Wiley VBScript Programmer's Reference, 3rd Edition
Contents
1. If YourName Then Greeting OK You don t want to tell me your name Else Greeting Hello amp YourName amp great to meet you End If MsgBox Greeting However this code is very difficult to read As a general rule of thumb you indent code by four spaces whenever a line or series of lines is subordinate to the lines above and below it For ex ample the lines after the If clause and the Else clause belong inside the If Else End If block so you indent them to visually suggest the code s logical structure Presentation while having no bearing whatsoever on how the computer or script engine handles your code is very important when it comes to how humans read it You should be able to look at the code and get a sense for how it is organized and how it works By seeing the indentations inside the If Else End If block you can not only read the code but also see the branching logic at that point in the code Indenting is only one element of programming style but learning and following proper style and layout is essential for any programmer who wants to be taken seriously The Else part of the block is optional Sometimes you want to test for a certain condition and if that condition is True execute some code but if it s False there s no code to execute For example you could add another If End If block to your script Chapter 1 A Quick Introduction to Programming Dim YourName Dim Greeting2. A Quick Introduction to Programming A chapter covering the basics of VBScript is the best place to begin this book This is because of the type of language VBScript is and the kind of users the authors see turning to it In this chapter you get a crash course in programming basics You might not need this chapter because you ve come to VBScript with programming skills from another language Visual Basic Visual Basic NET C C Delphi C and are already both familiar with and comfortable using programming terminology In that case feel free to skip this chapter and move on to the next one However if you come from a non programming background then this chapter will give you the firm foundation you need to begin using VBScript confidently If you re still reading chances are you fall into one of three distinct categories Q You re a Network Systems administrator who probably wants to use VBScript and the Windows Script Host or PowerShell to write logon scripts or to automate administration tasks Q You might be a web designer who feels the need to branch out and increase your skill set perhaps in order to do some ASP work Q You re interested in programming possibly Visual Basic or Visual Basic NET and want to check it out before getting too deeply involved Programming is a massive subject Over the years countless volumes have been written about it both in print and on the Internet In this chapter in a single paragraph
3. Q Organize the code in a logical manner All of these aid human readability and are covered later but clear concise comments are by far the most important However too much of a good thing is never good and the same is true for comments Over burdening code with comments doesn t help Remember that if you are scripting for the Web that all the code including the comments are downloaded to the browser so unnecessary comments may adversely affect download times You learn about some good commenting principles later in this chapter but for now just be aware of the fact that the comment in line 2 of the script is not really a good comment for everyday use This is because to any semi experienced programmer it is all too obvious that what you are doing is declaring Chapter 1 A Quick Introduction to Programming the YourName variable on the code line above However throughout this book you ll often see the code commented in a similar way This is because the point of the code is to instruct the reader in how a par ticular aspect of VBScript programming works and the best way to do that is to add comments to the code directly It removes ambiguity and keeps the code and comments together Also worth noting is that comments don t have to be on a separate line Comments can also follow the code like so Dim YourName initialize the variable YourName InputBox Hello What is your name ask for the user s name MsgBox Hello
4. amp YourName amp Pleased to meet you display a greeting This works in theory but it isn t as clear as keeping the comments on separate lines in the script Using Built in VBScript Functions OK back to the script Take a look at line 3 YourName InputBox Hello What is your name Here you are doing two things at once First you re initializing the variable You could do it directly like this YourName Fred However the drawback with this is that you re making the arbitrary decision that everyone is called Fred which is ideal for some applications but not for others If you wanted to assign a fixed value to a variable such as a tax rate this would be fine Dim TaxRate TaxRate 17 5 Because you want to do something that gives the user a choice you should employ the use of a function called InputBox This function and all the others are discussed in later chapters but for now all you need to know is that Input Box is used to display a message in a dialog box and it waits for the user to input text or click a button The InputBox generated is displayed in Figure 1 1 Cancel The clever bit is what happens to the text that the user types into the input box displayed it is stored in the variable YourName Chapter 1 A Quick Introduction to Programming Line 4 is another comment Line 5 is more code Now that you ve initialized this variable you re going to do something useful with it MsgBox i
5. YourName InputBox Hello What is your name If YourName Then Greeting OK You don t want to tell me your name Else Greeting Hello amp YourName amp great to meet you End If If YourName Fred Then Greeting Greeting amp Nice to see you Fred Had ILE MsgBox Greeting Q TheIf Else End If block can be extended through the use of the ElseIf clause and through nesting Nesting is the technique of placing a block of code inside of another block of code of the same type The following variation on your script illustrates both concepts Dim YourName Dim Greeting YourName InputBox Hello What is your name If YourName Then Greeting OK You don t want to tell me your name ElseIf YourName abc Then Greeting That s not a real name ElseIf YourName xxx Then Greeting That s not a real name Else Greeting Hello amp YourName amp great to meet you If YourName Fred Then Greeting Greeting amp Nice to see you Fred Ene IE mad IE MsgBox Greeting Once again seeing how the code has been indented helps you to identify which lines of code are subordinate to the lines above them As code gets more and more complex proper indenting of the code becomes vital as it will become harder to follow Q Even though the branching logic you are adding to the code tells the script to execute certain lines of code while not executing others all the code must s
6. t Overuse Them When writing code strive to make it as self documenting as possible You can do this by following the guidelines set out earlier However self documenting code is hard to achieve and no one is capable of 100 self documenting code Everyone writes code that can benefit from a few little scribbles to serve as reminders in the margins The coding equivalents of these scribbles are comments But how can you tell a good comment from a bad comment Generally speaking a good comment operates at the level of intent A good comment answers the questions Q Where does this code block fit in with the overall script Q Why did the programmer write this code The answers to these questions fill in the blanks that can never be filled by even the best most pedantic self documenting code Good comments are also generally paragraph level comments Your code should be clear enough that you do not need a comment for each and every line of code it contains but a comment that quickly and clearly describes the purpose for a block of code allows a reader to scan through the comments rather than reading every line of code The idea is to keep the person who might be reading your code from having to pore over every line to try and figure out why the code exists Commenting every line as you probably noticed with the earlier examples makes the code hard to follow and breaks up the flow too much Bad comments are generally redundant comments
