Microsoft WSH and VBScript Programming for the Absolute Beginner Part 32 pot

290 The Err object also provides two very useful methods. One of these methods is the Clear() method. This method clears out or removes the previous error, ensuring that the next time a script checks for an error, it will not get a false status (that is, it won’t see an already han- dled error). To use the Clear() method, place it at the end of your error-handling routine, as demonstrated in the previous example. VBScript automatically executes the Clear() method on several occa- sions, including • Whenever the On Error Resume Next statement executes • Whenever an Exit Sub statement executes • Whenever an Exit Function statement executes The second Err object method is the Raise() method. This method allows you to generate error messages to test your error-handling routines. Without this method, the only way that you could test your error-handling routines would be to deliberately introduce an error sit- uation into your code. This method is easy to use, as demonstrated by the following: Err.Raise(500) For example, if you save the previous statement as a script and run it, you will see the error message shown in Figure 9.8. To use the Raise() method, add it, along with the error number indicating the error that you want to generate, just before the error-handling procedure that you want to test in your script. After you have validated that the error handler is working as expected, remove the Raise() statement from your VBScript. Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Figure 9.7 A custom error message generated by a VBScript error handler. Figure 9.8 Using the Err object’s Raise() method to generate a test error. Reporting Errors The best solution for errors is to prevent them from occurring in the first place; however, that’s not always possible. The next best solution is to devise a way of dealing with errors, whether it be handling them or simply ignoring them. Another option is to report errors by recording them to a log file for later review. This allows you to come back and check to see whether any errors have occurred. This is important because many time users do not report errors when they occur, allowing errors to go on forever. By logging error messages, you create an audit log that you can come back to and review from time to time to identify and fix any errors that may have occurred. When logging error messages, you have two options. The first option is to create your own custom log file. The second option is to record error messages in the Windows application event log. The second option, however, is only available if the script is running on a com- puter running Windows NT, 2000, or XP. Creating a Custom Log File To create a custom log file, you must instantiate the FileSystemObject in your VBScript and then use it’s OpenTextFile() method to open the log file so that your script can write to it as demonstrated in the following example. On Error Resume Next Err.Raise(7) Set objFsoObject = WScript.CreateObject(“Scripting.FileSystemObject”) If (objFsoObject.FileExists(“C:\ScriptLog.txt”)) Then Set objLogFile = objFsoObject.OpenTextFile(“C:\ScriptLog.txt”, 8) Else Set objLogFile = objFsoObject.OpenTextFile(“C:\ScriptLog.txt”, 2, “True”) End If objLogFile.WriteLine “Test.vbs Error: “ & Err.Number & “, Description = “ & _ Err.Description & “ , Source = “ & Err.Source objLogFile.Close() In this example, the On Error Resume Next statement is used to allow the script to recover from errors and the Err.Raise(7) statement is used to simulate an “Out of memory” error. 291 Chapter 9 • Handling Script Errors 292 Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition The rest of the script logs the error in a file called ScriptLog.txt, located on the computer’s C: drive. If the file does not exist, it is created. Error messages are appended to the bottom of the file each time they are written, allowing a running history of information to accu- mulate. For more information about how to work with the FileSystemObject and its methods and properties, refer to Chapter 8, “Storing and Retrieving Data.” You can adapt the previous example as the basis for developing an error-logging routine in your VBScripts. Just copy and paste all but the first two lines into a function and call it when- ever errors occur. Just make sure that you call the function before clearing the error. Alter- natively, you can modify the example to use variable substitution and pass the function the error number and description as arguments. Be sure you always close any file that you open before allowing your script to terminate. If you don’t, you may have problems with the file the next time you want to open it because its end-of-file marker may be missing. Recording an Error Message in the Application Event Log An alternative to creating custom log files for your scripts is to record error messages in the Windows application event log. This is achieved using the WshShell object’s LogEvent() method: On Error Resume Next Err.Raise(7) Set objWshShl = WScript.CreateObject(“WScript.Shell”) objWshShl.LogEvent 1, “Test.vbs Error: “ & Err.Number & “, Description = “ & _ Err.Description & “ , Source = “ & Err.Source In this example, an “Out of memory” error has again been simulated, only this time, the error has been written to the Windows application event log using the WshShell object’s LogEvent() method. Only two arguments were processed. The first is a number indicating the type of event being logged. Table 9.5 lists the