JKarel Users Manual.
Contents
1. Copyright 2003 Heather Wellenius 21 6 1 Example using loop As an example of how a loop instruction becomes very useful the following method uses 2 loops one nested inside the other to allow a robot to walk around a square with sides that are 6 blocks long If a robot is placed at the origin 1 1 facing east and executes this method it will walk 6 blocks east to 1 7 and turn left then 6 blocks north to 7 7 and turn left then 6 blocks west to 7 1 and turn left and finally 6 blocks south to get back to 1 1 It will do a final turnLeft and end facing east void walkSquareOfLength 6 loop 4 loop 6 move burner ys Copyright 2003 Heather Wellenius 22 7 Conditional Instructions So far the JKarel programs we have written can only be applied to specific robot worlds Given different worlds executing a move pickBeeper or putBeeper instruction in the program would cause an error shutoff if the robot tried to walk into a wall or handle a beeper that wasn t there There are instructions in the JKarel language that allow a robot to look around and determine if a particular action can be performed given his surroundings By testing his surroundings a robot can determine if he is in a position to execute one of these instructions before actually executing it and avoid an error shutoff when the program is applied to different worlds Conditional instructions are used to test and are writte2. In a basic JKarel program a robot of type UrRobot is initialized and the instructions listed above are used to send a message to this robot The following format is used for the simplest JKarel programs that only include a main procedure class lt NewClassName gt main lt robotInitialization gt lt instruction gt lt instruction gt A new class name symbolized by lt NewClassName gt can be made up of any combination of letters digits and the underscore character _ but must begin with a letter and must not be one of the reserved words in JKarel A list of JKarel reserved words is provided at the end of this manual By convention class names begin with a capital letter The main procedure defines a task that a robot will do It begins with a robot initialization followed by any number of instructions When the main procedure ends the program ends Copyright 2003 Heather Wellenius 13 Robot initializations have a syntax that includes the name of the class or type of robot being created a name given to the robot and used to send messages to it the keyword new and information about the robot s initial street avenue direction and the number of beepers it is carrying The following initialization creates a robot of type UrRobot named jkarel and places him on 1 Street and 1 Avenue facing East and carrying 2 beepers UrRobot jkarel new UrRobot l1 1 East 2 Each activity that a robot can
3. lt methodName gt t lt instruction gt lt instruction gt j The rules for valid method names are the same as for valid class names letters digits and the underscore character are all valid characters to use but the name must begin with a letter by convention methods begin with a lowercase letter to differentiate them from classes Similar to class names method names must not be one of the JKarel reserved words Unlike instructions in the main procedure instructions in a method do not specify a robot name They simply provide the method name parentheses and semi colon as in the following move Copyright 2003 Heather Wellenius 15 The following example combines all of the elements of a JKarel program outlined so far in a program that creates a robot of type UrRobot and instructs it to sweep the beepers off of a flight of stairs 5 2 StairSweeper Example The following example illustrates the construction of a simple robot world and the program code that goes along with it The first step is to build the robot world Then a class 1s written for a new type of robot called a StairSweeper A main procedure is written that creates a StairSweeper robot and calls the StairSweeper methods to allow this robot to collect all of the beepers in the world Using the Vertical Wall Horizontal Wall and Beeper buttons on the toolbar build the robot world illustrated in Figure 4 It is composed of three stairs each topped
4. NA NAN End of Program Save this program file as StairSweeper jkl Note that the name of the class must be the same as the file name StairSweeper is the name of the class in this example so the file must be called StairSweeper jkl This class defines turning right as making three left turns since robots only know how to turn left It then defines climbing a stair as a sequence of four actions turning left to face north moving to get one block north turning right to face east and moving again to get one block east Note that it is necessary to define the method turnRight before using it in the method climbStair since using a class or a method before it has been defined is not Copyright 2003 Heather Wellenius 17 allowed Trace this sequence of actions for climbing a stair in the world you created to see that it works The main procedure creates a robot of type StairSweeper and then calls the methods climbStair andpickBeeper alternately to move into position and pick up the beeper on each stair Initially the robot is placed at the coordinates 1 1 facing east It is carrying no beepers at the beginning but by the end of the program it will be carrying 3 beepers This change in the robot s position direction and beeper count can be monitored in the Robot Log during program execution After saving this program file select compile m from the toolbar A message should appear to let you know that the