7. The code has to tell the script engine where it begins and where it ends Notice also that you ve given the function a clear useful name that precisely describes what this function does Giving your procedures good names is one of the keys to writing programs that are easy to read and maintain Notice also how there s a comment to the beginning of the procedure to describe only what it does not how the function does what it does The code that uses this function does not care how the function accomplishes its task it only cares about inputs outputs and predictability It is vitally important that you add clear informative comments such as this to the beginning of your procedures because they make it easy to determine what the function does The comment also performs one other valuable ser vice to you and any other developer who wants to call this function it says that the function may return a zero length string if the user does not enter his or her name 21 Chapter 1 A Quick Introduction to Programming Finally notice how in the second to last line the function name PromptUserName is treated as if it were a variable When you use functions as opposed to subprocedures which do not return a value this is how you give the function its return value In a sense the function name itself is a variable within the procedure Here is some code that uses the PromptUserName function Dim Greeting Dim VisitorName VisitorName Pr
8. amp contains amp WordLength amp characters 17 Chapter 1 A Quick Introduction to Programming 18 Figure 1 7 shows the final summary message generated by the code Notice how well the information is integrated Figure 1 7 Operators and Operator Precedence An operator acts on one or more operands when comparing assigning concatenating calculating and performing logical operations Say you want to calculate the difference between two variables X and Y and save the result in variable z These variables are the operands and to find the difference you use the subtraction operator like this Z X Y Here you use the assignment operator to assign the difference between X and Y which was found by using the subtraction operator Operators are one of the single most important parts of any programming language Without them you cannot assign values to variables or perform calculations or comparisons In fact you can t do much at all There are different types of operators and they each serve a specific purpose as shown in the following table Operator Purpose assignment The most obvious and is simply used for assigning a value to a variable or property arithmetic These are all used to calculate a numeric value and are normally used in conjunction with the assignment operator and or one of the comparison operators concatenation These are used to concatenate join together two or more diffe
9. asked it to do Just like functions procedure may or may not require input Chapter 1 A Quick Introduction to Programming Turning Code into a Function Some of the code that follows is from an example you used earlier in the chapter Here s how to turn code into a function Function PromptUserName This Function prompts the user for his or her name If the user enters nothing it returns a zero length string It incorporates various greetings depending on input by the user Dim YourName Dim Greeting YourName InputBox Hello What is your name If YourName Then Greeting OK You don t want to tell me your name ElseIf YourName abc Then Greeting That s not a real name ElseIf YourName xxx Then Greeting That s not a real name Else Greeting Hello amp YourName amp great to meet you If YourName Fred Then Greeting Greeting amp Nice to see you Fred End If macl I MsgBox Greeting PromptUserName YourName End Function The first things to take note of in the code are the first and last lines While not groundbreaking these are what define a function The first line defines the beginning of the function and gives it a name while the last line defines the end of the function Based on the earlier discussion of code blocks this should be a familiar convention by now From this you should begin to realize that a procedure is nothing but a special kind of code block
10. processes it and also handles saving to a file Q Code that handles file access and database access You ve been using procedures throughout this chapter but they have been procedures that VBScript provides for you Some of these procedures require input some don t Some of these procedures return a value some don t But all of the procedures you ve used so far MsgBox InputBox and so on are black boxes They perform one single well defined function and they perform it without you having to worry about how they perform their respective functions In just a moment you ll see how to extend the VBScript language by writing your own procedures Before you begin though it s time to get some of the terminology cleared up Procedure is a generic term that describes either a function or a subprocedure This chapter touched on some of this confusing termi nology earlier but a function is simply a procedure that returns a value Len is a function You pass it some text and it returns the number of characters in the string or the number of bytes required to store a variable back to you Functions do not always require input but they often do A subprocedure is a procedure that does not return a value You ve been using MsgBox as a subproce dure You pass it some text and it displays a message on the screen comprising of that text It does not return any kind of value to your code All you need to know is that it did what you
11. purposes throughout your script but don t do it This is one of the best ways to introduce very strange hard to track down bugs into your scripts Giving a vari able a good name that clearly defines its purpose will help prevent you from using it for multiple pur poses The moral here is that while reusing variables might seem like a total timesaver it isn t and can lead to hours of frustration and wasted time looking for the problem Always Lay Out Your Code Properly 28 Always remember that good code layout adds greatly to readability later Don t be tempted to save time early on by writing messy hard to follow code because as sure as day turns to night you will suffer if you do Without reading a single word you should be able to look at the indentations of the lines to see which ones are subordinate to others Keep related code together by keeping them on consecutive lines Also don t be frightened of white space in your code Separate blocks of unrelated code by putting a blank line between them Even though the script engine will let you avoid putting multiple statements on the same line Also remember to use the line continuation character _ to break long lines into multiple shorter lines The importance of a clean layout that visually suggests the logic of the underlying code cannot be overemphasized Chapter 1 A Quick Introduction to Programming Use Comments to Make Your Code More Clear and Readable but Don
12. the script a script s lifetime is the time from when it starts executing to the time it stops Unlike a variable a literal cannot change during the lifetime of the script Here is line 5 of the script again MsgBox Hello amp YourName amp Pleased to meet you An operator is a symbol or a word that you use within your code that is usually used to change or test a value Other operators include the standard mathematical operators and the equals sign which can actually be used in either a comparison or an assignment So far you ve used the equals sign as an assignment operator Later in this chapter you ll find out more about operators Now take a closer look at variables Remember how we said that a variable is a piece of reserved memory One question you might have is How does the computer know how large to make that piece of memory Well again in VBScript this isn t something that you need to worry about and it is all handled automatically by the VBScript engine You don t have to worry in advance about how big or small you need to make a variable You can even change your mind and the VBScript engine will dynamically change and reallocate the actual memory addresses that are used up by a variable For example take a quick look at this VBScript program Chapter 1 A Quick Introduction to Programming First declare the Variable Dim SomeVariable tTaitcialize wt with a value SomeVariable Hello Wor
