See for example Aho and Ullman 3 ch 1 Bornat 14 ch 1 Wilhelm and Maurer 137 ch 6 The description here is based mainly on the last of these which is the most detailed and up to date incorporating various terminological distinctions e g between a scanner and a screener deriving from DeRemer 37 that have become widespread since the earlier books were written 77 c Syntax Analysis which parses the output of the screener to produce a syntax tree for the source program determining its syntactic structure This tree may be stored explicitly within the compiler or it may be an implicit abstraction from the compiler s behaviour as it traverses the source code processing as it goes d Semantic Analysis decorating the syntax tree with semantic information such as type assignments and performing appropriate checks for consistency e Machine Independent Optimisation aiming to perform transformations on the decorated syntax tree to increase the efficiency of the resulting program f Address Assignment the first of the synthesis as opposed to analysis stages that take account of the specific machine architecture here involving such considerations as word length and the potential for data packing g Code generation which finally creates the object code for the target machine sele
102. ical ease requires inclusion of the circle and blot i e filled circle commands even though in Seymour Papert s original conception of Turtle Graphics circle is available only as a defined rather than as a primitive instruction see note 16 in 2 1 above Again thickness is presumably theoretically dispensable since one could draw multiple adjacent lines each of width 1 but its provision obviously makes life much simpler Most other Turtle Graphics primitives involve no such potential redundancy and are designed straightforwardly to extend its graphics capabilities The colour instruction is obviously required randcol less so though it greatly facilitates the creation of visually striking patterns by novice programmers The four instructions home setx sety and setxy provide absolute movement as do operations involving the global coordinate variables turtx and turty provided together with the direction colour and thickness variables turtd turte and turtt note that home here is not redundant because the home position is dependent on the Canvas dimensions and so does not involve specific coordinate settings 2 gt A similar point applies to the 25 However setx sety and setxy do involve some redundancy e g setx 100 is equivalent to though psychologically simpler than setxy 100 turty Educational
103. ikewise exit to quit from a procedure enabling a middle exit strategy which Soloway et al 125 identify as being helpful for novices Mutual recursion would be less restricted if the system had a forward or deferred directive cf note 44 Other additions that might be worth considering if only as selectable options to avoid potential overload for novices would be functions while and case Interactivity is currently not possible while a program is running except to halt it and output is restricted to geometrical shapes Other possibilities include fixed or program definable input boxes and buttons together with events and display of number strings Arbitrary text strings would require more radical revisions to the Turtle Machine and introduction of a string type would imply extension of the range of basic concepts conveyable by the system since type checking would then be essential Even if integers and booleans remain the only primitive types such checking might be a useful addition Arrays would also be valuable especially if visual output of numbers is facilitated since this would enable the system to be used to illustrate ideas of sorting and complexity If the Visual Compiler is extended accordingly e g with counting of primitive operations then some of the ideas from the systems reviewed by Chalk 27 might be worth considering 119 More fundamentally it would be desirable to incorpo
104. into account Apart from the various points already mentioned above the following criteria will be taken into account in marking your work e Program MUST run correctly without syntax errors e Program MUST be titled with your own username so if your username is DEP1ABC then the first line of your program should read PROGRAM deplabc and it should be saved under the filename DEP1ABC TGP e Text file MUST be saved under the filename PROGDESC TXT 116 e Program clearly written and formatted e g using the auto format facility and relatively easy to understand taking into account the complexity of what it does e Program should be annotated with comments to help indicate what is going on in different parts of it e Producing a visually interesting and maybe even entertaining result e Appropriate use of commands and structures to produce this result Items to be Submitted e TWO illustrative programs for Part I satisfying the criteria specified above and with names each reflecting their purpose so that the program title in the first line of the program is either EGPOLYLINE EGPOLYGON EGFOR EGREPEAT EGPROCEDURE EGRECURSION or EGCOLOUR as appropriate while the filename of the program file is either EGPOLYLINE TGP EGPOLYGON TGP EGFOR TGP EGREPEAT TGP EGPROCEDURE TGP EGRECURSION TGP or EGCOLOUR TGP e ONE main program for Part II satisfying the criteria above and with a program and filename corre
105. is false 1 e 0 this implies that count has reached a value greater than 200 and hence that the loop should terminate which is achieved by the IFNO 11 command in code line 3 which branches to the HALT at code line 11 This structure ensures that counting loops within Turtle conform to the Pascal standard in not operating at all if the initial value of the loop variable is already beyond the termination value The body of the loop consists of code lines 4 to 9 then line 10 pushes the loop variable count back onto the Stack increments it by 1 leaving the incremented value on the Stack and unconditionally jumps back to code line 2 where the new value gets stored and then tested continuing on to the next iteration of the loop 38 Tt may seem wasteful to load the count value onto the Stack then save it to the variable only to reload it again after the termination value The explanation for this will become clear shortly when we see future iterations of the loop via the JUMP command in code line 10 making use of the very same commands with the important exception of the preliminary loading of the initial value 54 Chapter 6 Turtle Machine Procedures 6 1 Procedure Calls and the Return Stack One of the most fundamental characteristics of procedures which helps to make them so valuable in modular programming is that they can be called from many different locations within the prog
106. ithmic power suitable to pave the way for graduation to Delphi Discussion in Chapter 3 To design program and progressively enhance a powerful robust and attractive programming environment conforming to the requirements implied by A1 and A2 a system which has proved in practice its suitability for the innovative and effective teaching of novices System illustrated in Chapter 2 and evidence of its practical value given in Chapter 4 To design a virtual Turtle Machine capable of supporting the language commands and structures implied by A2 including dynamic memory management dual parameter call methods and recursion and yet sufficiently simple to be potentially comprehensible to a non specialist student after a relatively modest investment of effort Virtual machine described in Chapters 5 and 6 which also aim to demonstrate by example how its behaviour can be explained at a relatively simple level 16 A5 A6 To design a compiler from the Pascal source code A2 to the Turtle Machine s PCode A4 operating by methods that are sufficiently simple to be explained without presupposing any significant prior knowledge of formal language or machine theory and sufficiently general that they can provide a practical introduction into the wider uses of abstract machines and compilation techniques Compiler described in Chapter 7 its operation divided into three levels each of which is sufficiently manageable
107. ivity Here I would suggest a strong similarity between the potential appeal of programming and that of playing chess as famously expressed by the great Siegbert Tarrasch in the preface to his last book The Game of Chess Chess is a form of intellectual productiveness therein lies its peculiar charm Intellectual productiveness is one of the greatest joys if not the greatest one of human existence It is not everyone who can write a play or build a bridge or even make a good joke But in chess everyone can everyone must be intellectually productive and so can share in this select delight I have always a slight feeling of pity for the man who has no knowledge of chess just as I would pity the man who has remained ignorant of love Chess like love like music has the power to make men happy If playing with Turtle can harness some of this power then the benefits may be considerable Scepticism about the value of this approach is most likely to be prompted by the thought that Turtle is just a toy system far too simple to be of significant use for introducing real programming But such scepticism is hard to maintain when faced with 30 I have not included statistics from the two Delphi courses that followed on from the Turtle based Programming Concepts core partly for reasons of space but also because they were optional making comparison with previous modules problematic In practice the vast majority of st
108. k black 10 LDIN 0 BLNK penup 11 PNUP movexy 140 40 12 LDIN 140 NEG LDIN 40 Mvxy bf 50 13 LDIN 50 PROC 1 END 14 HALT In the default situation shown here the procedure starts with PSPR HPCL and ends with HPRE PLPR ENDP the parameter is passed through the Program Stack at code lines 8 and 13 but no other information is passed by that route 47 Note also the option provided through the Compile menu to display the height of the Stack rather than the contents of the third Stack location in the last trace column 72 6 7 1 Doing Without the Procedure Return Stack Now suppose that we opt to compile the program without use of the Procedure Return Stack this means that the procedure return jump location must be passed via the Program Stack for which we use the LDRJ and PLRJ instructions respectively to load and pull it 48 Informal description of Assembler Decimal Hex command behaviour mnemonic code code load procedure return jump 224 E0 pull and make return jump 225 E1 Since however the address is intended to remain on the Stack after the parameters have been unpacked it is best to load it before the parameters This happens twice once when the procedure is called recursively at code line 8 and once when it is called from the main program at code line 13 PROCEDURE bf r integer 1 PSPR 1 HPCL 1 1 BEGIN 2 STVV Ll bf r 1 8 LDR
109. l abstraction But Turtle Graphics scores better on these three criteria in part because it delivers an obvious graphical product rather than being focused primarily on a task In addition to these general constructivist considerations there were a number of more specific reasons for favouring Turtle Graphics in the context of this project e Turtle Graphics is simple and intuitive and hence easy to grasp even for young children because it is based on a familiar type of metaphor cf Mayer 79 Brusilovsky et al 19 Pane and Myers 95 section 5 1 A turtle which need not be represented visually moves around the screen drawing as it goes and following straightforward English instructions such as forward left circle and colour This metaphor is so straightforward as virtually to guarantee that students will have no difficulty in understanding it e No doubt for obvious evolutionary reasons humans find visual comprehension far easier and more natural than for example mathematical as testified by the frequent use of visual language to describe our grasp even of abstract ideas With Turtle Graphics one can literally see the program working through its stages especially if it is possible to pause the processing at intermediate steps e Ifa Turtle Graphics system makes provision for loops then it is fairly easy to learn how to use it to produce attractive patterns far more intricate than w
110. l variables that are changed 10 12 a yellow blot is drawn using the new x and y coordinates xp and yp which yet again are aliases for xpos and ypos 13 14 screen updating takes place only here in the procedure aiming to give an illusion of smooth motion rather than flashing as the blots are drawn 15 16 a test is performed to see whether xp alias for xpos is within a ball s radius of the side of the Canvas if so then xv alias for xvel is inverted so that the ball will move back in the opposite direction Code line 17 which ends the procedure contains three commands First HPRE 1 releases the Heap space previously claimed by HPCL 1 7 as explained in 6 3 then PLPR pulls the procedure index from the Procedure Register Stack 6 5 finally ENDP returns control to the main program pulling the code line from which the procedure was called here 23 from the Procedure Return Stack Hence execution then continues from code line 24 where the test is made to determine whether the repeat loop should continue Eventually ypos will randomly drift to be less than 0 or more than 1000 at which point the IFNO branch back to line 23 will cease to operate and the program will terminate when it reaches the HALT instruction at code line 25 70 6 6 1 Note on the Trace Facility In closing this discussion recall from 2 3 and
111. larised into three fairly independent levels each being handled by a different mechanism which can be learned and understood separately as complementary compiling techniques Compilation subtask Mechanism involved in subtask PASS 1 Lexical Analysis Simple finite state machine FSM 7 3 Screening Tokenisation PASS 2 Parsing and Code Generation for 7 4 Program Block Structure Complex FSM plus counter 7 5 Control Structures Pushdown automaton PDA 7 6 Commands and Expressions Recursive descent makes reference to a parent procedure that has not yet itself been compiled Grune et al 47 point out that increased memory capacity has recently led to the development of broad compilers which read the entire program and then transform it removing any need for multiple passes pp 26 7 79 7 3 Lexical Analysis and Screening The first pass of the compiler involves the lexical analyser function lexanalyse which works through each line of the source code identifying the lexical items lexemes it contains and assigning an appropriate lexical type to each such item Categorising the lexemes in this way greatly facilitates the later processes by removing numerous complications For example at this stage the Pascal keywords or reserved words are picked out since the lexical analyser is integrated with a screener cf 7 1 above so that from now on the compiler when checking for
