Microsoft WSH and VBScript Programming for the Absolute Beginner Part 21 ppt

180 Do…While The Do While statement creates a loop that runs as long as a specified condition is true. VBScript supports two different versions of the Do While loop. The syntax for the first ver- sion of the Do While loop is as follows: Do While condition statements Loop Condition is expressed in the form of an expression, like this: intCounter = 0 Do While intCounter < 10 intCounter = intCounter + 2 Loop In this example, the expression (intCounter < 10) allows the loop to continue as long as the value of intCounter is less than 10. The value of intCounter is initially set to 0, but is increased by 2 every time the loop executes. As a result, the loop iterates five times. As the While keyword has been placed at the beginning of the loop, the loop will not execute if the value of counter is already 10 or greater. The syntax for the second format of the Do While statement is as follows: Do statements Loop While condition As you can see, the While keyword had been moved from the beginning to the end of the loop. Therefore, the loop will always execute at least once, even if the condition is initially false. Let’s look at another example of the Do While loop in action. In this example, the Do While loop is set up to collect names and phone numbers for an address book. The loop uses the VBScript InputBox() function to collect the names and phone numbers. The names and addresses are added to a variable string and formatted such that, when displayed, each entry is listed on a separate line. The user may enter as many names and numbers as he or she wishes. When done adding new address book entries, all he or she must do is type “Quit” as the final entry. Dim intCounter, strAddressBook, strAddressEntry intCounter = 0 Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Do While strAddressEntry <> “Quit” intCounter = intCounter + 1 strAddressEntry = InputBox(“Please type a name, a space, and then “ & _ “the person’s phone number”, “Personal Address Book”) If strAddressEntry <> “Quit” Then strAddressBook = strAddressBook & strAddressEntry & vbCrLf End If Loop MsgBox strAddressBook, ,”New Address Book Entries = “ & intCounter – 1 Figure 6.9 displays a list of four names entered as this script executed. Alternatively, you could have written this example as shown next. In this example the While keyword and its associated condition have been moved to the end of the loop. However, the script still operates exactly as in the previous example. Dim intCounter, strAddressBook, strAddressEntry intCounter = 0 Do intCounter = intCounter + 1 strAddressEntry = InputBox(“Please type a name, a space, and then “ & _ “the person’s phone number”, “Personal Address Book”) If strAddressEntry <> “Quit” Then strAddressBook = strAddressBook & strAddressEntry & vbCrLf End If Loop While strAddressEntry <> “Quit” MsgBox strAddressBook, ,”New Address Book Entries = “ & intCounter –1 181 Chapter 6 • Processing Collections of Data Figure 6.9 Using a Do While loop to collect new address book entries. 182 One of the dangers of working with loops is that you may accidentally create a loop that has no way of terminating its own execution. This is an endless loop . Endless loops run forever, needlessly consuming computer resources and degrading a computer’s performance. For example, look at the following: intCounter = 0 Do While intCounter < 10 intCounter = intCounter + 1 WScript.Echo intCounter Loop When executed this script counts from 1 to 10. Now look at the next script. intCounter = 0 Do While intCounter < 10 intCounter = intCounter - 1 WScript.Echo intCounter Loop It looks almost exactly like the previous example, only instead of incrementing the value of intCounter by 1, it increments the value of intCounter by –1, creating an endless loop. One way to protect against the creation of an endless loop is to put in a safety net, like this: intCounter = 0 Do While intCounter < 10 intCounter = intCounter – 1 intNoExecutions = intNoExecutions + 1 WScript.Echo intCounter If intNoExecutions > 99 Then Exit Do End If Loop As you can see, I added to the script a variable called intNoExecutions that I then used to keep track of the number of times that loop iterated. If the loop iterates 100 times, then something is wrong. So I added an If statement to test the value of intNoExecutions each time the loop is processed and to execute the Exit Do statement in the event that something goes wrong. Of course, there is no substitute for good program design and careful testing. Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Do…Until The VBScript Do Until statement creates a loop that executes as long as a condition is false (that is, until it becomes true). VBScript supports two versions of the Do Until statement. The syntax for the first version is as follows: Do Until condition statements Loop Let’s look at an example that demonstrates how this loop works. In this example, shown next, the script prompts the player to answer a question and uses a Do Until loop to allow the user up to three chances to correctly answer the question. Dim intMissedGuesses, strPlayerAnswer intMissedGuesses = 1 Do Until intMissedGuesses > 3 strPlayerAnswer = InputBox(“Where does Peter Pan live?”) If strPlayerAnswer <> “Neverland” Then intMissedGuesses = intMissedGuesses + 1 If intMissedGuesses < 4 Then MsgBox “Incorrect: You have “ & 4 – intMissedGuesses & _ “ guesses left. Please try again.” Else MsgBox “Sorry. You have used up all your chances.” End If Else intMissedGuesses = 4 MsgBox “Correct! I guess that you must believe in Faith, Trust “ & _ “and Pixy Dust!” End If Loop 183 Chapter 6 • Processing Collections of Data 184 In this example, the loop has been set up to execute until the value of a variable named intMissedGuesses becomes greater than 3. The variable is initially set equal to 1 and is incre- mented by 1 each time the loop executes, unless the player provides a correct answer, in which case the script sets the value of intMissedGuesses to 4 in order to arbitrarily terminate the loop’s execution. Figure 6.10 demonstrates the execution of this script by showing the pop-up dialog that appears if the player guesses incorrectly on his or her first attempt to answer the question. The syntax of the second form of the Do Until statement is as follows: Do statements Loop Until condition As you can see, the Until keyword and its associated condition have been moved from the beginning to the end of the loop, thus ensuring the loop executes at least once. While…Wend The While Wend statement creates a loop that executes as long as a tested condition is true. The syntax for this loop is as follows: While condition statements Wend The Do While and Do Until loops provide the same functionality as the While Wend loop. The general rule of thumb, therefore, is that you should use one of the Do loops in place of this statement. However, I’d be remiss if I failed to show you how this statement works, so take a look at the following example: Dim intCounter, strCountList Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Figure 6.10 Using a Do Until loop to provide the player with three chances to correctly answer a question. intCounter = 0 While intCounter < 10 intCounter = intCounter + 1 strCountList = strCountList & intCounter & vbCrLf Wend MsgBox “This is how to count to 10:” & vbCrLf & vbCrLf & _ strCountList, , “Counting Example” This example begins by initializing two variables. intCount is used to control the loop’s exe- cution. strCountList is used to build a formatted script containing the numbers counted by the script. The loop itself iterates 10 times. Figure 6.11 shows the output created by this example when run using the WScript execution host. Back to the Guess a Number Game Let’s turn our attention back to the Guess a Number game. In this game, the player is prompted to guess a randomly generated number between 1 and 100. Each time the player takes a guess, the script will check to see if the correct number was guessed. If not, the script will provide a hint to help the player on his or her next guess. Developing this script will enhance your knowledge and understanding of working with the Do Until loop. You will also work with the If statement, and learn how to work with a number of new built-in VBScript functions. Designing the Game The Guess a Number game begins by asking the player to guess a number between 1 and 100, and then helps the user guess the number by providing hints. This project has five steps. 185 Chapter 6 • Processing Collections of Data Figure 6.11 Counting to 10 using a While End loop. 186 These steps are 1. Add the standard documentation template and define any variables, constants, or objects used by the script. 2. Generate a random number between 1 and 100. 3. Create a loop that runs until the player either guesses the correct answer or gives up. 4. Test the player’s answer to see whether it’s valid. 5. Test the player’s answer to see whether it is too low, too high, or correct. As a kind of project bonus, once you have completed the Guess a Number game, I’ll show you how to create a VBScript desktop shortcut for it. I’ll also show you how to use shortcuts to configure the Windows Start Menu and Quick Launch toolbar. Beginning the Guess a Number Game Begin by creating a new script and adding your script template. ‘************************************************************************* ‘Script Name: GuessANumber.vbs ‘Author: Jerry Ford ‘Created: 10/19/02 ‘Description: This script plays a number-guessing game with the user ‘************************************************************************* ‘Initialization Section Option Explicit Next, create a constant and assign it the text message to be used in the title bar of the script’s pop-up dialogs. Const cGreetingMsg = “Pick a number between 1 – 100” Define four variables as shown. Use intUserNumber to store the player’s numeric guess. intRandomNo stores the script’s randomly generated number. strOkToEnd is a variable the script uses to determine whether the game should be stopped, and intNoGuesses keeps track of the number of guesses the player makes. Dim intUserNumber, intRandomNo, strOkToEnd, intNoGuesses Finally, set the initial value of intNoGuesses to 0, like this: intNoGuesses = 0 Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Generating the Game’s Random Number The following statements are next and are responsible for generating the game’s random number: ‘Generate a random number Randomize intRandomNo = FormatNumber(Int((100 * Rnd) + 1)) The Randomize statement ensures that a random number is generated each time the game is played. The last statement uses the following built-in VBScript functions to generate a number between 1 and 100. • Rnd(). Returns a randomly generated number. • Int(). Returns the integer portion of a number. • FormatNumber(). Returns an expression that has been formatted as a number. Creating a Loop to Control the Game Now you’ll need to set up the Do Until loop that controls the game’s execution. In this example, the loop executes until the value assigned to the strOkToEnd variable is set to yes. Do Until strOkToEnd = “yes” ‘Prompt users to pick a number intUserNumber = InputBox(“Type your guess:”,cGreetingMsg) intNoGuesses = intNoGuesses + 1 . . . Loop As you can see, the only statement inside the loop, for now, prompts the player to guess a number and keeps track of the number of guesses made by the player. Testing Player Input Now let’s put together the code that performs validation of the data supplied by the player. ‘See if the user provided an answer If Len(intUserNumber) <> 0 Then ‘Make sure that the player typed a number If IsNumeric(intUserNumber) = True Then 187 Chapter 6 • Processing Collections of Data 188 . . . Else MsgBox “Sorry. You did not enter a number. Try again.”, , cGreetingMsg End If Else MsgBox “You either failed to type a value or you clicked on Cancel. “ & _ “Please play again soon!”, , cGreetingMsg strOkToEnd = “yes” End If The first validation test is performed using the built-in VBScript Len() function. It is used to ensure that the player actually typed in a number before clicking on the OK button. If the player’s input is not 0 characters long, the game continues to the next test. Otherwise, an error message is displayed, and the value of strOkToEnd is set to yes, terminating the loop and ending the game. If the length test is passed, then the script performs a second valida- tion test on the player’s input. This time, the built-in VBScript IsNumeric() function is used to make sure that the player typed a number instead of a letter or other special character. If a number was typed, then the game continues. If a number was not typed, then an error message is displayed, but the game continues with the next iteration of the loop. Determine Whether the Player’s Guess Is High, Low, or Correct There are three more sets of statements that need to be added to the script. They will be inserted one after another, just after the If statement that performs the game’s second val- idation test. The first of these three sets of statements is shown here. It begins by verifying that the user’s guess matches the game’s randomly selected number. Then it displays a message congratu- lating the player, showing the random number, and showing the number of guesses that it took for the player to guess it. Finally, the value of strOkToEnd is set equal to yes. This ter- minates the loop and allows the game to end. ‘Test to see if the user’s guess was correct If FormatNumber(intUserNumber) = intRandomNo Then MsgBox “Congratulations! You guessed it. The number was “ & _ intUserNumber & “.” & vbCrLf & vbCrLf & “You guessed it “ & _ “in “ & intNoGuesses & “ guesses.”, ,cGreetingMsg strOkToEnd = “yes” End If Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition The second of the three sets of statements provides the player with help if his or her guess is too low. The value of strOkToEnd is set equal to no. This ensures that the loop that controls the game will continue. ‘Test to see if the user’s guess was too low If FormatNumber(intUserNumber) < intRandomNo Then MsgBox “Your guess was too low. Try again”, ,cGreetingMsg strOkToEnd = “no” End If Finally, the last collection of statements provides the player with help if his or her guess is too high. The value of strOkToEnd is set equal to no. This ensures that the loop that controls the game will continue. ‘Test to see if the user’s guess was too high If FormatNumber(intUserNumber) > intRandomNo Then MsgBox “Your guess was too high. Try again”, ,cGreetingMsg strOkToEnd = “no” End If The Final Result Let’s put all the pieces of the Guess a Number script together and see how it looks fully laid out. ‘************************************************************************* ‘Script Name: GuessANumber.vbs ‘Author: Jerry Ford ‘Created: 10/19/02 ‘Description: This script plays a number-guessing game with the user ‘************************************************************************* ‘Initialization Section Option Explicit Const cGreetingMsg = “Pick a number between 1 - 100” Dim intUserNumber, intRandomNo, strOkToEnd, intNoGuesses intNoGuesses = 0 189 Chapter 6 • Processing Collections of Data . wrong. Of course, there is no substitute for good program design and careful testing. Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Do…Until The VBScript Do Until. WSH and VBScript Programming for the Absolute Beginner, Second Edition Generating the Game’s Random Number The following statements are next and are responsible for generating the game’s random number: ‘Generate. If Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition The second of the three sets of statements provides the player with help if his or her guess is too low. The