13. word Do and ends with the line that starts with the word Loop The indentation should make this code block easy to identify This is the definition of the loop The code inside the loop will keep being executed until at the end of the loop the TryAgain variable equals No The TryAgain variable controls the loop The loop starts at the word Do At the end of the loop if the TryAgain variable equals Yes then all the code starting at the word Do will execute again Notice that the top of the loop initializes the TryAgain variable to No It is absolutely essential that this initialization take place inside the loop that is between the Do and Loop statements This way the variable is reinitialized every time a loop occurs If you didn t do this you would end up with what s called an infinite loop They are always bad At best the user is going to have to exit out of the program in an untimely and inelegant way because as the name suggests the loop is infinite At worse it can crash the system You want neither and you want to try to avoid both in your code 14 Chapter 1 A Quick Introduction to Programming Take a look at why the TryAgain No line is essential to preventing an infinite loop Going through the script line by line 1 Do 2 Do 3 Do This first line starts the loop This tells the script engine that you are starting a block of code that will define a loop The script engine will expect to fi
14. If YourName Then tests to see if the user typed in their name or at least some text If they typed something in the code immediately after the Else line will execute If they didn t type in anything or if they clicked the Cancel button then the YourName variable will be empty and the code after the If line will execute instead TryAgain No YourName InputBox Please enter your name If YourName Then MsgBox You must enter your name to continue TayAgain yes Else Greeting Hello amp YourName amp great to meet you End If If the user didn t type anything into the input box you will display a message informing them that they have done something you didn t want them to You then set the TryAgain variable the control variable to Yes and send them around the loop once more and ask the users 15 Chapter 1 A Quick Introduction to Programming for their name again wherein this time they will hopefully type something into the input box If the user did type in his or her name then you initialize your familiar Greet ing variable Note that in this case you do not change the value of the TryAgain variable This is because there is no need to loop around again because the user has entered a name The value of TryAgain is already equal to No so there s no need to change it 5 In the next line of code you encounter the end of the loop block What this Loop line is essen tially telling t
15. Modularization Black Boxes Procedures and Subprocedures 20 Modularization is the process of organizing your code into modules which you can also think of as build ing blocks You can apply the principles of modularity to create your own personal set of programming building blocks which you can then use to build programs that are more powerful more reliable easier to debug and easier for you and your fellow programmers to maintain and reuse When you take your code and divide it into modules your ultimate goal is to create what are known as black boxes A black box is any kind of device that has a simple well defined interface and that performs some discrete well defined function A black box is so called because you don t need to see what s going on inside it All you need to know is what it does what its inputs are and sometimes what its outputs are A wristwatch is a good example of a black box It has inputs buttons and outputs time and does a simple function well without you worrying about how the innards of the watch work in order to be able to tell the time The most basic kind of black box programmers use to achieve modularity is the procedure A procedure is a set of code that ideally performs a single function Good examples of procedures are Q Code that adds two numbers together Q Code that processes a string input Q Code that handles saving to a file Bad examples include Q Code that takes an input
16. appens if the registry keys I was expecting aren t there What happens if the user doesn t have the proper permission to carry out the operation If you don t know what might go wrong with a given operation find out through research or trial and error Get others to try out your code and get their feedback on how it worked for them on their system configuration and on their operating system Don t leave it up to your users to discover how well or not your script reacts to something unexpected A huge part of properly preparing for the unexpected is the implementation of proper error handling which is discussed in detail in Chapter 6 Chapter 1 A Quick Introduction to Programming Always Favor the Explicit over the Implicit When you are writing code constantly ask yourself Is my intent clear to someone reading this code Does the code speak for itself Is there anything mysterious here Are there any hidden meanings Are the variable names too similar to be confusing Even though something is obvious in your mind at the moment you are typing the code it doesn t mean it will be obvious to you six months or a year from now or to someone else tomorrow Always endeavor to make your code as self documenting as possi ble and where you fall short of that goal which even the best programmers do self documenting code can be an elusive goal use good comments to make things clearer Be wary of using too many generics in code su
17. ased to meet you Above we display a greeting containing the user s name Rightly so you re now probably wondering what all this code means Last time you were showed one line and now it s grown to six Chapter 1 A Quick Introduction to Programming All of the examples in this chapter are designed so that you can run them using the Windows Script Host WSH The WSH is a scripting host that allows you to run VBScript programs within Windows WSH allows you to try out these example programs for yourself You may already have WSH installed To find out type the previous example script into a text editor save the file as TEST VBS it must have the VBS extension and not a TXT and double click the file in Windows Explorer If the script runs then you re all set If Windows does not recognize the file then you need to download and install WSH from http msdn2 microsoft com en us library ms950396 aspx Using Comments You already know what the first line of code in the previous block does It declares a variable for use called YourName The second line in the code is a comment In VBScript any text preceded by the single quote character is treated as a comment which means that the VBScript engine completely ignores the text which begs the question why bother typing it in at all It doesn t contribute to the execution of the script right This is absolutely correct but don t forget one of the most important principles o
18. at to meet you macl IE This code presents the VBScript engine with an option that is based on what the user typed or didn t type into the input box The first line tests the input from the user It tests to see if the input that is stored in the variable YourName is a zero length string If it is the next line of code is run and the variable Greeting is assigned a string Figure 1 6 shows the message displayed if the user doesn t type his or her name into the InputBox Chapter 1 A Quick Introduction to Programming OK You don t want to tell me your name Figure 1 6 3 What happens if the user does as you expect type something into the input box Well this is where the next line comes in Else You can actually begin to read the code and in fact doing this helps it to make sense What the whole loop actually means is that if the value of variable YourName is a zero length string then assign the variable Greeting with one value however if it contains something else do some thing else assign Greeting a different value This doesn t protect your script from users enter ing data like numbers of non alphabet characters into the test box although you could code for all these conditions if you wanted to 4 The final line of the code uses the MsgBox function to display the value of the variable Greeting Notice that both lines of code assign a value to the Greeting variable However only one of these lines will actually