different types of events that are supported by Windows. The second argument was the message to be recorded. Figure 9.9 shows how the message will appear when viewed from the Event Viewer. TRAP Back to the Hangman Game Now that you’ve reviewed the basic steps involved in overcoming VBScript errors, let’s return to the Hangman game and begin its development. I’m going to cover the development of this game from a different angle than in previous chapters. By now, you should have a pretty good idea of how things work, and you should be able to read and understand the scripts that you’ll see throughout the remainder of this book (just in case, I’ll leave plenty of comments in the 293 Chapter 9 • Handling Script Errors Figure 9.9 Writing error messages to the Windows application event log using the WshShell object’s LogEvent() method. Value Description 0 Indicates a successful event 1 Indicates an error event 2 Indicates a warning event 4 Indicates an informational event 8 Indicates a successful audit event 16 Indicates a failed audit event TABLE 9.5 EVENT LOG E RROR I NDICATORS 294 code to help you along). This time, I’ll provide a much higher explanation of what is going on and offer suggestions for ways to test and develop this script one step at a time. I’ll also point out techniques that you can use to test and track the results of functions within the script, so that you can validate their operation without having to first complete the entire script. Designing the Game The overall design of the Hangman game is fairly complex. To simplify things, I’ll begin the game-development process by designing a flowchart, shown in Figure 9.10, which breaks the game down into distinct units, each of which is responsible for performing a unique task. Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Figure 9.10 A flowchart providing a high-level design for the Hangman game. In addition to the Initialization Section and Main Processing Section, this script is made up of 13 separate procedures. Therefore, you will develop this game in 15 steps, as follows: 1. Create a new script, adding your VBScript template and defining the variables, constants, and arrays that are used by this script. 2. Develop the controlling logic for the Main Processing Section. 3. Using the DoYouWantToPlay() function, create an introductory game splash screen and determine whether the user wants to play. 4. Assign a list of game words to an array using the FillArray() function. 5. Create a loop in the PlayTheGame() function that controls the actual flow of the game, collecting player guesses and calling other functions as required. 6. Retrieve a randomly selected game word using the RetrieveWord() function. 7. Display space-separated underscore characters representing each letter in the game word using the InitialDisplayString() function. 8. Using the FirstLevelValidation() function, validate the player’s input to make sure the player is providing valid guesses. 9. Using the SecondLevelValidation() function, test to determine whether the player has already tried guessing a letter before accepting it as input. 10. Using the TestLetterGuess() function, check to see whether the player made an incorrect guess. 11. Using the NonGuessedString() function, create a temporary string blanking out the letters correctly guessed by the player. 12. Using the CheckIfGameWon() function, check to see whether the player has guessed all the letters that make up the mystery word. 13. Using the FlipString() function, spin through the script created in step 11, and reverse the display of each character of the string (that is, now only show the correctly guessed letters). 14. Tell the player whether he or she won or lost using the DisplayGameResults() function. 15. Display information about the game as it finishes using the SplashScreen() function. Setting Up the Script Template and Initialization Section This portion of the script, shown here, should look pretty familiar to you by now, and does not require much explanation. As you can see from the code, this section consists of the script template and the definition of the script’s constant, variables, and array. 295 Chapter 9 • Handling Script Errors 296 ‘************************************************************************* ‘Script Name: Hangman.vbs ‘Author: Jerry Ford ‘Created: 02/30/02 ‘Description: This script demonstrates how to create a game of Hangman ‘ using VBScript and the WSH. ‘************************************************************************* ‘Initialization Section Option Explicit Const cTitlebarMsg = “VBScript HANGMAN” Dim strChoice, strGameWord, intNoMisses, intNoRight, strSplashImage Dim intPlayOrNot, strMsgText, intPlayAgain, strWrongGuesses Dim strRightGuesses, blnWordGuessed, intLetterCounter Dim strTempStringOne, strTempStringTwo, strWordLetter, strDisplayString Dim intFlipCounter, intRandomNo, strProcessGuess, blnGameStatus Dim strCheckAnswer Dim astrWordList(9) ‘Define an array that can hold 10 game words Putting Together the Logic for the Main Processing Section Like the other scripts that you have seen in this book, the logic located in the script’s Main Processing Section is very straightforward. It calls upon procedures that determine whether the user wants to play, loads the game words into an array, starts the game, and ultimately ends the game by displaying a splash screen and executing the WScript.Quit() statement. ‘Main Processing Section intPlayOrNot = DoYouWantToPlay() If intPlayOrNot = 6 Then ‘User elected to play the game FillArray() PlayTheGame() End