112. limitations on a turtle style system Roberts and Picard 109 and Bruce et al 18 have therefore developed alternative approaches which though somewhat inspired by Turtle Graphics are based instead around the idea of persisting graphical objects with their own internal state 12 See www otherwise com Jurtle html Another system which may be of a similar type since it is described as a graphics environment but requires an external compiler is Sparkling Light Software s 10 Jurtle requires prior installation of the standard Sun JDK Java Development Kit and integrates nicely with it once the location of the Java compiler has been set within Jurtle it hands over control seamlessly when necessary and displays the appropriate compiler messages within the programming environment The problem is that these messages are not designed to be suitable for a novice so as soon as things start to go wrong such a user is likely to feel overwhelmed and intimidated Here as a very mild example is the message given when the word forward is misspelt within BoxTurtle one of the simplest illustrative programs provided C Program Files Jurtle Examples BoxTurtle java 19 cannot resolve symbol symbol method forwad int location class BoxTurtle forwad size height 40 A Far worse however is what happens if the user happens to omit the which follows the line public void runTurtle within this
113. lphi and hence Turtle prefix convention of Here they are ordered according to their logical category and stack effects an organisation that should be evident from the Help file s PCode Reference Guide 35 Turtle Assembler Decimal Hex Turtle Assembler Decimal Hex instruction mnemonic code code instruction mnemonic code code fine oar e fse rink oun 1 so a Ear fe ES eee ee Se ee ae i ee ee Some of these instructions e g home require no parameters some require just one e g forward and some require two e g movexy or more i e canvas given this variety the simplest way of delivering these parameters in a uniform manner within the compiled machine code is by means of a Program Stack or just the capitalised Stack for short Such a stack also provides the simplest way of handling arithmetical calculations in reverse Polish fashion and this integrates easily with its use for parameter 35 In this and the next chapter all of the various PCode commands will be discussed with the sole exception of the NULL command code 0 which does nothing whatever but owing to constraints of space in most cases their precise behaviour is not specified For details of all PCode commands please consult the Help file section just referred to 46 passing Thus for example the command movexy 3 4 5 15 7 should involve the following sequence of
114. ly it is convenient to introduce simple x and y coordinate setting commands before students have been taught about the relevant global variables then setxy serves as a useful shorthand when both x and y coordinates are to be set 29 blank instruction which is not equivalent to any specific rectangle filling polygon command None of these four absolute movement commands involves drawing whereas forward and back are sensitive in this respect to the state of the pen controlled by penup and pendown Such sensitivity also applies to drawxy but not to movexy both of which provide relative movement The remaining Turtle primitives are canvas to set the Canvas dimensions dynamically pause invaluable for simple debugging as well as visual effects update and noupdate which are needed only to mitigate the display speed limitations of computer hardware and the related group of polyline polygon remember and forget these last four give a great deal of power and obviate the need for more specific polygon drawing or filling routines In contrast with the case of circle note that there is no compelling argument here for dedicated triangle square or rectangle procedures etc because the versatile polyline and polygon commands do not involve any increased psychological complexity identifying
115. m 100 for the most positive available answer to 0 for the most negative 2000 2001 2002 ACOM 2001 2002 2003 Norm The lectures were well structured and organised 87 0 84 3 85 7 70 The lecturer was always clear and audible 90 0 88 4 89 3 70 The lecturer made good use of examples etc 86 8 89 3 70 The lecturer made things easier to understand 81 7 79 8 66 The lecturer helped to make things interesting 83 4 86 9 62 The lecturer seemed responsive and approachable 89 0 85 8 90 5 67 The coursework was well designed and appropriate 83 8 82 9 76 8 67 I feel this work has taught me useful skills 81 0 78 0 75 6 73 The lecture handouts or booklet were very helpful 90 0 84 1 83 9 72 It is apparent that these are extremely positive results and this impression is confirmed when they are compared against the overall responses for the 27 highly varied components that have been delivered within the ACOM IT Skills programme over the period which have averaged around the approximate figures shown in the last column Even though the ACOM programme as a whole is seen as extremely successful with students Programming Concepts has consistently come at the top of its feedback league Only one of the other 26 components has ever exceeded it over this time namely the small option IT Poli
116. mitting a special test for null statements following then else and do In structures such as if x 1 then lt actionl gt else lt action2 gt or for x 1 to 10 do lt action3 gt it is standardly permissible for any of the lt action gt statements to be absent i e null and these structures can occur embedded within other structures for example repeat for x 1 to 10 do nothing if y lt 100 then y y 2 else nothing until y gt 100 The Turtle compiler differs from standard Pascal in treating null statements as illegal if they are immediately followed by a semicolon since in novice programs such semicolons are almost invariably misplaced But other null statements within these structures need to be compiled appropriately and for this purpose the Turtle compiler makes the following test as soon as each new lexeme is read If either the new lexeme is in until end and the top pending operation on the stack is in poTo poDownTo or the new lexeme is in unt il end else and the top pending operation is in polfThen polfElse then the lexeme pointer is decremented by 1 so the new lexeme will be read again next time round the loop and processing continues within the pushdown automaton from immediately below the Process Command node as though a command had just been dealt with READ next lexeme End of block ltEnd Stack READ empty lt expr gt Process 1tThen
117. n the first built in illustrative program seen in 2 1 above for example forward 450 LDIN 450 FWRD pause 1000 LDIN 1000 WAIT right 90 LDIN 90 RGHT thickness 9 LDIN 9 THIK 48 Suppose however that we wish to compile a statement that operates on a global variable for example forward turty which moves the turtle forward by a distance corresponding to its current y coordinate Here we take advantage of the fact that turty is a predefined global variable and so can be discussed without worrying about the details of variable declarations This command is obviously similar to the forward 450 compiled above except that here the parameter to FWRD has to be loaded onto the Program Stack from the storage address of the global variable turty rather than being loaded as a fixed integer Global variables are stored sequentially at the bottom of a structure called the Heap which begins with the five predefined globals turtx turty turtd turtt and turtc representing the current turtle x coordinate y coordinate direction pen thickness and colour and then continues with any global variables that may be defined within the running program The remainder of the Heap is used for the storage of local variables which we ll come to later see 6 2 4 Heap structure lt any local variables declared within procedures that are currently active gt lt
118. n 2002 03 and are followed by an example coursework Plan for First Lecture on Turtle a b c d e In every lecture apart from the last emphasise that they shouldn t yet be worrying about the coursework their concern should be simply to master what s been covered in the lecture concerned and to have a go at the exercises that take them further Also they shouldn t need to make lots of notes because all the details are in the Help file however do emphasise when you make a general engineering point that they should be noting it down e g advice for debugging tips for systematic working etc that aren t in the Help Start with the module handout which provides syllabus and administrative details but will need modification each semester to reflect lecture times etc When commenting on the structure emphasise that the Pascal language taught in the core is the same language that is required for Delphi so it s not just a toy The first lecture s objective is to introduce the Turtle system to a level where students can explore it for themselves and have a go at the first 4 exercises Don t tell them that they re expected to complete the first 4 exercises before the next lecture rather say this sort of thing You ll benefit far more from the next lecture if you ve at least had a go at exercises to 4 But don t worry if you get stuck and don t spend ages trying to solve p
119. n and Information Technologies 2 pp 65 83 5 23 20 Brusilovsky P A Kouchnirenko P Miller and I Tomek 1994 Teaching programming to novices a review of approaches and tools in T Ottmann and I Tomek eds Educational Multimedia and Hypermedia Proceedings of ED MEDIA 94 Vancouver Association for the Advancement of Computing in Education pp 103 10 23 21 Burton Philip J and Bruhn Russel E 2003 Teaching programming in the OOP era ACM SIGCSE Bulletin 35 2 pp 111 14 7 22 Callear David 2000 Teaching programming some lessons from Prolog 1 LTSN ICS Conference Edinburgh 7 23 Calloni B and Bagert D 1994 ICONIC programming in BACCI vs textual programming which is a better environment ACM SIGCSE Bulletin 26 1 pp 188 92 117 24 Ca as J J T Bajo and P Gonzalvo 1994 Mental models and computer programming International Journal of Human Computer Studies 40 pp 795 811 96 25 Cantu Marco 1997 Comparing OOP languages Java C Object Pascal published on the Web at www marcocantu com papers ooplang htm 7 26 Caspersen Michael E and Christensen Henrik Baerbak 2000 Here there and everywhere on the recurring use of Turtle Graphics in CS1 Proceedings of the 4 Australian Computing Education Conference ACE2000 Melbourne pp 34 49 9 27 Chalk Peter 2000 Webworlds Web based modeling environm
120. n assembled by the Maryland Collaborative for Teacher Preparation see www towson edu csme mctp Essays html 4 Ben Ari 12 outlines how an educational paradigm can impact on attitudes to ontology and epistemology as well as methodology and pedagogy He then goes on to suggest how a more sober constructivism can bring positive benefits to computer science education 5 The last two are Experience WITH the knowledge construction process i e learning how to learn and Metacognition i e reflecting on one s learning to generate more effective learning strategies Both of these involve higher level educational goals which do not apply in the early stages c a Authentic Learning Tasks learning tasks should be embedded in problem solving tasks that are relevant to the learner b Interaction interaction is the primary source material for the cognitive constructions that people build to make sense of the world One particularly influential approach ultimately deriving from Vygotsky 134 is Cognitive Apprenticeship theory which emphasises the active role of the teacher in supporting the learner The teacher first provides a model of expert performance in the task and actively coaches the learner in acquiring the target skills and knowledge then gradually removes this support forcing the learner to become increasingly independent Such a removable support is commonly referred to a
121. n it finishes through either of the end transitions from state PROC The condition of the procedure stack also determines the choice between the two possible end transitions from state PROC which are shaded in Figure 16 on page 83 to indicate the deviation here from a pure finite state machine to what is in effect a l counter machine the simplest kind of pushdown automaton with a single stack and only one symbol in its stack alphabet If the procedure stack is empty then a transition is made to ProgSemi but otherwise to ProcSemi since the latter indicates a move into a parent procedure rather than to the main program 7 4 1 Generation of Procedure Code Although code generation plays relatively little role in the processing of block structure it does feature when procedures start and finish Just as any parameter or variable name declaration triggers the addvar routine so any procedure name declaration will call the routine addproc lexnum integer at the point of transition to state ProcName This routine checks the name for non duplication and initialises a new procedure record primed so that parameters and variables can be bound to it by any immediately subsequent calls to addvar and addvtype These bindings then influence how code is generated when the procedure body is reached with the transition to state PROC which calls the routine procbegin lexnum integer this code deals with the Turtle Machine s activation