19. ch as x y and z as variable names and Function1 Function2 and Function3 as function names Instead make them explicit Use variable names such as UserName and TaxRate When naming a variable use a name that will make it clear what that variable is used for Be careful using abbreviations Don t make variable names too short but don t make them too long either 10 16 characters is a good length but ideal length is largely a matter of preference Even though VBScript is not case sensitive use mixed case to make it easier to distinguish multiple words within the variable name for example UserName is easier to read than username When naming procedures try to choose a name that describes exactly what the procedure does If the procedure is a function that returns a value indicate what the return value is in the function name for example PromptUserName Try to use good verb noun combinations to describe first what action the procedure performs and second what the action is performed on for example SearchFolders MakeUniqueRegistryKey or LoadSettings Good procedure names tend to be longer than good variable names Don t go out of your way to make them longer but don t be afraid to either Fifteen to thirty characters for a procedure name is perfectly acceptable they can be a bit longer because you generally don t type them nearly as much If you are having trouble giving your procedure a good name that might be an indicatio
20. e script engine that this is what you want to do by also using the name of the SomeVariable variable on the right side of the equals sign and then concatenating its existing value with an additional value using the ampersand amp operator Adding onto the original value works with num bers too as opposed to numbers in strings but you have to use the operator instead of the amp operator Dim SomeNumber SomeNumber 999 MsgBox SomeNumber SomeNumber SomeNumber 2 MsgBox SomeNumber SomeNumber SomeNumber 999 MsgBox SomeNumber Chapter 1 A Quick Introduction to Programming Here are the resulting message boxes generated by this code The first is shown in Figure 1 3 Figure 1 3 The second message box is shown in Figure 1 4 Figure 1 4 The final message box is shown in Figure 1 5 Figure 1 5 Chapter 1 A Quick Introduction to Programming You can store several different types of data in variables These are called data types and so far you ve seen two O String Q Integer You ve also seen a single precision floating point number in the tax rate example We ll be covering all of them later on in the book For now just be aware that there are different data types and that they can be stored in variables Flow Control When you run a script that you have written the code executes in a certain order This order of execution is also known as flow In simple scripts such a
21. ence and there are no hard and fast rules for code after all only you know what you want it to do Only you know if parts are going to be reused later Only you know how complex something will turn out However always keep an eye out for possible modularization Use the Hungarian Variable Naming Convention You might hear programmers especially C programmers mention this quite a bit While this is a bit out of scope of this introductory discussion it is still worth mentioning nonetheless The Hungarian naming convention involves giving variable names a prefix that indicates what the scope and data type of the variable are intended to be So as not to confuse matters the Hungarian convention was not used in this chapter but you will find that most programmers prefer this convention Properly used it makes your programs much clearer and easier to write and read See Chapter 3 for more on Hungarian notation variable prefixes The standard prefixes for scope and data types are in Appendix B Don t Use One Variable for More Than One Job This is a big no no and a common mistake of both beginner and experienced programmers alike but the fact that experienced programmers might have a bad habit does not make it any less bad Each variable in your script should have just one purpose It might be very tempting to just declare a bunch of generic variables with fuzzy names at the beginning of your script and then use them for multiple
22. execute in any one running of the script This is because the If Else End If block makes an either or decision Either a given condition is True or it is False There s no way it can be neither not a string that contains text nor a zero length string or both a zero length string that contains text If it is True then the script engine will execute the code between the If and Else statements If it is False then it will execute the code between the Else and End If statements So what the complete script does is test the input and then executes different code depending on the result of that test and hence the term branching Using this technique allows your script to adapt to the unpredictable nature of the input Compare the intelligent script to the following one which looks pretty lame Dim YourName Dim Greeting YourName InputBox Hello What is your name Greeting Hello amp YourName amp great to meet you MsgBox Greeting This script is just plain dumb because it does not contain any branching logic to test the input so when the user does something unpredictable such as clicking the Cancel button or not entering any name at all the script does not have the ability to adapt Compare this to your intelligent script which is capable of adapting to the unpredictability of input by testing it with If Else End If branching 11 Chapter 1 A Quick Introduction to Programming 12 Before you move on to
23. f programming It is not just computers that may have to read script It is equally important to write a script with human readers in mind as it is to write with the computer in mind Of course none of this means you should for one moment forget that when you write scripts you must do so with the computer or more specifically the script engine in mind If you don t type the code cor rectly that is if you don t use the proper syntax the script engine won t be able to execute the script However once you ve written some useful scripts you ll probably need to go back to make some changes to a script you wrote six months or a year ago If you didn t write that code with human readers as well as computers in mind it could be pretty difficult to figure out what you were thinking and how you decided to solve the problems at the time you wrote the script Things can get worse What happens when you or one of your coworkers has to make some changes to a script you wrote many months ago If you did not write that script to be both readable and maintainable others who use your code will encounter difficulties deciphering it no matter how well written the actual computer part of the code is Adding comments to your code is just one part of making sure code is clear and readable There are many other things that you can do Q Choose clear meaningful variable names Indent code for clarity Q Q Make effective use of white space
24. fact a variable can store pretty much anything you want it to store Behind the scenes the variable is a reserved section of the computer s memory for storing data Memory is temporary things stored there are not stored permanently like they are when you use the hard drive Because memory is a temporary storage area and variables are stored in the computer s memory they are therefore also temporary Your script will use variables to store data temporarily that the script needs to keep track of for later use If your script needs to store that data permanently it would store it in a file or database on the computer s hard disk To make it easier for the computer to keep track of the millions of bits of data that are stored in memory at any given moment the memory is broken up into chunks Each chunk is exactly the same size and is given a unique address Don t worry about what the memory addresses are or how you use them because you won t need to know any of that to use VBScript but it is useful to know that a variable is a reserved set of one or more chunks Also different types of variables take up different amounts of memory In your VBScript program a variable usually begins its lifecycle by being declared or dimensioned before use Chapter 1 A Quick Introduction to Programming It is not required that you declare all of the variables you use By default VBScript allows you to use undeclared variables However it s