5. current time along with their position direction and the number of beepers they are carrying The final display in the Robot Log after execution of the StairSweeper program is the following alex Street 4 Avenue 4 Facing East Beepers 3 Copyright 2003 Heather Wellenius 19 This identifies the robot alex as the only robot in the world at this time and places him at the coordinates 4 4 facing East and carrying 3 beepers Try using the step button to step through the program again Press M1 to see the execution of each statement Copyright 2003 Heather Wellenius 20 6 Repetition Instructions Repeating the same instruction or a sequence of instructions several times 1n a program can lead to very long program code Repetition instructions are helpful in specifying that a block of code a group of instructions set apart by curly braces 1s to be executed a certain number of times The keyword loop is used and the format for a Loop block is the following loop lt positiveNumber gt lt instruction gt annon j The number of times to repeat the instructions within this block of code is specified within the parentheses indicated by lt positiveNumber gt The turnRight method from the StairSweeper example provides an example of when the 1oop instruction could be used Instead of listing the same instruction three times a loop is used to indicate this repetition vord turnRignt TOO ts Lurnbere
6. i j lex climbStair Executing on alex move i a Execution Executing on alex turnRighti alex pickBeeper Robot and Executing on alex turnLeft alex turn0ff Executing on alex turnLeft i Error Logs Executing on alex turnLeft i Executing on alex movef Current Line 17 Figure 2 JKarel Application Window There are two parts to a JKarel program the JKarel program code and a graphical robot world in which to run the program code Once a world is constructed and the code is written then the program may be compiled and run The following sections describe how robot worlds Copyright 2003 Heather Wellenius 5 are built how JKarel programs are compiled and executed and how information that is displayed in each of the logs can be used to debug program code Copyright 2003 Heather Wellenius 6 4 1 Building a Robot World Initially a robot world has only streets and avenues no walls or beepers It has a western barrier to the left of 1 Avenue and a southern barrier below 1 Street It is possible to execute a JKarel program in this empty world and in such a world a robot would be able to travel almost anywhere since there are no obstacles for it to run into except the western and southern barriers However since a world without walls and beepers does not provide a basis for many challenging JKarel programs most programs rely on the construction of a more complex robot world
7. with walls and beepers This allows for the creation of interesting tasks for a robot to undertake The tools for drawing walls and beepers are located on the toolbar above the graphical world b 2 Clicking on one of the three toolbar objects representing a vertical wall a horizontal wall and a beeper and then clicking anywhere in the grid draws the selected object in the robot world at the coordinates clicked Clicking multiple times on an intersection with the beeper tool allows multiple beepers to be drawn on that corner The number of beepers at an intersection is denoted by the number drawn in the middle of the beeper To erase an object that has been drawn in the world select the eraser 2 as well as the object type to be erased W oA In the picture above the button for drawing a vertical wall is selected as well as the eraser In this configuration clicking on any vertical wall that has been drawn in the world will now erase that wall When a robot world has been completed select Save World As under File in the menu World files should be saved with a wld extension A saved world file can be re opened edited and saved again any time changes are made to it Copyright 2003 Heather Wellenius 7 4 2 Writing a Program JKarel program code is written in the editor pane on the right side of the application window All program files must be saved before they can be compiled and executed and must hav