If SplashScreen() WScript.Quit() Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition 297 Chapter 9 • Handling Script Errors At this point in the script, you have enough code in place to run your first test and see whether there are any syntax errors. For now, I recommend that you go ahead and define a procedure for each of the preceding functions, placing a MsgBox() function that simply displays the name of the function inside each one. Save and execute the script and make sure that the pop-up dialogs all appear when they should. You can leave the functions as they are until you are ready to complete them. Using the WScript.Quit() method, as I did in this section, is not required. Script execution would have ceased after the display of the splash screen anyway. I added this statement for the sake of clarity, and to prevent any statements that I might have inadvertently left outside of a function in the Procedure Section from accidentally being executed. Building the DoYouWantToPlay() Function You’ve seen functions very similar to this one in previous chapters. All the DoYouWantToPlay() function does is display a clever graphic and ask the user if he or she wants to play a game of Hangman. Function DoYouWantToPlay() ‘Display the splash screen and ask the user if he or she wants to play ‘Display the splash screen and ask the user if he or she wants to play strSplashImage = Space(100) & “***********” & vbCrLf & _ “W E L C O M E T O “ & Space(68) & “*” & Space(18) & “*” & _ vbCrLf & Space(100) & “0” & Space(18) & “*” & vbCrLf & _ “V B S c r i p t H A N G M A N !” & Space(50) & “—||—” & _ Space(15) & “*” & vbCrLf & Space(99) & “/” & Space(1) & “\” & _ Space(17) & “*” & vbCrLf & Space(120) & “*” & vbCrLf & Space(120) & _ “*” & vbCrLf & space(113) & “ ******* “ & vbCrLf & _ “Would you like to play a game?” & vbCrLf & “ “ DoYouWantToPlay = MsgBox(strSplashImage, 36, cTitlebarMsg) End Function This is a good place to pause and perform another test of your script to ensure that this func- tion looks and works like it should. This test allows you to evaluate the operation of all the controlling logic in the Main Processing Section. TRICK TRICK 298 Building the FillArray() Function The FillArray() function, shown next, simply loads a list of words into an array. Later, another procedure will randomly select a game word from the array. Function FillArray() ‘Add the words to the array astrWordList(0) = “AUTOMOBILE” astrWordList(1) = “NETWORKING” astrWordList(2) = “PRACTICAL” astrWordList(3) = “CONGRESS” astrWordList(4) = “COMMANDER” astrWordList(5) = “STAPLER” astrWordList(6) = “ENTERPRISE” astrWordList(7) = “ESCALATION” astrWordList(8) = “HAPPINESS” astrWordList(9) = “WEDNESDAY” End Function You can’t perform much of a test on this function at this point, but you can always save and run the script again to see whether you have any syntax problems. You should create a tem- porary script, copy this function into it, and then create a For Next loop that processes and displays the contents of the array to ensure that the function is loading as expected. Next delete the For Next loop and add the following statements to the beginning of the temporary script: Dim astrWordList(9) ‘Define an array that can hold 10 game words FillArray() Save this script again. A little later, I’ll show you how to modify and use this temporary script to perform another test. Building the PlayTheGame() Function The PlayTheGame() function, shown next, controls the play of the Hangman game. When I developed this function, I wrote a few lines, stopped and tested it, and then wrote some more. Specifically, each time I added a call to an external function I stopped, wrote the func- tion that I called, and then did a test to be sure that everything worked before continuing. However, it would take me too long to guide you through every step along the way. Instead, Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition I’ll leave it up to you to follow this basic process, and will instead focus on the development of the other functions that make up the script, most of which are called from within the PlayTheGame() function. Function PlayTheGame() ‘Initialize variables displayed by the game’s initial pop-up dialog intNoMisses = 0 intNoRight = 0 strWrongGuesses = “” strRightGuesses = “” ‘Get the game a mystery word strGameWord = RetrieveWord() ‘Call function that formats the initial pop-up dialog’s display string strDisplayString = InitialDisplayString() strTempStringOne = strGameWord ‘Let the player start guessing Do Until intNoMisses = 6 ‘Collect the player’s guess strChoice = InputBox(vbCrLf & vbTab & strDisplayString & vbCrLf & _ vbCrLf & vbCrLf & “No. of Misses: “ & intNoMisses & _ “ “ & vbTab & “Incorrect:” & strWrongGuesses & vbCrLf _ & vbCrLf & vbCrLf & _ “Type a letter and click on OK.” , cTitleBarMsg) ‘Determine if the player has quit If strChoice = “” Then Exit Function End If strProcessGuess = FirstLevelValidation() 299 Chapter 9 • Handling Script Errors . Errors 292 Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition The rest of the script logs the error in a file called ScriptLog.txt, located on the computer’s C: drive. If the. VBScript. Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Figure 9.7 A custom error message generated by a VBScript error handler. Figure 9.8 Using the Err object’s. 9.10, which breaks the game down into distinct units, each of which is responsible for performing a unique task. Microsoft WSH and VBScript Programming for the Absolute Beginner, Second Edition Figure