25. he program is well written an item such as a button for saving a file which the user expects to have code behind it will indeed have code behind it How Top Down and Event Driven Work Together 24 When a GUI based program starts there is almost always some top down style code that executes first This code might be used to read a setting stored in the registry prompt the user for a name and pass word load a particular file at startup or prompt to take the user through setup if this is the first time the application has been run and so on Then a form typically comes up The form contains all the menus buttons lists and fields that make up the user interface of the program At that point the top down style coding is done and the program enters what is known as a wait state No code is executing at this point and the program just waits for the user to do something From here on it s pretty much all about events Chapter 1 A Quick Introduction to Programming When the user begins to do something the program comes to life again Suppose the user clicks a but ton The program raises the Click event for the button that the user clicked The code attached to that event starts to execute performs some operations and when it s finished the program returns to its wait state As far as VBScript is concerned the event driven model is used heavily in scripting for the Web Scripts that run inside of HTML web pages are all based on event
26. he script engine is If the TryAgain variable equals Yes at this point then go back up to the Do line and execute all that code over again If the user entered his or her name then the TryAgain variable will be equal to No Therefore the code will not loop again and will continue onto the last line Do TryAgain No YourName InputBox Please enter your name If YourName Then MsgBox You must enter your name to continue TryAgain Yes Else Greeting Hello amp YourName amp great to meet you End If Loop While TryAgain Yes MsgBox Greeting MsgBox Greeting If the user did not enter his or her name then TryAgain would be equal to Yes which would mean that the code would again jump back to the Do line This is where the reinitialization of the TryAgain variable to No is essential because if it wasn t done then there s no way for TryAgain to ever equal anything but Yes And if TryAgain always equals Yes then the loop will keep going around and around forever This results in total disaster for your script and for the user Using the For Next Loop 16 In this kind of loop you don t need to worry about infinite loops because the loop is predefined to execute only a certain number of times Here s a simple if not very useful example Dim Counter MsgBox Let s count to ten Ready Hor Counter OO MsgBox Counter Next MsgBox Wasn t that fun This loop is similar to
27. ick Introduction to Programming 10 Dim YourName Dim Greeting YourName InputBox Hello What is your name If YourName Then Greeting OK You don t want to tell me your name Else Greeting Hello amp YourName amp great to meet you mac IE MsgBox Greeting Walking through the code you do the following 1 You declare the two variables that you are going to use Dim YourName Dim Greeting YourName InputBox Hello What is your name You ask the user for some input again using the InputBox function This function expects one required parameter the prompt text the text that appears on the input box It can also accept several optional parameters Here you only use the one required parameter Note that the parameter text that you passed Hello What is your name is displayed as a prompt for the dialog box The InputBox function returns the value that the user types if any If the user does not type anything or clicks the Cancel button both do the same thing then InputBox returns a zero length string which is a strange kind of programming concept that basically means that it returns text that doesn t actually contain any text Your script stores the result of the InputBox function in the YourName variable 2 Youcome to the actual loop you re going to use If YourName Then Greeting OK You don t want to tell me your name Else Greeting Hello amp YourName amp gre
28. inputs and outputs are predictable and remain unchanged the code will work just fine Top Down versus Event Driven Before you leave this introduction to programming it may be helpful to point out that you will encounter two different models of programming in this book top down and event driven programs The differences between the two have to do with the way you organize your code and how and when that code gets executed at runtime As you get deeper into programming in general and VBScript in particular this will become clearer so don t be alarmed if it doesn t completely sink in right now 23 Chapter 1 A Quick Introduction to Programming Understanding Top Down Programming So far in this chapter you ve written very simple top down style programs The process is simple to follow Q Write some code Save the code in a script file Use Windows Script Host to execute the script The Script Host starts executing at the first line and continues to the last line Q Q Q Q Ifa script file contains some procedure definitions such as your PromptUserName function then the Script Host only executes those procedures if some other code calls them Q Once the Script Host reaches the last line of code the lifetime of the script ends Top down programs are very useful for task oriented scripts For example you might write a script to search your hard drive for all the files with the extension htm and copy all the names and fi
29. ld MsgBox SomeVariable Change the value of the variable to something larger SomeVariable Let s take up more memory than the previous text MsgBox SomeVariable Change the value again SomeVariable Bye MsgBox SomeVariable Each time the script engine comes across a variable the engine assigns it the smallest chunk of memory it needs Initially the variable contains nothing at all so needs little space but as you initialize it with the string Hello World the VBScript engine asks the computer for more memory to store the text But again it asks for just what it needs and no more Memory is a precious thing and not to be wasted Next when you assign more text to the same variable the script engine must allocate even more mem ory which it again does automatically Finally when you assign the shorter string of text the script engine reduces the size of the variable in memory to conserve memory One final note about variables Once you ve assigned a value to a variable you don t have to throw it away in order to assign something else to the variable as well Take a look at this example Dim SomeVariable SomeVariable Hello MsgBox SomeVariable SomeVariable SomeVariable amp World MsgBox SomeVariable SomeVariable SomeVariable amp Goodbye MsgBox SomeVariable Notice how in this script you each time keep adding the original value of the variable and adding some additional text to it You tell th
30. le locations to a file formatted in HTML to act as a sitemap Or you might write a script that gets executed every time Windows starts and which randomly chooses a different desktop wallpaper bitmap file for that session of Windows Top down programming is perfect for these kinds of scripts Understanding Event Driven Programming Event driven code is different and is useful in different contexts As the name implies event driven code only gets executed when a certain event occurs Until the event occurs the code won t get executed If a given event does not occur during the lifetime of the script the code associated with that event won t be executed at all If an event occurs and there s no code associated with that event then the event is essentially ignored Event driven programming is the predominant paradigm in Windows programming Most of the Windows programs you use every day were written in the event driven model This is because of the graphical nature of Windows programs In a graphical user interface GUI you have all sorts of buttons drop down lists fields in which to type text and so on For example the word processor program Microsoft Word is totally jam packed with these Every time a user clicks a button chooses an item in a list or types some text into a field an event is raised within the code The person who wrote the pro gram may or may not have decided to write code in response to that event However if t