122. nd indicate their type but such prefixes are ignored here except when explicitly quoting code Most of the transitions between states in this FSM involve more than a single lexeme moving from ProcVName to ProcVSemi for example requires a colon followed by an identifier followed by a semicolon But it would be wasteful and confusing to add states to the FSM corresponding to each of the intermediate stages of such a sequence because once the colon has been identified the only legal continuation is an identifier e String Type EntryState ExitState followed by a semicolon Hence this program program Start 1 REA i triangles identifier part of the compiler is implemented in _ ProgSemi PROCEDURE procedure ProgSemi i a way that directly mirrors the _ triangle identifier ProcName ProcName ProcBkt structure of the FSM as illustrated by siz identifier ParName that part of the Delphi Pascal code 9 identifier ParType ParlType ProcSemi itself part of a larger case statement Seam begin ProcSemi PROC z if if nanar z which handles the situation where the syntax state is ProcVName Figure 15 FSM states in the Syntax table All statements within the body of any procedure are processed in state PROC begin program y id ProgSemi begin Var All statements in the body id of the main program are processed in state PROG gt amp end 2 id i begin Prog
123. ng the Help file 1 Cycling colours RANDCOL as a random number generator and use of MOD to cycle note that MOD is also used in the Combining structures illustrative program to produce the steps so it s worth pointing out how useful it can be for any oscillating pattern 2 Using booleans more randomness and boolean variables 113 Using POLYGON with FORGET mention POLYGON and POLYLINE both of which are relatively likely to be useful for all students and explain how FORGET can be used to extend their power 3D effects with colour just use this as an opportunity to draw attention to Turtle s ability to handle colours in more powerful ways than have been shown before making reference to the Help system for anyone who wants to follow this up g Finish off by discussing the coursework and associated administrative arrangements and by reminding students about the final lecture on the Turtle compiler to which all are invited but for which attendance is not compulsory advertise it as of particular interest for those who want to know what goes on under the bonnet of a real computer Illustrative Programming Concepts Coursework This illustrates the typical content of a recent Turtle Graphics Programming coursework Web page omitting cross references links and general advice on submission method deadlines plagiarism help sessions newsgroups example submissions and other
124. ngles Manual termination is particularly important if the trace facility is operative to enable this select Trace on run from the Compile menu which automatically selects also the Analysis tables setting making visible a set of tab controls along the bottom of the Canvas Now again click on RUN then HALT the program after a few seconds and click on the tab control labelled PCode Turtle Graphics Programming System c PJR Millican 2000 2003 jol x File Edit Layout Compile Options Help Y 186 Direction 120 Thickness 1 Colour B PROGRAM triangles Display decimal Machine Code decimal Assembler instructions PROCEDURE triangle size int C hexadecimal Machine Code C hexadecimal Assembler instructions BEGIN if size gt 2 then begin forward size triangle size 2 right 120 forward size triangle size 2 Perre E right 120 forward size triangle size 2 right 120 end END Cycle BEGIN 1 14 1 LDIN 100 PaU 5 aso 100 movexy 100 150 2 14 3 NEG Pd U 5 070 100 triangle 256 3 14 4 LDIN 150 Pa U 5 070 150 100 END 4 14 6 MVXY PAU 5 0 0 5 15 1 LDIN 256 Pd U 5 070 256 24 This program is explained in E E ST T 25 the online help under 8 1 3 HPCL 1 1 PU 6 1 1 256 26 Procedures and Parameters 9 2 1 stvv 1 1 Pa U A 1 1 27 It illustrates recursion in 10 3 1 LDVV 1 1 Pd U 1 1 256 28
125. ngly the previous value of this pointer must be saved to be restored when the current instance of the procedure terminates Since there is no limit in principle to the depth of recursion and hence to how many such previous values may be involved the most efficient way of storing them is in a stack structure Rather than have a separate stack for each procedure the most elegant way of achieving this is to maintain a single Heap Control Stack of which the Heap Top Pointer is the topmost value Then the required result can be achieved by specifying the behaviour of HPCL and HPRE as follows HPCL First exchange the value currently on top of the Heap Control Stack i e the Heap Top Pointer with the Procedure Heap Pointer for the 44 Tn standard Pascal two procedures can be defined as mutually recursive using the forward directive but this is not provided here because forward has a very different traditional meaning within Turtle Graphics It would be possible to define an alternative directive the word deferred might be appropriate but since simple mutual recursion can already be achieved in Turtle by nesting the procedures this complication and the inevitable inconsistency with Pascal as implemented within Delphi and other systems seems best avoided 60 procedure concerned this both sets the Procedure Heap Pointer appropriately and also saves its previous value on the Heap Control Stack Then
126. nt just below the seven Heap locations that doball needs in order to hold or in the case of reference parameters to indirect its seven local variables With the necessary Heap locations now claimed code line 2 performs the task of unpacking the Program Stack so that all seven local variables are appropriately set up As already mentioned in 6 4 1 this unpacking must take place in reverse order because of the last in first out stack structure For neatness the same reverse ordering is extended to genuine local variables although these do not require any use of the Stack hence the first operation in code line 2 is the one that deals with ymove the seventh local variable whose initialisation to 0 involves a new PCode instruction Informal description of Assembler Decimal Hex command behaviour mnemonic code code zero local variable ZERO 27 Thus ZERO 1 7 sets the seventh local variable of the first procedure to 0 without requiring any values to be passed via the Program Stack This command is not strictly necessary for the operation of the Turtle Machine since a sequence such as LDIN 0 STVV 1 7 would have a similar result but use of ZERO significantly shortens the initialisation sequence for procedures that have genuine local variables The rest of code line 2 involves unpacking from the Stack either variable addresses in the case of the three reference parameters or integ
127. obvious way of helping students to develop an adequate mental model of computer execution is to provide an easily accessible virtual machine to study one that can handle the problematic looping and recursive control flow structures but which in other respects is as straightforward to program as possible The natural objection to this suggestion is that learning about virtual machines and low level code so far from reducing the problem may prove even more confusing and unpopular with students than learning about high level programming Certainly low level code is in some obvious respects more complex than the high level equivalent but on the other hand it is less abstract and more obviously mechanical which may tip the balance if the student s main problem lies in acquiring an adequate notional model of execution At any rate there is some evidence that even novice students can cope with machine code if due care is taken and appropriate learning tools provided For example the breadth first curriculum described by Tucker et al 131 developed on the basis of the ACM IEEE Computing Curricula 199 report 1 successfully combined the teaching of programming with an introduction to computer organisation using a simple machine and assembly language simulator called Marina 64 This simulator was seen as playing a key role in facilitating the computer organisation section p 54 helping to make it one of the most popular in the curricul
128. of recursion The Programming Core was a fun and interesting one very interesting It was a very good module thanks Programming was a new experience for me but one I have enjoyed despite it being so time consuming I look forward to pursuing more programming options later in my university life Turtle graphics in built help and examples were extremely useful I found I learnt a lot just by playing around with the illustrative programs The lectures were also very useful well explained and relevant Happy customer Would have been interesting to study the applications of recursion in greater detail as this was a fascinating area of the course Many thanks I thought this was a great set of lectures I found them useful interesting and thoroughly enjoyable 36 Of the total of 38 comments provided over 2000 2002 30 were clearly positive and only 3 negative with one of these expressing uncertainty over what the coursework was really looking for and the others concerning the availability of demonstrators Other comments included suggestions for the course namely that more time should be spent on basic mathematics repeat loops or procedures and parameters and three recommendations for improvements to the Turtle system itself namely inclusion of a visual grid making the Are you sure quitting prompt more selective and adding keyboard shortcuts The last two of these suggestions have since been acted on 4 3 How Well Di
129. of the procedure as described in 6 3 and 6 5 claiming Heap space and pushing the Procedure Register and the initialisation of local variables and unpacking of parameters as illustrated in 6 6 Finally when the procedure finishes with one of the end transitions mentioned above the procend lexnum integer routine is called to insert the PCode required to terminate the procedure releasing Heap space popping the Procedure Register and performing an appropriate return jump etc 86 7 5 Parsing and Code Generation for Program Control Structures The middle level of parsing and code generation is concerned with the statement flow control and grouping structures that lie within the main program body or within the body of individual procedures these mainly involve the keyword combinations if then else repeat until for do and begin end Unlike the top level this parsing task goes well beyond what can be dealt with by a simple FSM or counter machine in that the various keyword structures can be intermixed and nested to an arbitrary degree Hence we resort instead to a pushdown automaton PDA that operates a stack of pending operations yet another abstract machine whose practical behaviour and usefulness can be nicely illustrated by this form of compiler The overall behaviour of this automaton is represented by Figure 17 on page 87 4 54 Figure 17 is slightly simplified in o
130. onals Sleeman et al 121 Hoc 56 Putnam et al 107 and general difficulties with tracing flow of control Perkins et al 102 Rogalski and Samurcay 112 precisely the sorts of problems that might be alleviated if students spent time experimenting with systems of this type In the context of such problems with flow of control it is hardly surprising that recursion is widely viewed as 31 Many more references could easily be given for example Shackelford and Badre 115 and those in Deek 34 and in the first paragraph of Sims Knight 119 For consideration of why programming is so difficult see e g Guzdial 49 Jenkins 62 Moser 85 and the references they cite 32 Two useful reviews of such studies are provided by Winslow 138 and Robins et al 111 33 Here I focus on Turtle s role as a vehicle for introductory program development and execution but we shall see later in 8 1 1 that it has another role which can help in dealing with these problems There is considerable evidence that students problems with control structures derive in large part from an inability to conceive an appropriate model of the computer s behaviour see for example the reviews by Pane and Myers 95 pp 30 32 Robins et al 111 pp 149 53 Turtle is distinctive amongst introductory programming systems in incorporating an explicit virtual machine which is designed to enable even non specialist users though admittedly not total
131. onvey the subject s and fun p 204 scope beauty genius 101 References Each item is followed by a bracketed list of the pages where reference is made to it within the thesis 1 ACM IEEE Joint Curriculum Task Force 1991 Computing Curricula 1991 New York ACM Press 97 2 Adair G 1984 The Hawthorne effect a reconsideration of the methodological artifact Journal of Applied Psychology 69 pp 334 45 39 3 Aho Alfred V and Ullman Jeffrey D 1979 Principles of Compiler Design Addison Wesley originally published in 1977 by Bell Telephone Laboratories 76 77 4 Aiken Alexander 1996 Cool a portable project for teaching compiler construction ACM SIGPLAN Notices 31 7 pp 19 24 99 5 Alfonseca Manual and Ortega Alfonso Representation of fractal curves by means of L systems 1996 ACM SIGAPL APL Quote Quod 26 4 pp 13 21 119 6 Applebe Bill 1979 Teaching compiler development ACM SIGCSE Bulletin 11 1 pp 23 7 99 7 Ariga Taeko and Tsuiki Hideki 2001 Programming for students of information design ACM SIGCSE Bulletin 33 4 pp 59 63 9 8 Atwood J W Jr M M Burnett R A Walpole E M Wilcox and S Yang Steering programs via time travel 1996 IEEE Symposium on Visual Languages Boulder Colorado pp 4 11 118 9 Baldwin Doug 1996 Discovery learning in Computer Science ACM SIGCSE Bulletin 28 1 pp
132. operations Push 3 onto the Stack Push 4 onto the Stack Push 5 onto the Stack Multiply the top two Stack values leaving the result i e 20 in their place Add the top two Stack values leaving the result i e 23 in their place Push 15 onto the Stack Push 7 onto the Stack Subtract the top Stack value from the next leaving the result i e 8 in their place Execute MVXY pulling its two parameters i e 23 and 8 from the Stack Since any student who aspires to understand this sort of thing obviously needs to have grasped the concept of a stack the simplest possible structure for the desired virtual Turtle Machine is one based on an explicit hardware stack architecture taking advantage of this to eliminate as far as possible the need for any other parameter storage registers As we shall see such a simplification is indeed entirely feasible The use of a stack based architecture avoids any need to consider hardware operations at any lower level i e we can ignore the issue of how the Stack is to be implemented and this enables us to determine very straightforwardly the required behaviour both of the Turtle Graphics primitive machine code commands and also the various arithmetic and Boolean operators These must operate on the top Stack value or the top two and in the latter case must treat these in an appropriate order so that SUBT for example subtracts the top value from the next as in the italicised e