8. 2003 Heather Wellenius 26 7 4 if else Instruction Sometimes certain actions should be performed only if a certain condition is true and other actions should be performed only if that same condition is false The if e1se instruction allows this distinction to be made Be Errore LeGLear move turnLeft In the preceding example a robot needs to either move forward if there is no wall blocking his path or turn left to avoid crashing into a wall The if instruction allows a set of instructions to be executed if a condition is true and else allows a different set of instructions to be executed if the condition 1s false Copyright 2003 Heather Wellenius 2 8 Another Repetition Instruction The repetition instruction loop allows a block of code to execute a specific number of times As we saw in the previous section it is sometimes preferable to use tests to determine whether actions should execute Similarly it is often preferable to use a test to determine how many times a block of code will be executed The while instruction allows repetition of a block of code as long as a test remains true It has the format while lt test gt t lt instruction gt lt instruction gt The test in a while instruction is just like a test in an if instruction However the test is re evaluated after every repetition of the instructions and these instructions continue to be executed until the test returns fals
9. 3 tab of the log panes By default only the first error encountered in a program is displayed since subsequent errors are often related to the first error and it is usually easier to fix one error at a time This option can be changed so that all errors are displayed by selecting View All Errors under View in the menu bar Figure 3 illustrates a compiled program that resulted in two syntax errors The option to View All Errors is selected so all of these errors are listed in the Error Log at the bottom left corner of the JKarel application window and can be associated with specific lines of code Copyright 2003 Heather Wellenius 9 displayed in the editor The current line number indicating the cursor s location in the editor pane is displayed below the editor pane Once all errors have been corrected the program will compile successfully and will be ready for execution Copyright 2003 Heather Wellenius 10 4 4 Executing a Program A JKarel program file that has successfully compiled is ready for execution If the program relies on a particular robot world configuration in order to execute successfully ensure that this world has been created or opened before beginning execution of the program There are two ways to execute a program playing straight through from beginning to end or stepping through one statement at a time To execute the entire program from start to finish select p Play To step through the program cod
10. JKarel A Stepping Stone to Object Oriented Programming in Java User s Manual 2003 Heather Wellenius 1 Installing JKarel JKarel depends upon the Java 2 Runtime Environment Standard Edition 1 3 1 or later Mac OS X comes with this installed but on Windows machines you can test whether this is installed by opening a Command Prompt window and typing java version If you don t have version 1 3 1 or later a current J2SE can be downloaded from http java sun com j2se downloads html Unzip JKarel zip into a directory of your choice The directory JKarel will be created containing all files that are needed to run the JKarel application Copyright 2003 Heather Wellenius 2 Running JKarel JKarel has been tested to run on Microsoft Windows 98 2000 XP and Mac OS X It may also run on other operating systems that have a Java 2 Runtime Environment see nstalling JKarel but these are currently untested and the user must assume all responsibility for problems that occur To run JKarel simply double click JKarel jar located under the JKarel directory Alternatively JKarel can be run from a Command Prompt window Change to the JKarel directory and type java jar JKarel jar It is recommended that you read the next few pages of this User s Manual once you have started the JKarel application for an overview of the various elements of the JKarel programming environment Copyright 2003 Heather Wellenius 3 3 The JKarel Rob
11. compile has completed successfully If there are any errors found in the program file they will be displayed in the Error Log Look for the line numbers specified in the error message s and correct the mistake s in the code Save the program file again and re compile it When the program has been successfully compiled select the play B button Watch the execution in the world you built When execution finishes successfully the application should appear as in Figure 5 with the robot pictured on the top stair facing east eked Dy x File View Project Help Slow Se ala ES I j2j rs stair wid StairSweeper jki O DE e aS E a class StairSweeper extends UrRobot i void turnRight turnLefti turnLeft turnLeft Success x climbStair all 2 Execution ended successfully rnLefti mel E El a 1 rnRight Fr car vel main StairSweeper alex new StairSweeper 1 1 East 0 alex climbStair alex pickBeeper alex climbStair alex pickBeeper alex climbStair alex pickBeeper alex turn ff Executing o on alex Hitna Executing on alex turnLefti Executing on alex move Executing on alex turnRight Executing on alex turnLefti Executing on alex turnLefti Executing on alex turnLefti z Executing on alex move f i Executing on alex pickBeeper i f xecuting on alex turnO0fft Pee off now Figu