31. looping you should know a few other things about If Else End If The block of code containing the If Else End If is known as a block of code A block is a section of code that has a beginning and an end and it usually contains keywords or state ments at both the beginning and the end In the case of If Else End If the If statement marks the beginning of the block while the End If marks the end of the block The script engine requires these beginning and ending statements and if you omit them the script engine won t understand your code and won t allow your script to execute Over the course of this book you will encounter many different types of code blocks in VBScript To confuse matters the term block of code is often used informally to describe any group of lines of code As a rule block of code will refer to lines of code that work together to achieve a result E Notice that the lines of code that are inside the block itself are indented by four spaces This is an extremely important concept but not for the reason you might think This indenting has nothing whatsoever to do with the script engine it doesn t care whether you add four spaces 44 spaces or none at all This indenting is for the benefit of any humans who might be reading your code For example the following script is completely legal and will execute just fine Dim YourName Dim Greeting YourName InputBox Hello What is your name
32. meaning they repeat what the code itself already tells you Try to make your code as clear as possible so that you don t need to repeat yourself with comments Redundant comments tend to add clutter and do more harm than good Reading the code tells you the how reading the comments should tell you the why Finally it s a good idea to get into the habit of adding tombstone or flower box comments at the top of each script file module class and procedure These comments typically describe the purpose of the code the date it was written the original author and a log of modifications Kathie Kingsley Hughes 22 Feb 2007 This script prompts the user for his or her name It incorporates various greetings depending on input by the user Added alternative greeting Changed variable names to make them more readable Summary In this chapter you took a really fast paced journey through the basics of programming The authors tried to distill a whole subject at least a book into one chapter You covered an awful lot of ground but also skimmed over or totally passed by a lot of stuff However the information in this chapter gave you the basics you need to get started programming with VBScript and the knowledge and confidence you need to talk about programming with other programmers in a language they understand 29
33. n that the procedure is not narrow enough a good procedure does one thing and does it well That said if you are writing scripts for web pages to be downloaded to a user s browser it is sometimes necessary to use shorter variable and procedure names Longer names mean larger files to download Even if you sacrifice some readability to make the file smaller you can still take time to create descriptive names With web scripts however you may encounter instances where you don t want the code to be clear and easy to understand at least for others You ll look at techniques that you can employ to make scripts harder for script snoopers to follow while still allowing you to work with them and modify them later see Chapter 17 Modularize Your Code into Procedures Modules Classes and Components As you write code you should constantly evaluate whether any given code block would be better if you moved it to its own function or subprocedure Q Is the code rather complex If so break it into procedures Q Are you using many Ands and Ors inanIf End If statement Consider moving the evaluation to its own procedure 27 Chapter 1 A Quick Introduction to Programming Q Are you writing a block of code that you think you might need again in some other part of the script or in another script Move it to its own procedure Q Are you writing some code that you think someone else might find useful Move it This isn t a sci
34. nd a loop statement somewhere further down in the script This is similar to the If End If code block because the script engine expects the block to be defined with beginning and ending statements The Do statement on a line all by itself means that the loop will execute at least once Even if the Loop While statement at the end of the block does not result in a loop around back to the Do line the code inside this block will be executed at least one time Moving on to the second line of code you initialize the control variable It s called the con trol variable because it ultimately controls whether or not the code block loops around again You want to initialize this variable to No so that by default the loop will not loop around again Only if a certain condition is met inside the loop will you set TryAgain to Yes This is yet another strategy in an ever vigilant desire to expect the unexpected Tey Again Nee The next line of code should look familiar You use the Input Box function to ask the user to enter a name You store the return value from the function in the YourName variable Whatever the user types unless they type nothing will be stored in this variable Put another way the script receives some external input and remember that we said input is always unpredictable TryAgain No YourName InputBox Please enter your name 4 Do In the next part of the code you test the input The line
35. omotUserName If VisitorName lt gt Then Greeting Goodbye amp VisitorName amp Nice to have met you Else Greeting I m glad to have met you but I wish I knew your name macl I MsgBox Greeting If you are using Windows Script Host for this code bear in mind that this code and the PromptUserName function itself must be in the same vbs script file 22 Dim PartingGreeting Dim VisitorName VisitorName PromptUserName If VisitorName lt gt Then PartingGreeting Goodbye amp VisitorName amp Nice to have met you Else PartingGreeting I m glad to have met you but I wish I knew your name End If MsgBox PartingGreeting Function PromptUserName This Function prompts the user for his or her name It incorporates various greetings depending on input by the user Dim YourName Dim Greeting YourName InputBox Hello What is your name If YourName Then Greeting OK You don t want to tell me your name ElseIf YourName abc Then Greeting That s not a real name ElseIf YourName xxx Then Greeting That s not a real name Chapter 1 A Quick Introduction to Programming Else Greeting Hello amp YourName amp great to meet you If YourName Fred Then Greeting Greeting amp Nice to see you Fred End If End If MsgBox Greeting PromptUserName YourName End Function As you can see calling the PromptUserName function is prett
36. on as they re used In VBScript whenever you have a piece of information that you need to work with you declare a vari able using the exact same syntax you saw a moment ago At some point in your script you ll need to do something with the memory space you ve allocated yourself otherwise what would be the point of declaring it And what you do with a variable is place a value in it This is called initializing the vari able Sometimes you initialize a variable with a default value Other times you might ask the user for some information and initialize the variable with whatever the user enters Alternatively you might open a database and use a previously stored value to initialize the variable When we say database we don t necessarily mean an actual database but any store of data it might be an Internet browser cookie or a text file that we get the data from If you are dealing with small amounts of data a cookie or text file will suffice but if you are dealing with a lot of data you need the performance and structure that a database offers Initializing the variable gives you a starting point After it has been initialized you can begin making use of the variable in your script Here s a very simple VBScript example Dim YourName Above we dimensioned the variable YourName InputBox Hello What is your name Above we ask for the user s name and initialize the variable MsgBox Hello amp YourName amp Ple