133. ould be feasible by hand Hence such a system can appear useful even to students who have no interest in computation or information processing as such e Although very straightforward Turtle Graphics is not intrinsically limited Both novices and more advanced students can find a challenge within such a system because patterns of arbitrary complexity can be generated with more or less the same simple tools as are used by beginners e When a Turtle Graphics program goes wrong it is relatively easy to see where it has gone wrong and in what way thus greatly easing the pain of debugging e Likewise in Turtle Graphics it is quick and straightforward to test what if scenarios changing the program and identifying how the output has changed as a result McConnell 82 pp 52 3 stresses the value of such active learning 7 The virtues of using graphics to mediate the learning of programming are also stressed with supporting references and a brief survey by Cooper Dan et al 31 32 though Naps et al 88 argue that such benefits do not necessarily extend to visualisation of more abstract algorithms e Turtle Graphics gives ample scope for personal creativity one of the greatest satisfactions to be derived from programming though largely closed off from beginners in typical training situations e g where they are set a fixed information processing task Not only is this enjoyable but there is evidence t
134. p 535 44 1 3 134 Vygotsky L S 1934 Thought and Language published in English translation by MIT Press 1962 4 135 Warford J Stanley 1999 BlackBox a new object oriented framework for CS1 CS2 ACM SIGCSE Bulletin 31 1 pp 271 5 7 136 Wichmann B A 1970 PL516 An Algol like assembly language for the DDP 516 UK National Physical Laboratory Report CCU9 at www series16 adrianwise co uk software exp15 16 p1516 p1516 pdf 12 137 Wilhelm Reinhard and Maurer Dieter 1995 Compiler Design Addison Wesley 76 77 138 Winslow L E 1996 Programming pedagogy A psychological overview ACM SIGCSE Bulletin 28 3 pp 17 22 41 43 139 Wolfram Stephen 2002 A New Type of Science Wolfram Media 2 140 Zelkovitz M V B Kowalchack D Itkin and L Herman 1989 A SUPPORT tool for teaching computer programming in R Fairley and P Freeman eds Issues in Software Engineering Education Springer Verlag pp 139 67 117 109 Appendix A Lecture Plans and Example Coursework The following sections spell out the plan for each of the four compulsory lectures in the Programming Concepts component designed to cover enough programming concepts and syntax to enable students to meet the Delphi system for the first time in Lecture 5 The lecture plans are in note form addressed to the lecturer mostly copied verbatim from those produced for the sessio
135. ped to consolidate the material coveredin the lectures and it was good to be able to try out specific tasks to make sure that you weren t having any problems Drew 1 0 0 100 0 with the work Overall the lectures were brilliant the best lectures I ve ever been toll Garry 0 0 0 z 2 it was difficult to complete the coursework as there was hardly ever any programming demonstrators to help Jill 0 0 0 and no one else semmed to know anything about turtle so it was very difficult to resolve any problems also the Martin 0 0 0 booklet was not very easy to understand i found that it complicated things and did not make much sense Pete 2 0 0 100 0 3 The only difficult aspect of this coursework was formulating an idea for a program for which we could apply the Peter 10 0 0 100 0 relevant programming Apart from this the course was excellent hd Sarah 0 0 0 Figure 7 The automated student feedback analysis program 34 took advantage of the second part to emphasise specific points The feedback was analysed using a Feedback Reporter program which broadly mirrored the structure of the students feedback interface see Figure 7 In assessing this student feedback most weight must be put on the overall appraisal radio button responses because they are so much more representative than the written comments Over the three sessions in question the results were as follows averaging each question s responses on a scale that ranges fro
136. procedure instance because in a recursive program the same procedure can have many instances each with its own local variables For simplicity I shall here usually refer simply to procedures but take for granted that procedure instances are understood where appropriate 59 The inverse operation releasing the claimed Heap space when the procedure terminates might seem to be equally straightforward simply requiring that the Heap Top Pointer be reduced by the same amount that it was previously incremented However this crude method will fail to restore the Procedure Heap Pointer also to its own previous value which though unproblematic if the just terminated procedure is no longer operative is disastrous if another instance of that procedure is still active a common situation if the procedure is recursive The obvious refinement of this crude method would be to decrement the Procedure Heap Base Pointer also by the same amount as the Heap Top Pointer and this will indeed work if the only recursion in the program involves single procedures that call themselves However a Turtle program can contain two procedures that are mutually recursive as long as one is a sub procedure of the other 4 and in such a case maintenance of the Heap structure through the possibly alternating recursive calls requires a far more robust approach The upshot of all this is that when a procedure is called and its Procedure Heap Pointer is updated accordi
137. program If you just throw in a random jumble of drawing instructions you will end up with a program that looks a mess and is unlikely to produce a visually interesting result Always allow plenty of time for debugging and tidying up your program This often takes much longer than working out the general ideas behind the program 117 Appendix B Possible Future Developments The development and refinement of a system such as Turtle is potentially endless because there are so many useful features that could be added to the editor the environment the Visual Compiler and the help resources indeed it would be relatively straightforward to fill an entire thesis with a discussion of the possibilities Since space does not permit any such extended discussion I shall confine myself here to merely listing a few of these together with references to some relevant literature that suggests possible alternative approaches Deek and McHugh 35 provide a particularly useful survey of many varied systems for supporting the learning of programming making reference to most of those mentioned below The Editor Currently the system s editor is relatively basic and it could usefully be enhanced with such features as search and replace syntax highlighting and multiple file processing to mention only the most obvious possibilities More radical suggestions might include a syntax tree based code generator such as that incorporated within the
138. push onto the Heap Control Stack the calculated new value for the Heap Top Pointer which is equal to the old value plus the number of local variables in the procedure HPRE Remove the top element of the Heap Control Stack i e the Heap Top Pointer Then exchange the new top element with the Procedure Heap Pointer for the procedure concerned this restores the previously saved values of both the Heap Top Pointer and the Procedure Heap Pointer It should be clear from these descriptions that they are indeed inverse operations HPRE thus defined restores exactly the Heap situation prior to the corresponding HPCL and because all the relevant values are saved on the Heap Control Stack this will remain true no matter how many recursive or other procedure calls are executed 6 4 Dealing with Parameters Pascal procedures can have two different types of parameter those that are called by value and those that are called by reference or value parameters and reference parameters for short The syntactic difference between the two is minimal in that the latter are simply prefixed by the keyword VAR hence the informal term VAR parameters But the two need to be handled by quite different mechanisms 6 4 1 Value Parameters Value parameters behave essentially as ordinary local variables except that their initial value on entry to the procedure is set to the value of the corresponding a
139. r than integer However in practice students taught using the system have moved on to Delphi before detailed discussion of types has become appropriate so it 26 orientation with turtles as objects and therefore potentially multiple cf Appendix B below 3 2 Arithmetical and Boolean Operators The four basic arithmetical operators are accommodated though is interpreted as integer division i e div given that reals are not permitted The inclusion of the complementary operator mod is obviously desirable too playing a particularly valuable role in the creation of alternating patterns as exemplified in the illustrative programs Combining structures and Cycling colours The arithmetical comparison operators lt gt lt gt lt gt are likewise included yielding the boolean values 1 true or 0 false as in 3 1 2 above Use of these values enables the Boolean operators not and or and xor to be incorporated as bitwise arithmetical operators also without ambiguity since thus interpreted the Boolean and arithmetical results correspond the point here being that the twos complement binary representation of 1 has all bits set 3 3 Program Structures Clearly the basic conditional structure if then else is essential and is sufficiently general
140. rall approach but the lack of full syntax analysis somewhat restricted the complexity of programming constructs that were permissible and made it extremely difficult to generate appropriate error messages for all syntactic failures Moreover the system made no provision for procedures an absolute essential if it was to act as a pathway into Delphi or consequently for recursion greatly limiting both its scope for genuinely stimulating experimentation and also its potential as an introduction to any programming concepts beyond the most basic Having established that a fundamental redesign of the pilot system s internal operation would be required for further progress it seemed an obvious next step to make a virtue of necessity and develop the system not only as an introduction to the craft of programming but also to the concepts of programming language systems such as compilation machine code and dynamic memory management Having come through the route of assembler programming myself I find it regrettable albeit very understandable that as the realm of applications has expanded this fundamental aspect of the discipline has tended increasingly to be pushed out of modern syllabuses so that even many specialist programming students today remain virtually ignorant of the underlying mechanisms with consequent detriment to their understanding of other areas for example data structures and complexity theory Hence my decision to develop this
141. ram code without disrupting the local flow of control A procedure acts as a self contained unit running its course and then immediately returning control to the location from which it was called which can vary from occasion to occasion Hence the implementation of procedures within PCode requires a form of flow management more flexible than the fixed jumps and branches discussed in the previous chapter To cope with the potential flexibility of calling locations the PCode must be able to save the appropriate return location when each procedure is called and to jump back to it when the procedure terminates A further complication is that procedures can be nested or recursive with one procedure calling another and that yet another and so on without theoretical limit thus numerous procedures or instances of a single procedure can all be active simultaneously This in turn implies that the sequence of pending return locations corresponding to the hierarchy of procedures currently in progress cannot be stored in fixed addresses but must be stored in a stack structure last in first out since the last procedure to start is always the first to finish The implementation of procedures therefore requires that the Turtle Machine should incorporate a Procedure Return Stack or Return Stack for short Operation of the Procedure Return Stack clearly requires at least two PCode commands one when each procedure or more accurately p
142. rate However such extensions would have taken more time than was available in this project and their implementation is anyway perhaps best postponed until practical experience has been gained of the system s use in teaching computing concepts and relatively basic compilation to enable more reliable judgements to be made as to whether additional system complexity is likely to carry too great a cost in comprehensibility and accessibility 3 1 Data Types 3 1 1 Numbers Any Turtle Graphics system must make use of numbers most obviously to specify lengths or angles and here integers are indispensable They are simpler to handle than real numbers more efficient both in respect of storage and processing speed but also they are precise enabling conditional tests for equality to be performed on them without having to worry about rounding inaccuracies Related to this is the fact that integers can be divided precisely into bytes and thus conveniently used for representing a wide range of colours in the standard 24 bit manner as explained in 3 4 2 below The question still arises however whether any other numeric types should be permitted within the system A case could be made both for real numbers notably to enable greater accuracy in recording fractional coordinates within intricate recursive patterns and also for bytes to reflect the limited range of the virtual machine code instructions The latter case is however rat