12. e The following example uses the predicate next ToABeeper to test if there is a beeper on the same corner as the robot As long as there is at least one beeper the robot picks it up and tests for another beeper This results in all beepers being picked up before the method ends Note that this example works even if there are no beepers on the corner void clearCornerOfBeepers while nextToABeeper pickBeeper Copyright 2003 Heather Wellenius 28 Reserved Words The following list of words contains keywords in the JKarel language none of which can be used as class names robot names or method names boolean class East else extends false if import loop main North return South true void West while References Bergin J Stehlik M Roberts J amp Pattis R 1997 Karel A gentle introduction to the art of object oriented programming New York John Wiley amp Sons Copyright 2003 Heather Wellenius 29
13. e select pi Step It is possible to step through a portion of a program and then play to the end or play from the beginning and then select the step button part of the way through in order to move slowly through the last few lines of the program The step button can also be thought of as a pause in execution At any point during play or step execution the stop button M may be selected to terminate execution of the program A slider is provided to allow playback of a program in a range of speeds It can be adjusted during execution to fast forward through a repetitive portion of code or slowly advance through a troublesome part rast eo Siow During execution there are two logs that display runtime information The Execution Log 1 tab in the log pane displays each statement as it is executed The Robot Log 2 tab in the log pane lists all robots that currently exist in the world and provides their current location direction and the number of beepers they are carrying These logs can be useful in debugging a program that has compiled successfully but is still not executing in the desired manner Program code is highlighted line by line in the editor as it executes in order to indicate the statement that is currently executing By following the highlighted code while stepping slowly through the program and by watching the execution in both the graphical world and the Execution Log errors can more easily be detected Copyr
14. e a jkl file extension Each program file provides a class definition that may have a main procedure to be executed and or a set of new robot instructions More about the JKarel programming language will be introduced starting in section 5 Karel Primitive Instructions Copyright 2003 Heather Wellenius 4 3 Compiling a Program Before a JKarel program can be executed it must be compiled Selecting the compile button a will compile the current version of the program that is visible in the editor pane JKarel program files may be compiled at any time as they are being written since compiling can help identify program errors Note that when you compile the program will automatically be re saved all x Iez m sow Oea stair wid StairSweeper jki lclass stairSweeper extends UrRobot J void turnRight turnLeft i turnLefti turnLefti i void climbStair turnLeft novel turnRighti novej 16 ff Y2 R J4 in Exec Robot Log Error Log Syntax Error in StairSweeper jkl Expected semi colon at line 7 alex climbStair Syntax Error in StairSweeper jkl Expected right parenthesis at line 13 alex pickBeeper StairSweeper alex new StairSweeper 1 1 East 0 alex climbStair alex pickBeeper alex climbStair alex pickBeeper alex turnO0ff j Figure 3 Compile Errors Errors that are found during compiling appear in the Error Log the
15. ent JKarel combines several tools within its programming environment to facilitate the development of JKarel programs As illustrated in Figure 2 it provides e aprogram editor for writing JKarel program code e a grid area for building new robot worlds and viewing the graphical execution of JKarel programs e log panes for viewing error messages and observing runtime information The toolbar above the graphical world provides buttons that allow you to place walls and beepers within the robot world Above the editor there are buttons for saving compiling executing stepping through and stopping the execution of a JKarel program iojxi File View Project Help tle elo a w E a gt m _stair wid _ StairSweeper jki a Aa ea Sa S NY a Graphical Py tf fof ff class StairSweeper extends UrRobot Za hiii T yyy i 5 f ZA TTT E E g Program AA turnLefti Editor iN Eee turnLeft A a FA diea TTT T T T W sia csar INF AH 5 turnLefti dee a ST Le E turnRight i ISi S ISE S O ee eee nover ai Execution Log 7 a are Execution Log CENS Error Lo 5 l StairSweeper alex new StairSweeper 1 1 East 0 cCACeCaCINg ON AICA CALINIC ECTY x 7 i f Executing alex move alex cliabS5talri Executing alex pickBeeper 5 alex pickDeEReE z Execut ig on alex climbStair alex solisb rair z kecuting on alex turnLefti alex pickBeeper