37. ons to be evaluated first and A becomes equal to 165 Organizing and Reusing Code So far the scripts you ve worked with have been fairly simple in structure The code has been all together in one unit You haven t done anything all that complicated so it s easy to see all the code in just a few lines The execution of the code is easy to follow because it starts at the top of the file with the first line and then continues downward until it reaches the last line Sometimes at certain points choices redirect the code using branching or sections of code are repeated using loops However when you come to writing a script that actually does something useful your code is likely to get more complex As you add more code to the script it becomes harder to read in one chunk If you print it on paper your scripts will undoubtedly stretch across multiple pages As the code becomes more complex it s easier for bugs and errors to creep in and the poor layout of the code will make these harder to find and fix The most common technique programmers use to manage complexity is called modularization This is a big fancy word but the concept behind it is really quite simple This section defines some terminology used when organizing and reusing code and then discusses how to write your own procedures by turning code into a function You then learn a few advantages of having procedures 19 Chapter 1 A Quick Introduction to Programming
38. r Dim WordLength Dim WordBuilder WordLength Len VBScript is great For Counter 1 to WorcLengta MSGBOX Much VeSeriptelwonGreat ACCOUN EE WordBuilder WordBuilder amp Mid VBScript is great Counter 1 Next MsgBox WordBuilder For example the phrase VBScript is great has exactly 18 letter spaces If you first calculated the number of letters in the phrase you could use that number to drive a For Next loop However this code uses the VBScript Len function to calculate the length of the phrase used Inside the loop it uses the Mid function to pull one letter out of the phrase one at a time and display them separately The position of that letter is controlled by the counter variable while the number of letters extracted is defined by the length argument at the end It also populates the WordBuilder variable with each loop adding each new letter to the previous letter or letters rebuilding the phrase Here s a variation of the last example here giving the user the opportunity to type in a word or phrase to use proving that there s nothing up your sleeve when it comes to knowing how many times to loop the code Dim Counter Dim WordLength Dim InputWord Dim WordBuilder InputWord InputBox Type in a word or phrase to use WordLength Len InputWord For Counter 1 to WordLength MsgBox Mid InputWord Counter 1 WordBuilder WordBuilder amp Mid InputWord Counter 1 Next MsgBox WordBuilder
39. rent expressions comparison These are used for comparing variables and expressions against other variables constants or expressions logical These are used for performing logical operations on expressions all logical operators can also be used as bitwise operators bitwise These are used for comparing binary values bit by bit all bitwise operators can also be used as logical operators Chapter 1 A Quick Introduction to Programming When you have a situation where more than one operation occurs in an expression the operations are normally performed from left to right However there are several rules Operators from the arithmetic group are evaluated first then concatenation comparison and finally logical operators This is the set order in which operations occur operators in brackets have the same precedence UO n Mod amp O lt lt gt lt ls Q Not And Or Xor Eqv Imp This order can be overridden by using parentheses Operations in parentheses are evaluated before operations outside the parentheses but inside the parentheses the normal precedence rules still apply Take a look at the following two statements A 5 6 7 8 A 5 amp 0 7 8 They look the same but they re not According to operator precedence multiplication is performed before addition so the top line gives A the value 55 6 7 42 5 8 55 By adding parenthe ses you force the additi
40. s One script may execute when the page is loaded while another script might execute when the user clicks a link or graphic These mini scripts are embedded in the HTML file and are blocked out in a syntax very similar to the one you used to define the PromptUserName function in the previous section An Event Driven Code Example As you progress through the second half of this book the finer points of event driven programming will become much clearer to you However just so you can see an example at this point type the following code into your text editor save the file with a HTM extension and then load it into Internet Explorer 6 if you are running Internet Explorer 6 7 and you are running this file off your desktop you might have to dismiss some security warnings and allow Activex lt html gt lt head gt lt title gt Simple VBScript Example lt title gt lt Scripe lenquage Voscripre gt Sub ButtonClicked window alert You clicked on the button End Sub Sri pt lt head gt lt body gt lt DbUEEON Meme BULEoOnI tyoe BULTON onciluck Buttone licked Glick Me If You Can burton lt body gt lt Ten Figure 1 8 shows the result of clicking the button on the page In this case it s only a message box but it could be much more Coding Guidelines It s a really good idea to get into healthy programming habits right from the beginning As you continue to hone your programming skills and pos
41. s another built in VBScript function that you will probably use a lot during the course of your VBScript programming Using the MsgBox function is a good way to introduce the programming concept of passing function parameters also known as arguments Some functions don t require you to pass parameters to them while others do This is because some functions take the Date function as an example this returns the current date based on the system time do not need any additional information from you in order to do their job The MsgBox function on the other hand dis plays a piece of information to the user in the form of a dialog box such as the one shown in Figure 1 2 Figure 1 2 You have to pass MsgBox a parameter because on its own it doesn t have anything useful to display in fact it will just bring up a blank pop up box The MsgBox function actually has several parameters but for now you re just going to look at one All of the other parameters are optional parameters Understanding Syntax Issues Take another look at line 5 and you ll probably notice the ampersand amp The ampersand is a VBScript operator and is used to concatenate join pieces of text together To concatenate simply means to string together This text can take the form of either a literal or a variable A literal is the opposite of a variable A variable is so named because it is exactly that a variable and can change throughout the lifetime of
42. s the ones you looked at so far the statements simply execute from the top down The script engine starts with the first statement in the script executes it moves on to the next one and then the next one and so on until the script reaches the end The execution occurs this way because the simple programs you ve written so far do not contain any branching or looping code Branching Take a look at a script that was used earlier Dim YourName Above we initialized the variable YourName InputBox Hello What is your name Above we ask for the user s name and initialize the variable MsgBox Hello amp YourName amp Pleased to meet you Above we display a greeting containing the user s name If you save this script in a file with a vbs extension and then execute it using the Windows Script Host all of the statements will be executed in order from the first statement to the last Note that it was previously mentioned that all of the statements will be executed However this isn t what you always want There is a technique that you can use to cause some statements to be executed and some not depending on certain conditions This technique is called branching VBScript supports a few different branching constructs and they are covered in detail in Chapter 5 but here we only cover the simplest and most common one which is the If Else End If construct Take a look at this modified code example Chapter 1 A Qu