143. rate additional language syntaxes notably that of Java which could be combined with moving to an object oriented approach The notion of a turtle as an object is very natural and would open the door to multiple turtles which can interact as explored by Resnick 108 with his development of StarLogo Modern systems are increasingly composed of interacting objects with independent threads of control and object orientation would enable programming to be taught in the spirit of this new paradigm as strongly advocated by Stein 129 under the motto computation as interaction A less fundamental but more easily manageable change would be to allow Java syntax but remain procedural taking advantage of the restriction to introduce intertranslatability between Pascal and Java syntax which might in itself have a significant educational value for novices The Compiler Leaving aside changes to the compiler implied by the suggestions above it might be useful to extend the range of error messages and warnings also adding corrective hints of the type described by Lewis and Mulley 73 for example to warn students of apparent identifier keyword conflicts duplicate identifiers in nested scopes variables not used or not initialised etc The Illustrative Programs and Tutorial Material If new features are added such as those listed under The Language above then it will obviously be appropriate to update the illustrative programs and cor
144. rawpause BEGIN colour green blot 100 pause 1000 colour red forward 450 pause 1000 right 90 thickness 9 colour blue pause 1000 forward 300 END Note that comments in curly green blot pause 1 sec red line 90 degrees thickness 9 blue line brackets have absolutely no effect on the program They can run over several lines just as this one does As this program runs watch the turtle settings shown just above the Canvas 0 x Colour B Xx 800 Y 50 Direction 90 Thickness 9 Canvas Syntax Expressions Declarations PCode Figure 1 Turtle running the drawpause illustrative program As shown in the picture above the left hand section of the screen is the Programming Area which incorporates a simple editor while the right hand section is the Canvas on 18 which graphics are created by the drawing statements in the program gt The syntax of the program is essentially that of Pascal except that the system includes built in drawing procedures some of which are common to virtually all Turtle Graphics implementations e g forward right though others are specially provided to take advantage of this system s particular facilities e g blot pause The Help file contains a detailed walk through of this particular program in its third section entitled The Program this
145. remainder of the thesis is structured as follows Chapter 2 provides a very quick tour of the Turtle system from the user s point of view Then Chapters 3 and 4 together cover use of the system for introducing programming to novices with the former focusing on the choice of programming language structures and commands and the latter on practical experience of three years teaching using the system or its earlier prototype Chapter 4 s conclusion effectively sums up the first half of the thesis reflecting on the overall value of the Turtle system in the learning and teaching of introductory programming Chapters 5 to 7 then turn from introductory programming to the Turtle virtual machine its PCode virtual machine code and its compiler Chapter 5 explains the general design of the Turtle Machine and the way in which Turtle Graphics commands and simple Pascal control structures can be represented within PCode Chapter 6 builds on this to tackle the far more complex subject of procedures digging deeper into the Turtle Machine to show how Pascal procedures can be handled by it including recursion and parameter calling methods Finally Chapter 7 explains the workings of the Turtle compiler which translates the user s Pascal source code into equivalently functioning Turtle PCode in conformity with the design explained in the previous two chapters It is important to note that Chapters 5 to 7 are deliberately written in a more di
146. responding tutorial Help file sections accordingly Apart from this an interesting possibility is to develop the discussion of recursion into the area of fractal patterns perhaps linking this with the theory of Lindenmayer systems L systems for the display of which Turtle Graphics is already known to be a useful vehicle Bridges 16 Alfonseca and Ortega 5 Proulx 106
147. rest comparator available within Leeds University is the module COMP1150 Introduction to Programming on the PC which ran as an elective for a similar range of students between 1993 and 1999 also teaching Pascal Obviously over the last decade there has been considerable change not only in computer systems and staff teaching allocations but perhaps even more so in the typical 37 profile of a Leeds student more IT literate but arguably less elite mathematically weaker and more used to spoon feeding so such comparisons must be viewed with some caution But if we focus on the last three years of COMP1150 over which period it was taught by two different lecturers with a single package namely Borland Pascal for Windows the contrast with the Turtle Graphics course also taught by two different lecturers over the relevant period is very striking Figure 8 Grade profile for the 1996 99 course teaching Borland Pascal Borland Pascal for Windows 1996 99 20 15 4 10 4 i 1 i Th 0 ool T all T T IF E NO AMDAOHORDAAHAANAHAANDANNAA QY Q AN D P a Fyen ae ooN ROO Q O omo OMHROMDAHAMHDAH OOM O GOH OO AAA HY THO HO KH OO GOKRKK KR DO BO WO Figure 9 Grade profile for the 2000 03 course teaching Turtle Graphics Pascal Turtle Graphics Pascal 2000 03 20 15 4 10 5 0 NO DANO RDAHAANAHAANDANANANAA HM Q oe Sn en Se Se ee a Oe ee Oe ee ae Pee ae S
148. rious ways of improving its appropriateness for self learning by young users independently of any formal tuition viii Abstract The Turtle Graphics Programming System developed for this MSc Turtle for short is designed to enable non specialist students to learn a range of fundamental concepts of programming and Computer Science in a straightforward but fairly rigorous manner within a user friendly environment that makes the first steps as easy as possible while providing scope for advanced experimentation Chapter 1 explains the overall form of the system why it is based on Turtle Graphics why it uses Pascal source code and why it incorporates a virtual machine and a self contained compiler to translate that source code into Turtle Machine PCode The chapter ends with an outline of the remainder of the thesis and a list of what I take to be the project s main achievements and novel contributions Chapter 2 provides a quick tour around the finished system including its Visual Compiler displays that aim to make the Turtle Machine s inner workings transparent Chapter 3 then explains in detail why its Pascal source language has the commands and structures that it does bearing in mind the system s primary role as a learning system for introductory programming Chapter 4 reviews its actual performance in this role drawing on student feedback and assessments to establish its effectiveness Chapters 5 to 7 cov
149. roblems if you encounter them this is likely to do more harm than good Next week Pll be going over what you need to do for these exercises but what I say will mean more to you if you ve at least made an initial try at them even if you get virtually nowhere with them Also advise them that the exercises provide a valuable fail safe in case they mess up the coursework they will be invited to submit their exercises along with the coursework and in this case the exercises might enable them to pass even if their coursework fails however this will not compensate for failure to make a serious attempt at the coursework Show them how to access and start up the system and emphasise that it can be downloaded for home use Show them the Help system especially the exercises section Read some of this through and show them how it advises to start with Illustrative program 1 pointing out that there are other illustrative programs too Using this program as an illustration informally cover all the points in the section on The Program which is linked from the exercises Help and afterwards draw their attention to the fact that everything you ve said is there in the Help section so they can read it through for themselves at leisure Particular points to note when 110 f g h running the program include the various areas of the screen especially the program window Canvas RUN button status bar examples
150. rocedure instance is called and one when it terminates The command that calls a procedure must perform two distinct functions transferring control to the code line corresponding to the start of the procedure s body and pushing the appropriate return location i e the code line from which the procedure was called onto the Procedure Return Stack The command that terminates the procedure should then pop this same return location from the Return Stack and transfer control accordingly back to the end of the code line from which the procedure was called These two commands are as follows 55 Informal description of Assembler Decimal Hex command behaviour mnemonic code code call procedure PROC 32 end procedure ENDP 33 6 2 Local Variable Storage the Heap Top Pointer and Procedure Heap Pointers 5 2 above explained how global variables are stored at the bottom of the Heap and accessed through the instructions LDVG and STVG by means of an index number that gives their address within the Heap Local variables are also stored in the Heap above the globals but what makes their processing more complicated is that they do not reside there permanently but are created and destroyed in turn as the procedure instance to which they belong first starts and then terminates The basic principle for referencing local variables is quite similar to that for globals in that each variable has an index number which in
151. ror messages if illegal characters appear in the sequence most other error messages produced by the lexical analyser concern characters that are illegal wherever they appear such as amp But the most complicated type to deal with is tIdK a temporary place holder in effect filling the gap between pure syntax analysis and screening which indicates a sequence of alphanumeric characters starting with a letter when the character sequence is complete it is looked up in the keyword list to establish its nature Another place holder type is 6699 ItMinus signifying a character which could be either a unary negative as in 66 x 1 or a binary subtraction as in x y z This ambiguity however is resolved not by the lexical analysis but by the recursive descent phase of the later syntax analysis cf 7 6 especially note 61 7 4 Parsing the Block Structure of the Program The second pass of the compiler integrates syntax analysis and code generation controlled by the function syntax which is called repeatedly while working through the sequence of lexemes created by the first pass As outlined in 7 2 this task is broken down into three levels the first of which involves parsing the program s large scale structure identifying the blocks that constitute its procedures sub procedures and main body and dealing with their associated declarations including scope issues This top level of proc
152. s Stack position Item on the Corresponding formal top 1 Program Stack parameter within procedure 1 Absolute Heap address of global yp reference parameter variable ypos 9 2 Absolute Heap ess of global xv reference parameter variable xvel 8 3 Apoi Heap aadress of global xp reference parameter variable xpos 7 4 Integer 65535 or FFFF the col value parameter colour code of yellow Current value of global variable b l t bgcol 20480 or 5000 Beya ie Paramet Er Integer 30 size value parameter Note that all six items on the Stack are integers but the top three are destined to be used as Heap addresses rather than as integer values in their own right To see how this happens we must now turn our attention to the code lines of the procedure itself The first thing that happens after procedure doball is entered at code line 1 is that PSPR 1 pushes the value of 1 onto the Procedure Register Stack the 1 here signifying that the first procedure in the program is now active Then HPCL 1 7 claims space on the Heap for the seven local variables of this first procedure these are the six formal parameters in the table above plus the one genuine local variable ymove HPCL 1 7 does this by adjusting the various Heap pointers as described in 68 6 3 above this ensures in particular that Procedure Heap Pointer 1 is set up to poi
153. s scaffolding within the educational literature c Encourage Voice and Ownership in the Learning Process students should be allowed to choose the problems they will work on and the teacher should serve as a consultant to help students to generate problems which are relevant and interesting to them Turtle Graphics is an excellent exemplar of this paradigm on all three criteria First it replaces traditional number or text based programming exercises with creative graphical design something which is interesting and motivating to a far higher proportion of students Secondly it provides a good vehicle for interactive learning precisely because the tasks it involves are ones that can easily be appreciated and discussed moreover a Turtle Graphics system with a simplified environment and built in primitives designed to facilitate the creation of graphics can itself serve as a scaffold to develop the skills needed for more advanced systems Finally the creativity implied by graphical design positively invites students to generate their own ideas our human visual imagination ensures that there is no difficulty whatever in having an entire class all working on their own individual problems 6 Along with Turtle Graphics perhaps the best known microworld approach to introductory programming is that of Karel the Robot Pattis 99 which in particular gives an excellent basis for learning about procedura
154. s 1991 Pascal User Manual and Report ISO Pascal Standard fourth edition revised by Andrew B Mickel and James F Miner Springer Verlag originally published as the Pascal User Manual and Report by Jensen and Wirth 1974 11 66 Kahney H 1989 What do novice programmers know about recursion in Soloway and Spohrer 126 pp 209 28 42 67 Keller John M 1983 Motivational design of instruction in Charles M Reigeluth ed Instructional Design Theories and Models An Overview of the Current Status Lawrence Erlbaum Associates pp 383 434 42 68 Kessler C M and Anderson J R 1989 Learning flow of control recursive and iterative procedures in Soloway and Spohrer 126 pp 229 60 42 69 Krishnamurthy E V 1983 Introductory Theory of Computer Science Macmillan 82 70 Kurland D M R D Pea C Clement and R Mawby 1989 A study of the development of programming ability and thinking skills in high school students in Soloway and Spohrer 126 pp 83 112 3 42 71 Levy Dalit 2001 Insights and conflicts in discussing recursion a case study Computer Science Education 11 pp 305 22 42 72 Lewis C and Olson G M 1987 Can principles of cognition lower the barriers to programming in in Olson Sheppard and Soloway 93 pp 248 63 23 105 73 Lewis Stuart and Mulley Gaius 1998 A comparison between novice and experienced