16. ere 3 Le Pacangsourh LUPE gt LUPNbeL TL 4 if facingWest turnLeft turnLeft turnLeft Copyright 2003 Heather Wellenius 25 7 3 Predicates New predicates can be written in any class that extends class Robot The format of a predicate is exactly the same as that of a regular method in JKarel except that the return value is boolean instead of void A very simple predicate might look like this boolean frontIisBlocked return frontiIisClear There are three parts to the instruction in this predicate The keyword return is used to send either a true or false value back to whoever called this predicate In order to determine whether true or false should be returned frontIsClear is evaluated The symbol means not and negates whatever value follows it Putting these pieces all together this predicate will return true if front is not clear and return false 1f front is clear The next predicate example illustrates the use of a conditional to determine what value will be returned If next ToABeeper evaluates to false the predicate should always return false In this case the keyword false may be used in the return instruction instead of a call to another predicate Similar usage of the keyword true 1s also allowed in return statements boolean next ToAbeeper AND TronelsBlocked if nextToABeeper return frontIsClear return false Copyright
17. ight 2003 Heather Wellenius 11 4 5 Printing a Program There are two ways you can print a JKarel program file from within the JKarel application and from any simple text editor PC users can use the print amp icon on the JKarel toolbar This will print the program in the editor pane to the default printer connected to your computer There are inconsistencies with this way of printing depending on the printer selecting more than one copy may print the copies continuously without page separation And some printers may need a manual form feed to complete the printing Mac users and those who have trouble printing from within the JKarel application should use the second method of printing Open a JKarel program or copy and paste a program from the JKarel editor pane into a text editor such as Notepad or Simple Text and print it from within this text editor Copyright 2003 Heather Wellenius 12 5 JKarel Primitive Instructions UrRobot 1s the most basic type of robot in the JKarel language It includes five primitive instructions for performing the actions that are available to robots class UrRODOL void move A robot moves forward one block Void turnLert A robot turns 90 degrees to the left void pickBeeper A robot picks up a beeper from an intersection to put in its beeper bag void putBeeper A robot puts a beeper onto an intersection from its beeper bag void eUrniOrt o IA A robor Turns iteelr OFT
18. n using the keyword i f if lt test gt t lt instruction gt Stidi j A test indicated above by lt test gt 1s a method that determines if something 1s true or false Methods used for testing called predicates can send back a boolean value true or false When the test within an if instruction returns true the instructions within the curly braces are executed However if the test returns false then the instructions within the curly braces are skipped and execution continues with the instruction immediately after the entire block of code that comprises the if instruction The next section describes a more advanced class of robot defined in JKarel that knows how to determine information about its surroundings using predicates Copyright 2003 Heather Wellenius 23 7 1 Robot class JKarel defines two classes of robots for use in programs and as the basis for new JKarel classes We have looked at class UrRobot which defines the most basic robot actions Class Robot which extends UrRobot defines another type of robot A Robot can do everything an UrRobot can do as well as provide information about its surroundings It can indicate 1f it is standing next to a beeper or next to a robot if it is facing in a particular direction or carrying any beepers and 1f the path in front of it is clear of a wall enabling it to move forward The methods in class Robot are provided in the definition below Like class UrRobot
19. ot World JKarel presents a robot world similar to the world of Karel Bergin Stehlik Roberts amp Pattis 1997 composed of a grid of streets and avenues There are one thousand streets and one thousand avenues in the robot world Streets run horizontally from west to east while avenues run vertically south to north Robots move from one street intersection to the next and each step is considered one block Horizontal walls and vertical walls placed across a street or an avenue block a robot s path preventing direct movement between neighboring intersections Beepers are objects in the robot world that robots can pick up carry around and put down Each robot carries a beeper bag to hold beepers that it finds along its path As robots travel around the world they can pick up any beepers that they find on intersections and put down any beepers that they are carrying in their beeper bag Figure 1 illustrates a JKarel world containing vertical and horizontal walls two beepers and a robot The number inside a beeper object indicates how many beepers are on an intersection Figure 1 JKarel World A basic robot in the JKarel world can move forward one block turn left pick up a beeper and put down a beeper It can also turn itself off when it has finished executing a program These basic commands will be introduced in section 5 Karel Primitive Instructions Copyright 2003 Heather Wellenius 4 4 The JKarel Programming Environm