43. sibly learn multiple languages these habits will serve you well Your programs will be easier for you and your fellow developers to read understand and modify and they will also contain fewer bugs When you first start writing code you have to concentrate so hard on just getting the syntax correct for the computer that it may be easy for you to forget about all the things you need to do in order to make sure your code is human friendly as well However attentiveness early on will pay huge dividends in the long run 25 Chapter 1 A Quick Introduction to Programming iy te simple VBScript Example i You clicked on the button Figure 1 8 Expect the Unexpected Always remember that anything that can happen probably will happen The idea here is to code defensively preparing for the unexpected You don t need to become totally fixated on preparing for all contingencies and remote possibilities but you can t ignore them either You especially have to worry about the unexpected when receiving input from the user from a database or from a file 26 Whenever you re about to perform an action on something ask yourself questions such as Q COO ODO What could go wrong here What happens if the file is flagged read only What happens if the file isn t there What happens if the user doesn t run the program from the right folder What happens if the database table doesn t have any records What h
44. strongly recommended that you get into the good habit of declaring all of the variables you use in your scripts Declaring variables before use makes code easier to read and to debug later Just do it By declaring variables you also give them a name in the process Here s an example of a variable declaration in VBScript Dim YourName By doing this you are in fact giving the computer an instruction to reserve some memory space for you and to name that chunk YourName From now on the computer or more accurately the VBScript engine keeps track of that memory for you and whenever you use the variable name YourNane it will know what you re talking about Variables are essential to programming Without them you have no way to hold all the data that your script will be handling Every input into the script output from the script and process within the script uses variables They are the computer s equivalent of the sticky notes that you leave all over the place with little bits of information on them All the notes are important otherwise why write them but they are also temporary Some might become permanent so you take a phone number and write it down in your address book or contact list while others are thrown away after use say after reminding you to do something This is how it works with variables too Some hold data that you might later want to keep while others are just used for general housekeeping and are disposed of as so
45. the previous loop The beginning loop block is defined by the For statement and the end is defined by the Next statement This loop is different because you can predetermine how many times it will run in this case it will go around exactly ten times The line For Counter 1 to 10 essentially tells the script engine Execute this block of code as many times as it takes to count from Chapter 1 A Quick Introduction to Programming 1 to 10 and use the Counter variable to keep track of your counting When you ve gone through this loop ten times stop looping and move on to the next bit of code Notice that every time the loop goes around including the first time through the Counter variable holds the value of the current count The first time through Counter equals 1 the second time through it equals 2 and so on up to 10 It s important to note that after the loop is finished the value of the Counter variable will be 11 one number higher than the highest value in your For statement The reason for this is that the Counter variable is incremented at the end of the loop after which the For statement tests the value of index to see if it is necessary to loop again Giving you a meaningful example of how to make use of the For Next loop isn t easy because you haven t been exposed to much VBScript just yet but here s an example that shows you don t need to know how many times the loop needs to run before you run it Dim Counte
46. till be interpreted by the script engine including the code that s not executed If any of the code that s not executed contains any syntax errors the script engine will still produce an error message to let you know 13 Chapter 1 A Quick Introduction to Programming Looping Branching allows you to tell the script to execute some lines of code but not others Looping on the other hand allows you to tell the script to execute some lines of code over and over again This is particularly useful in two situations Q When you want to repeat a block of code until a condition is True or False Q When you want to repeat a block of code a finite number of times There are many different looping constructs but this section focuses on only two of them Q The basic Do Loop While loop Q The basic For Next loop Using the Do Loop While Loop This section takes a look at the Do Loop While construct and how it can be used to repeatedly execute a block of code until a certain condition is met Take a look at the following modification of the example script Dim Greeting Dim YourName Dim TryAgain Do ievAgGe it Nou YourName InputBox Please enter your name If YourName Then MsgBox You must enter your name to continue lryAgain Yes Else Greeting Hello amp YourName amp great to meet you macl I Loop While TryAgain Yes MsgBox Greeting Notice the block of code that starts with the
47. we might end up introduc ing several unfamiliar concepts We ll be moving pretty fast but if you read along carefully trying out your hand at the examples along the way you ll be just fine Also do bear in mind that there will be a lot that we don t cover here such as LJ Architecture Q System design Q Database design Chapter 1 A Quick Introduction to Programming Q Documenting code Q Advanced testing debugging and beta testing Q Rollout and support Think of this chapter as a brief introduction to the important building blocks of programming It certainly won t make you an expert programmer overnight but it will hopefully give you the know how you ll need to get the most out of the rest of the book Variables and Data Types In this section you ll quickly move through some of the most basic concepts of programming in particular Q Using variables Using comments Q Q Using built in VBScript functions Q Understanding syntax issues Using Variables Quite simply a variable is a place in the computer memory where your script holds a piece or pieces of information or data The data stored in a variable can be pretty much anything It may be something simple like a small number like 4 something more complex like a floating point number such as 2 3 or a much bigger number like 981 12932134 Or it might not be a number at all and could be a word or a combination of letters and numbers In
48. y straightforward Once you have written a procedure calling it is no different than calling a built in VBScript procedure Advantages to Using Procedures Procedures afford several key advantages that are beyond the scope of this discussion However here are a few of the most important ones E Code such as that put in the PromptUserName function can be thought of as generic meaning that it can be applied to a variety of uses Once you have created a discreet well defined generic function such as PromptUserName you are free to reuse it any time you want to prompt users for their name Once you ve written a well tested procedure you never have to write that code again Any time you need it you just call the procedure This is known as code reuse When you call a procedure to perform a task rather than writing the code in line it makes that code much easier to read and maintain Increasing the readability and therefore the manageability and maintainability of your code is a good enough reason to break a block of code out into its own procedure When code is isolated into its own procedure it greatly reduces the effects of changes to that code This goes back to the idea of the black box As long as the procedure maintains its predictable inputs and outputs changes to the code inside of a procedure are insulated from harming the code that calls the procedure You can make significant changes to the procedure but as long as the
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