155. s also required to contain at least two procedures preferably both with parameters each of which should have a clearly identifiable purpose Ideally these two procedures should each do a single job which is fairly easy to describe and understand but which is at least moderately complex so each procedure should contain at least 5 10 commands A procedure of this kind is particularly valuable because not only does it make the program easier to understand by dividing it up into easily understandable tasks but also it can save you work in the long run because the very same procedure once written and checked can be copied into any other Turtle program where you need to do the same job and if you got it right first time won t need to be re written or re checked The specification of each procedure s purpose and a brief description of the program s intended behaviour and of any issues you have encountered in its development should be written in a text file and submitted with your programs The point of this requirement is first to show that you have thought about the design of your program and what it is intended to do secondly to show that you understand what your procedures do and how they contribute to the program s overall structure and thirdly to give you an opportunity to mention any aspects of your work that you are particularly pleased with or any difficulties that you have spent time on and that you would like the markers to take
156. s are significantly different in behaviour from value parameters in that instead of specifying a value to be given to the corresponding formal parameter within the procedure they specify a variable for which the formal parameter is to act as an alias within the procedure Thus the actual parameter to the procedure must be a variable and it is in effect the address of this variable that gets passed into the procedure rather than its value It follows that handling reference parameters requires a completely different set of PCode instructions from those that apply to value parameters The two most basic are Informal description of Assembler Decimal Hex command behaviour mnemonic code code load reference parameter LDVR 13 store reference parameter STVR 23 62 which within the procedure do more or less the same job for reference parameters that LDVV and STVV do for value parameters that is they provide access to the relevant formal parameter within the procedure s code lines enabling that parameter to be loaded onto the Program Stack or saved from the Program Stack much like any normal variable Obviously the method by which LDVR and STVR operate however is very different because in order to identify which address on the Heap should be loaded or stored respectively a process of indirection needs to take place whereby the actual parameter s storage address is looked up in a register w
157. s beyond it they can try running it instead on the larger Canvas the other virtue of the large Canvas is that it s centred around the origin and this can be nice if they re doing clever symmetrical patterns Plan for Second Lecture on Turtle a b c d e f Start by making sure that everyone has the module handout and repeating the most important administrative and don t worry stuff Again run quickly through the material covered in Lecture 1 adjusting your pace to take account of how many people said they didn t have the handout i e had missed Lecture 1 If very few missed then this can be significantly quicker than if lots did But the repetition will be useful to build up confidence for everyone so it s not wasted time Work through Exercises 1 and 2 quite briskly Work through Exercise 3 more slowly showing them how to do the faces with a FOR loop but probably leave the repeat loop for those who are keen to try for themselves Make reference to the two relevant illustrative programs and point out how they work Introduce variables as labelled boxes and point out that although all the variables we ll be dealing with in Turtle are integers declaration of variable types is essential once they get on to Delphi Even in Turtle booleans are available for keenies if they want to explore that for themselves Draw attention to the importance of indenting and its relation to program
158. same program Not only is there no correct diagnosis of the error the first error message given refers to a missing semicolon rather than a missing brace but also the messages run to well over 100 lines with no fewer than 38 errors identified in what is a very short program This particular type of problem would not have been quite so serious in a system based on the Delphi compiler which generally gives very helpful error messages but even if it had been possible to overcome the accompanying complexity of the Delphi context cf 1 3 above the same fundamental issue would remain that messages appropriate to the expert programmer and therefore typically generated by any industrial strength standalone compiler will often be unsuitable or unhelpful for the novice It was clear therefore that the envisaged Turtle Graphics Programming System would have to provide its own syntactic error handling Turtlebox system for C though given the characteristics of that language and its compilers this seems less likely to be suitable for novice programmers 11 The very first version of the Turtle system which was piloted in a self standing three week option course that made no claim to lead on to higher things did not incorporate a compiler but made do instead with a relatively superficial interpreter This proved sufficiently successful e g in respect of student results and feedback to give confidence in the ove
159. scursive manner than might be expected in a typical thesis because their role is not only to explain the system s design but also crucially to illustrate how that design can be put across in a way that would be relatively accessible to a non expert in Computer Science These aspects of the Turtle system have not yet been used in practice to teach the concepts of machine code or compilation but the system is intended to provide a basis for doing so In the absence of such road testing the best way of demonstrating the system s potential for the task is to manifest by example how it can indeed serve as a vehicle for explanation at roughly the appropriate level 4 14 However if used for teaching the system would be supplemented by additional materials such as definitions of the relevant abstract machines and data structures which are clearly not necessary here 14 Chapter 8 concludes the thesis starting by drawing together the threads of the previous chapters to reflect on the system s value in the learning and teaching of Computer Science complementing the exclusive emphasis on novice programming which was the theme of Chapter 4 s conclusion It ends by summing up the project s achievements with reference to 1 7 below Appendix A contains the lecture plans and coursework used when teaching the Turtle system in Leeds University Appendix B collects together some ideas for future development and Appendix C contains the en
160. sed since 2000 to introduce programming within formal taught modules for elective students and since 2001 as the basis for the Programming Concepts core of a full 10 credit module which then led on to Delphi programming So it is appropriate here to review this experience with the system before going on to a wider discussion of its place within the educational curriculum 4 1 Brief Outline of the Course The Programming Concepts component consists of four compulsory lectures followed by a final optional lecture which gives a tour of the Visual Compiler Full lecture plans for the compulsory lectures together with an example coursework are provided in Appendix A The lectures are built almost entirely around the exercises provided in the Turtle Help file and in that sense resemble example classes rather than typical formal lectures 1 The first lecture introduces the Turtle system and the idea of an algorithm running through simple drawing commands and using the opportunities afforded by the first two exercises to familiarise students with relevant illustrative programs help resources and other Turtle facilities that they might find useful 2 The second lecture builds on the first quickly reviewing what it covered and then moving on to introduce the idea of a variable and of a counting loop combining these with a discussion of program layout systematic program development and debugging techniques e g using pauses and
161. shdown automaton used Pascal to parse the cf 7 3 5 program together with the corresponding indentation calculations for the Edit menv s auto indent facility An expressions analysis tallying the various types of statement that have been used grouped together by category and referenced by line number this is particularly useful for marking student assignments that may require them to demonstrate competence with an appropriate range of commands as with the coursework in Appendix A A list of declarations showing both Line Lexeme String Type Entry State ExitState Indent a 1 1 PROGRAM program Start 1 1 2 triangles identifier 1 3 7 ProgSemi 3 4 PROCEDURE procedure ProgSemi 2 3 5 triangle identifier ProcName 3 6 ProcName ProcBkt 3 7 size identifier ProcBit ParName 3 8 ParName 3 9 integer identifier ParType 3 10 ParType 3 11 ProcSemi 4 12 BEGIN begin ProcSemi PROC 2 5 13 if if PROC 3 5 14 size identifier 5 15 gt gt 5 16 2 integer 5 17 then then PROCT 6 18 begin begin PROCT PROC TB 4 7 identifier PROC TB 5 19 forward Figure 4 Syntax Analysis Display Expression Count Program Lines FORWARD 0 BACK 0 MOVEXY 6 12 14 15 16 26 41 DRAWXY 0 Relative Movement 6 LEFT 0 RIGHT 0 Direction 0 HOME 0 SETX 0 SETY 0 SETXY 1 39 Absolute Movement 1 CIRCLE 0 RLOT 2 7534 Variables including global turtle properties TURTX TURTY TURTD
162. sponding to your own username as explained in detail above e ONE plain text file called PROGDESC TXT describing your main program as explained in detail above e Ifyou wish the programs that you have done for the self teach exercises in the Turtle Graphics Help file Note a that to be taken into account these exercise programs must be named correctly e g TGPX10 TGP and b that they will not usually count towards your mark unless they are needed to provide a safety net e g where your main submitted program goes seriously wrong in some way in which case evidence of having done the exercises competently could enable you to pass nevertheless Advice on Approaching Your Work You should plan what you are going to do in Part II of the coursework before you create your program Get plenty of scrap paper to hand and think about what kind of design or effect you want to produce Think about what you want your procedures to do what global and local variables you will need and so on As you begin to write your code annotate it with comments so that you can keep track of where you are up to You will find the coursework much easier if you have a clear idea of what you are trying to do in your program This will also make it easier for you to write up your text file You should also bear in mind that when your coursework is marked the sections of your code will be examined in the context of how they contribute to the overall workings of the
163. standard learning resources etc Outline The coursework for this component falls into two parts Part I Choose two from the following six features of Turtle Graphics Pascal a b c d e f POLYLINE or POLYGON FOR DO loops REPEAT UNTIL loops PROCEDURES with parameters non recursive Recursion Colour codes and colour handling Then illustrate each of your two chosen features with a short example program preferably of less than 20 lines each and aiming to satisfy the criteria given below e g correctly named clarity interest relevance Note that these short example programs should be quite distinct from your main Part II program you will not be given credit for merely extracting parts of that program and submitting them for Part I 114 Part II Construct a reasonably sized program at least 50 100 lines altogether which creates an interesting design of your choice using every one of the types of command and structure mentioned in the Exercises page of the Turtle Graphics Help file details of exactly what this requirement amounts to are given below together with the criteria by which your program as a whole will be judged Your design might be abstract or it could be a picture And it might incorporate movement but it need not however it should have a reasonable degree of complexity produced in a clearly comprehensible way by making use of loops and procedural structures T
164. t Hawthorne Science 183 pp 922 32 39 99 Pattis Richard E 1981 Karel the Robot A Gentle Introduction to the Art of Programming Wiley 4 18 23 100 Pea R D 1987 LOGO programming and problem solving in Eileen Scanlon and Tim O Shea eds Educational Computing Wiley pp 155 60 3 101 Pemberton Steven and Daniels Martin 1982 Pascal Implementation The P4 Compiler and Interpreter Ellis Horwood 12 102 Perkins D N C Hancock R Hobbs F Martin and R Simmons 1989 Conditions of learning in novice programmers in Soloway and Spohrer 126 pp 261 79 41 103 Piaget Jean The Origin of Intelligence in the Child 1936 published in English translation by Routledge amp Kegan Paul 1953 3 104 Piaget Jean The Child s Construction of Reality 1937 published in English translation by Routledge amp Kegan Paul 1955 3 105 Poulton John 2003 Managing diversity in introductory programming classes Using Logo as a diagnostic tool 3 LTSN ICS one day conference on the Teaching of Programming 43 106 Proulx Viera K 1997 Recursion and grammars for CS2 ACM SIGCSE Bulletin 29 3 pp 74 6 119 107 Putnam R T D Sleeman J A Baxter and L K Kuspa 1989 A summary of misconceptions of high school Basic programmers in Soloway and Spohrer 126 pp 301 14 41 107 108 Resnick Mitchel 1994 Turtles Termites and Traf