20. perform is also known as a method All instructions in a main procedure following the robot initialization include the name of the robot a period or dot and the name of the method as in the following jkarel move The method name is followed by parentheses Every instruction of this kind ends with a semi colon and it is good programming practice to place each instruction on a new line A complete program moving a robot three blocks east in the robot world and then turning off looks like this class Move3Blocks main UrRobot kate new UrRobot l 1 East 0 kate move kate move kate move kate tC Urnorr r Copyright 2003 Heather Wellenius 14 5 1 Writing New Instructions A class definition can also define new methods in addition to a main procedure In order to define new methods a class must extend an existing class definition using the following format class lt NewClassName gt extends lt OldClassName gt t lt newMethodDefinition gt lt newMethodDefinition gt main t lt robotInitialization gt lt instruction gt lt instruction gt UrRobot is the class that will usually be extended in your first programs since it defines the basic robot instructions introduced so far If a class 1s extended that does not exist a compile error will be generated New method definitions are included at the top of the new class above the main procedure and have the following format void
21. re 5 Successful Execution of StairSweeper Copyright 2003 Heather Wellenius 18 After execution has completed the Execution Log should display the text below The different levels of indentation represent methods called from within other methods Each statement also identifies the object upon which a command is being executed which may be useful in later programs that initialize two or more robots Instantiated StairSweeper alex Bxecuting on alex climbStarr Executing on alex turnLeft Executing on alex move Executing on alex turnRight Executing on alex turnlLeft Executing on alex turnLeft Executing on alex turnlLeft Executing on alex move Executing on alex pickBeeper PXeCUUING on alex climbstalr Executing on alex turnLeft Executing on alex move Executing On abex curnRrghu Executing on alex turnLeft Executing on alex turnLeft Executing on alex turnlLeft Executing on alex move Executing on alex pickBeeper Executing on alex climbStair Executing on alex turnlLeft Executing on alex move Executing on alex turnRight Executing on alex turnLeft Executing on alex turnlLeft Executing on alex turnLeft Executing on alex move Executing on alex pickBeeper Executing on alex turnoff TUZNING Ose WOW wx The Robot Log also displays runtime information pertaining to the current state of the robot s in the world It lists all robots in the world at the
22. the Robot class may also be extended in order to create new classes with other predicates based on the eight predicates provided here Class Robot boolean boolean boolean boolean boolean boolean boolean boolean frontIsClear Is a robot s path blocked by a wall nextToABeeper jf Te amp robot on a Corner wath a beeper next ToARobot ii LS a robot Of a Corner with another FF LODOL facingNorth i Ls eS robor Lacing north facingSouth Is a robot facing south facingEast Is a robot facing east facingWest Is a robot facing west anyBeepersInBeeperBag Does a robot have any beepers in its beeper bag Copyright 2003 Heather Wellenius 24 7 2 Example using if The following example demonstrates how predicates are used in if instructions to enable a robot to face north regardless of his current direction When this method executes 1f the robot is facing east he must only turn left once but if he is facing south he must turn twice and if he is facing west he must turn three times If he 1s already facing north when this method 1s invoked nothing will happen to change the direction he faces Note that the method in this example can only be invoked on a robot of type Robot not a robot of type UrRobot since an UrRobot is not able to use the predicates defined in class Robot such as facingEast or facingSouth void faceNorth if facingEast GUY w
23. with one beeper This world can be saved with any name but for the purposes of this example we will call it stair wld This file is also included with the JKarel program and can be opened from File in the menu bar Figure 4 Robot World for StairSweeper Program Now we will create a new type of robot called a StairSweeper which defines two new methods turnRight and climbStair Select New Program File from File in the menu bar and copy into it the following lines of code or select Open Program File and open the included copy of StairSweeper jk1 Copyright 2003 Heather Wellenius 16 New class to define a robot that will be able to climb stairs and pick up a beeper from each stair class StairSweeper extends UrRobot Turning right is accomplished rf WEE SB eT GUNo void EuUrnRegiuy BUCH EeL Et turnLeft turnLeft Climbing a stair assumes the robot begins facing towards the stair and ends facing in this same direction after climbing to the next stair void clLimbStair turnLeft move CU EN REO J J move The main procedure creates a StairSweeper Jf obot and has te climb lt 3 stairs picking up a beeper from each main StairSweeper alex new StairSweeper 1 1 East 0O alex climbStair alex pickBeeper alex climbStair alex pickBeeper e l e 7 7 7 7 alex climbStair alex pickBeeper alex curcnort I a 7 N SS NA NA
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