165. tNot then as the syntax diagram indicates this should be followed by a factor Hence in this case the function calls itself from the following lexeme with result factor start 1 and when this returns generates the single PCode instruction pcNeg or pcNot corresponding to the operator lexeme The point here is that the recursive call if successful will not only parse the 61 Note that the code dealing with tMinus disambiguates this to tNeg unary negation because 94 factor starting at lexeme start but will also generate PCode designed to leave that factor s result on the Program Stack as in the previous paragraph Hence that result can be negated as implied in the source code by applying an appropriate PCode negation operator i e pcNot or pcNeg that acts on the top Program Stack value Finally if the lexeme with which the factor starts is a left bracket the system expects this to be followed by an expression and then a right bracket So the function expression start 1 is called in order to identify and generate PCode for the expression in question If this fails 0 will be returned and passed on also as the return value of the factor function 0 being generally used to signify a syntactic error But if parsing of the expression succeeds the value returned by expression start 1 will be the index of the next lexeme after the end of the identified expression accordingly a check is then made to ensure that this le
166. tack FALSE is represented as 0 and TRUE as 1 though any non zero value will be treated as TRUE in conditional processing Turtle also explicitly recognises the boolean type e g in procedure or variable declarations as shown in the flashlights illustrative program which can be helpful for explaining and motivating type declarations in a system where otherwise these might seem to be pointless No other data types are recognised because integers and Booleans suffice for the fundamental concepts necessary to the system s practical teaching role as a preparation for Delphi though there is something to be said for introducing arrays in particular not only to provide more conceptual range and power for users but also to develop the possibility that the Visual Compiler provides for the practical illustration of complexity considerations e g by enabling machine code cycles to be counted within sorting algorithms Another natural suggestion would be to move in the direction of object 21 1 is a more appropriate choice than 1 because the four byte twos complement hexadecimal representation of 1 is FFFFFFFF which equals the bitwise NOT 0 cf 3 2 below 22 Turtle could do with further development here since there is currently no type compatibility checking between Booleans and integers and hence little benefit to be gained from declaring variables or procedure parameters as boolean rathe
167. te how all this is implemented here is the code that compiles a factor enclosed within a function of the same name function factor start integer integer begin case lexitems start lextype of ltId ltInt ltHex begin addloadstack numcommands start result starttl end continued on next page 93 ltMinus begin lexitems start lextype ltNeg disambiguate result factor start l1 addpcode numcommands pcNeg end 1tNot begin result factor start l1 addpcode numcommands pcNot end ltLbkt begin result expression startt1 if result gt 0 then begin if lexitems result lextype l1tRbkt then inc result else result 0 end end else result 0 end end function factor When the function is called start specifies the index of the next lexeme to be processed i e the lexeme which is expected to be the beginning of a factor As the syntax diagram indicates a factor can be simply an identifier integer or hexadecimal integer in any of these cases the procedure addloadstack numcommands start is called to make appropriate checks e g that an identifier is a defined variable and if these checks are passed to generate the necessary PCode for loading the relevant item onto the Program Stack Then start is returned as the value of the factor function to enable syntactic analysis to continue from the next lexeme If the next lexeme is of lexical type tMinus or I
168. the Program Stack for transfer into the procedure body Three instructions are needed because a procedure call can occur not only from the main program but also from another procedure one of whose own variables or parameters might therefore be aliased by the reference parameter within the called procedure Hence the actual input parameter could be any one of a global variable LDAG gt a local variable or value parameter of the calling procedure LDAV or a reference parameter of the calling procedure LDAR Whichever of these is involved 45 1 DAG is in fact equivalent to LDIN since the Heap address of the n global variable is just n But for the sake of clarity the two uses are differentiated by using separate instructions 64 however the same instruction STAR is used within the procedure code to store the actual parameter s Heap address in the procedure s corresponding formal parameter indirection register 6 5 The Procedure Register Stack We saw earlier in 2 3 that the Turtle system incorporates a trace facility which displays each PCode command as it is executed together with relevant contextual information including the numeric index of whichever procedure if any is running at any particular time and how many procedures are active However the instructions introduced so far are not designed to keep track of this information and it cannot be deduc
169. ther special provision is needed to facilitate such things as multi colour counting loops used to cycle colours in the cyclecolours illustrative program For this purpose the numbers 1 to 8 which following the BGR convention would all appear more or less jet black with no blue or green and only a minimal intensity of red are specially distinguished as not following that convention instead they represent the eight standard colours in the order listed above this order being derived from the traditional Turtle Graphics ordering It is these special codes that are randomly chosen by the randcol command and this also has the virtue of enabling randcol to be used as a simple random number generator as shown in two of the illustrative programs Cycling colours and Using Booleans Having now examined the Pascal language structures that are supported by the Turtle system we shall see in Chapter 4 how it has performed as a vehicle for introducing this limited language within elective modules at the University of Leeds where it has served as a prelude to Delphi Pascal 31 Chapter 4 Using Turtle to Teach Introductory Programming The Turtle system is designed primarily as a learning rather than a teaching system in that it aims to encourage students to experiment and thus discover for themselves the creative pleasure of programming in particular by working through the self teaching exercises But it has been u
170. this point in the pushdown automaton it is possible for several pending operations to be dealt with in turn reflecting the fact that where conditional and counting loop structures are nested a number of them can terminate together As the program is parsed the in Type Entry State _ PDA stack is shown in the i to s identifier i do do PROC F Syntax table of the Visual if if PROC F 4 ok identifier Compiler following PROC then then PROC FT right identifier PROC FT 5 cc or PROG depending on the ic r f FSM state This table also else else PROC FE 4 home identifier PROC FE PROC 5 shows the calculated indents PROC PROC movexy identifier PROC 3 which are to be used if the program is auto formatted Figure 18 PDA stack and indents in the Syntax table 60 Though yet again since the extent of the else part is not yet known this JUMP instruction is given a dummy argument which will in due course be backpatched as in a above Note that this treatment of conditionals provides a resolution of the notorious dangling else problem which besets grammar based parsers since any else will automatically be paired with the nearest previous eligible if in accordance with both standard and Delphi Pascal e g if lt a gt then if lt b gt then lt c gt else lt d gt will be disambiguated as if lt a gt then begin if lt b gt then lt c gt else lt d gt end
171. tics and Society and that only in 2002 03 Although the students written feedback must be viewed with considerable caution because of its unsystematic and unrepresentative nature it gratifyingly conforms very 35 well with the positive impression given by the overall statistics above with many comments expressing positive enjoyment of the learning experience I found this by far the most enjoyable thing I have done in ACOM so far I found this to be a very tough yet very enjoyable module With practice all of the concepts become clear and the results are very satisfying I thought Peter Millican s teaching was excellent The online Turtle Graphics help was very helpful Handouts weren t really necessary as all of the necessary information was contained within the program itself I found myself quite keen on doing the exercises and coursework but this may have been because it gave me a break from my other homework A good programming starter program I found this core both stimulating and enjoyable Peter Millican s approach to teaching programming makes the subject less mind boggling Very well structured and well taught Peter made it very easy to understand the concepts of the programming language being taught A superb introduction to computer programming I d recommend it to anyone from the very young to the very old Peter Millican s performance of The Cat in the Hat to elucidate recursion just has to be s
172. tire contents of the system s online Help file with independent page numbering and cross references to those page numbers replacing the online hyperlinks to facilitate use in printed form The Turtle system itself which includes both the software and the online Help file is available from www leeds ac uk jcom turtle 15 1 7 Achievements and Novel Contributions My initial aim in this project was to produce a tool for a job namely to enhance the teaching of introductory programming and I had no particular focus on creating something novel for its own sake However both my studies at the time and subsequent reviews of the literature have confirmed that the Turtle system does indeed represent a genuine innovation not so much in the technologies that it builds on but rather in the way that these technologies are combined and presented The main achievements and novel contributions represented by this work are I believe as follows Al A2 A3 A4 To provide a strong case for the development of an integrated programming environment for novices and non Computing students in particular combining standard programming structures simple graphics primitives comprehensive help facilities and a standalone user friendly compiler Case presented in 1 1 1 4 supplemented in 4 4 To select a suitable subset of the Pascal language combining overall simplicity with considerable graphical and algor
173. to make case unnecessary More delicate is the choice of conditional iterative structure between Pascal s repeat until and while both are similarly expressive in combination with if then else so only one of them is required within a system whose primary aim is simplicity In some respects while is preferable since in technical programming situations initial non satisfaction of a loop condition typically implies total non performance of the loop However in Turtle Graphics null loops are relatively rare and personal experience indicates that beginners tend to find the idea of a loop that is always performed at least once rather more intuitive perhaps because it is easier to think of repeating an operation already concretely performed than to think of a has sufficed to use the mere possibility of Booleans to illustrate how variable types can differ and hence to explain why more advanced systems such as Delphi require type declarations 27 loop that is entirely conditional see Soloway et al 125 Rogalski and Samur ay 112 There is also a more particular confusion about the Pascal while statement deriving from its natural language meaning leading some novices to expect that the loop condition will be tested continuously rather than once per iteration Sleeman et al 121 Bonar and Soloway 13 Finally when sequences of operations are involved as is usually the case when dr
174. to the Clipboard or to a file using the File menu Finally ask the students to complete Exercises 5 to 8 before the next lecture and invite them to have a go at Exercises 9 to 12 Plan for Fourth Lecture on Turtle a b c d e f Start by reminding students briefly about what was covered last time making reference to the relevant illustrative programs and especially Help sections Run the REPEAT loop and Combining structures illustrative programs to show the kind of thing that s going to be covered in this lecture Work through Exercises 9 to 11 following the order of presentation given in the Help file which includes reference to the Programming Essentials section for the syntax and meaning of if statements Draw attention to the commands UPDATE and NOUPDATE which are mentioned in connection with Exercise 8 but are particularly important when simulating smooth motion Having shown how to produce smooth motion of a single object mention and demonstrate but do not discuss the illustrative programs on Reference VAR parameters and Multiple bouncing balls which ambitious students might wish to follow up themselves with the subsection on Value and Reference Parameters from the Help section on Procedures and Parameters Briefly draw attention to the material covered in the remaining illustrative programs and how this can be followed up usi
175. ton L D and Oberski I 1998 Learning and conceptions of understanding in History and Science lecturers and new graduates compared Studies in Higher Education 23 pp 43 58 98 91 Nulty Duncan D and Barrett Mary A 1996 Transitions in students learning styles Studies in Higher Education 21 pp 333 45 6 92 Olsen K A 1988 The DSP system a visual system to support teaching of programming in Proceedings of IEEE Workshop on Visual Languages Pittsburgh pp 199 206 117 93 Olson G M S Sheppard and E Soloway eds Empirical Studies of Programmers Second Workshop Norwood New Jersey Ablex contains 43 46 72 94 Palumbo D B 1990 Programming language problem solving research a review of relevant issues Review of Educational Research 60 pp 65 89 3 95 Pane John F and Myers Brad A 1996 Usability issues in the design of novice programming systems Technical Report CMU CS 96 132 Carnegie Mellon University 5 43 98 96 Pane John F Brad A Myers and Leah B Miller 2002 Using HCI techniques to design a more usable programming system Proceedings of the IEEE 2002 Symposia on Human Centric Computing Languages and Environments Arlington Virginia pp 198 206 41 118 97 Papert Seymour 1993 Mindstorms second edition Basic Books first edition published in 1980 3 18 28 98 Parsons H M 1974 What happened a
176. uch promising results 28 Indeed if anything this makes the comparison more favourable since the earlier course was an integrated module making early dropouts far more likely than in the Turtle component where students were not anyway committed to continuing with programming beyond the first four weeks 29 A sort of Hawthorne Effect Mayo 81 ch 3 might be suspected here whereby the mere act of studying students performance can serve to improve it but in fact this supposed phenomenon is considered rather dubious in the light of later studies e g Parsons 98 Adair 2 and besides the Turtle component with its self learning materials put very little emphasis on student monitoring 40 4 4 Conclusion The Value of Turtle as a Vehicle for Introducing Programming Clearly the Turtle system s ambitions in respect of introducing novices to programming are very limited it aims to provide a good basis for the first month or so of learning and makes no attempt to convey software engineering principles of any sophistication Turtle s value must accordingly be measured against this limited ambition and by that standard the evidence given above strongly suggests that it is a success Not only do novice students cope with it and many choose to go on to more serious programming in Delphi generally with success but they are excited and enthused at discovering for themselves the pleasure of intellectual creat
177. udents have passed with an overall average around 56 60 and feedback has been very positive mostly 75 85 41 evidence of how badly many students fare when taught programming in a traditional manner Thus McCracken et al 83 highlight concerns expressed by many computer science educators about their students lack of programming skills often focused on basic mastery of fundamental skills and give supporting references together with research indicating that such concerns are fully warranted In the abstracts to two recent papers Chalk et al 28 and Jenkins 61 both give considerable prominence to a depressing assessment of the current situation There is a national crisis in the teaching and learning of programming and The graduating student who professes a complete inability to write the simplest program is commonplace Likewise according to Jenkins and Davy 64 many students appear to be totally unable to grasp the basic concepts leading them later in their studies to insist that they want to avoid programming at all costs 3 But what is particularly striking about all this is the level of expertise at which the students concerned are faltering as shown by a wide variety of studies 2 Variable assignment and initialisation are often misunderstood du Boulay 41 Samurgay 114 while particularly common bugs are those associated with loops du Boulay 41 Spohrer et al 128 and conditi
178. um pp 36 7 39 Moreover the authors stress that this was not at the cost of reducing the course to a shallow overview The key factor is to organize the presentation well introducing as much of it as is necessary to give students a firm understanding of its principles Avoid introducing a topic in a survey fashion p 54 Turtle is clearly a potential vehicle for such a presentation 64 Another interesting example is the PIPPIN machine described by Decker and Hirschfield 33 which is explicitly designed for use with novice students though its associated compiler Rosetta is limited to the compilation of an assignment statement involving simple expression evaluation 98 8 1 2 Deep Understanding Leaving aside the possible value of the Turtle Machine to novices it has more obvious potential in helping relatively advanced students to knit together the various aspects of the discipline which according to many writers is the key to robust deep understanding e g Marton and S lj 78 Gibbs 44 And again this seems to be closely related to having an adequate notional machine for it is striking how both lecturers and students identify deep understanding with having a mental picture of what is going on Newton et al 90 pp 48 51 cf Robins et al 111 pp 140 151 Such a mental picture is far easier to acquire within a system whose workings are based on a systematic and intuitive metaphor cf
179. und the language LOGO because Turtle Graphics systems focused on the learning of other languages such as C Delphi Java or Visual Basic are usually provided as mere units or classes within a standard programming environment The closest approximation I have found to the sort of system envisaged in this project which became available recently albeit only in a beta version that lacks a user manual and most of the planned lessons is Otherwise Software s Jurtle for Java Though a commercial system this is evidently targeted for the educational market being user friendly reasonably priced and also provided in an unlicenced version that allows temporary trial of the product 10 See for example the LOGO Foundation website at http el media mit edu logo foundation logo 11 A Web search can very quickly find for example the GrWin Graphics Library for Fortran and C C the turtle classes in Donovan 40 for C and the many Turtle Graphics Windows components on various Delphi and Visual Basic freeware and shareware sites Turtle Graphics is now enjoying something of a resurgence in the teaching of introductory Java as for example in Martin 77 Caspersen and Christensen 26 Schaub 116 Slack 120 and Ariga and Tsuiki 7 This emphasis on early graphics is presumably fostered by Java s association with Web applets but unfortunately its standard stateless redraw every time mechanism imposes
180. ut Robins et al point out that this can be overcome when they develop an adequate mental model Many studies have noted the central role played by a model of an abstraction of the computer often called a notional machine to provide a foundation for understanding the behaviour of running programs 111 p 149 cf p 158 Du Boulay 41 accordingly identifies the absence of an adequate notional 63 Studies listed by Robins et al in this connection include du Boulay 41 du Boulay et al 42 Mayer 79 Hoc and Nguyen Xuan 58 Mendelsohn et al 84 and Cafias et al 24 The paper by Ben Ari 12 is also particularly interesting because he links the philosophy of constructivism cf 1 2 above with the need to teach a model based on the unfamiliarity of algorithmic mechanisms within the student s pre existing conceptual framework You have to construct a viable model that will enable you to predict the outcome of any operation on the model The relevance for CSE is that courses must explicitly address the construction of a model If the student does not bring a preconceived model to class then we must ensure that a viable hierarchy of models is constructed and refined as learning progresses p 260 97 machine as one of the five main sources of difficulty in programming pp 283 4 But if so then the two teaching aspects of the Turtle system are quite intimately connected after all For one
181. ven further arguing on this sort of basis that programming is now critical to FlTness section 3 1 though this conclusion is of course controversial e g Urban Lurain and Weinshank 132 133 e For students of many academic disciplines there is value in acquiring an understanding of algorithmic thinking quite independent of its potential practical usefulness to them Students of Philosophy History and Philosophy of Science or Cultural Studies for example can thus gain an appreciation of a pervasive modern mode of thinking that now profoundly influences our culture in radically novel ways cf Dijkstra 39 This influence is also significant in a vast range of disciplines where algorithmic models are widely used from social sciences such as Economics Political Theory and Psychology to hard sciences such as Biology and even fundamental Physics e In some disciplines algorithmic models are sufficiently accessible to provide ways of learning about the domain by active development and experimentation Shafto 117 Harel and Papert 52 Lippert 76 cf Guzdial 49 A simple Turtle Graphics example would be to give a Physics student the task of writing a program to simulate the flight of cannonballs under gravity So there are strong reasons for wishing to make basic programming concepts available to non specialist students The main aim of the software described in this thesis was to provide a way of doing so which
182. w proceed to explore the system s use as a vehicle for the learning and teaching of introductory programming focusing first in Chapter 3 on its Turtle Graphics Pascal source language and then moving on in Chapter 4 to see how it has performed in practice within taught modules at the University of Leeds 17 In Figure 6 like Figure 5 the ballsteps illustrative program is used since this gives a more interesting display than the triangles program used in Figure 4 23 Chapter 3 The Turtle Graphics Pascal Source Language The idea of starting students on a simplified subset of a programming language is very familiar from the educational literature 8 Sometimes the emphasis here is on self contained mini languages such as that used by Karel the Robot 99 see also the discussion by Brusilovsky et al 19 and sometimes on progressive introduction of more structures within a standard language extending the relevant subset as the students develop e g Brusilovsky et al 20 DePasquale 36 However given Turtle s first intended role as an introduction to programming concepts for total beginners designed to lead on to the far more sophisticated Delphi environment after only four lectures it was clearly sensible to choose a subset of Pascal rather than any specially invented language and to fix that subset with initial learning in mind rather than to attempt to cater for the students later development Far
183. xample above 3 the calculated result of each operation must then replace the used values on top of the Stack Working out the specific behaviour of each particular instruction from 36 This is a virtue rather than a drawback since it means that the Turtle system provides a useful vehicle for introducing and illustrating the tremendous practical value of this vital data structure 37 All this applies equally to the relevant Turtle Graphics machine code commands from the previous table as illustrated by MVXY in the italicised example 47 this general pattern is then very simple they are merely listed here but see the PCode Reference Guide in the online Help file for details if desired Turtle Assembler Decimal Hex Turtle Assembler Decimal Hex operator mnemonic code code operator mnemonic code code LESS 210 p2 5 2 Command Parameters Global Variables and the Heap An issue not covered in the previous section is how the various command parameters or numbers to be operated upon are to be loaded or pushed onto the Program Stack in the first place Where they are straightforward integers rather than variables this is accomplished using the LDIN machine instruction Informal description of Assembler Decimal Hex command behaviour mnemonic code code load integer onto Stack LDIN 10 So we can now derive the appropriate compilation of the various simple statements i
184. xeme is indeed a right bracket in which case inc result increments the function s return value to point to the lexeme after that bracket Note that brackets in themselves unlike identifiers integers negation operators etc do not yield any specific PCode instructions They simply ensure that the PCode generated by the expressions that they enclose corresponds appropriately to the sequence of processing indicated by the bracketing hierarchy 7 6 1 Code Generation for Commands and Procedure Calls The processing of individual Turtle Graphics commands assignments and procedure calls comes into play whenever the pushdown automaton illustrated on page 87 encounters an identifier at the beginning of its main loop Assuming the program is syntactically correct there are then three possibilities which are checked in turn the other possible interpretation of tMinus namely tSubt for binary subtraction cannot occur as the first lexeme of a factor See 7 3 for a brief discussion of this ambiguity 62 The order of processing shown below allows for the possibility that a variable or procedure will be declared with a name which is also a Turtle Graphics instruction and if so the identifier will be interpreted accordingly when that name is used within the scope of the declaration i e Turtle Graphics instructions are not reserved words 95 e The identifier is a recognised and in scope variable name in which case it should be
185. y clearly what use is being made of the feature being illustrated and or how that feature assists in creating the visual effect Part II The main component of the coursework is to construct a relatively large program at least 50 100 lines altogether which creates an interesting design of your choice using every one of the types of command and structure mentioned in the Exercises page of the Turtle Graphics Help file this means that it should contain at least one instance from each of the following groups and you will be given credit for showing yourself able to use a variety of commands so it s a good idea to aim to use all of these commands and structures at least once but don t feel you have to do this if doing so wouldn t fit smoothly into your program 115 relative movement FORWARD BACK MOVEXY DRAWXY direction LEFT RIGHT absolute movement HOME SETX SETY SETXY shapes CIRCLE BLOT POLYLINE POLYGON pen settings COLOUR RANDCOL THICKNESS drawing control PENUP PENDOWN NOUPDATE UPDATE PAUSE procedures PROCEDURE with parameters for loops FOR TO FOR DOWNTO repeat loops REPEAT UNTIL conditionals IF THEN ELSE variable assignment You may also want to consider using the following BLANK CANVAS to control the appearance of the canvas REMEMBER FORGET to control the behaviour of POLYLINE and or POLYGON Arithmetical relational and boolean operators Your program i

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