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

The syntax for the first ver-sion of the Do...Whileloop is as follows: Do While condition statements Loop Conditionis expressed in the form of an expression, like this: intCounter = 0 Do

The Do Whilestatement creates a loop that runs as long as a specified condition is true VBScript supports two different versions of the Do Whileloop The syntax for the first ver-sion of the Do Whileloop is as follows:

Do While condition

statements

Loop

Conditionis 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 2every time the loop executes As a result, the loop iterates five times

As the Whilekeyword 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 Whilestatement 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 Whileloop 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

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

Figure 6.9

Using a

Do While loop

to collect new

address book

entries.

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 intCounterby 1, it increments the value of intCounterby –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 Ifstatement to test the value of intNoExecutionseach time the loop is processed and to execute the Exit Dostatement in the event that something goes wrong Of course, there is no substitute for good program design and careful testing

The VBScript Do Untilstatement 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 Untilstatement 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 Untilloop 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

In this example, the loop has been set up to execute until the value of a variable named intMissedGuessesbecomes 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 intMissedGuessesto 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 Untilstatement is as follows:

Do

statements

Loop Until condition

As you can see, the Untilkeyword and its associated conditionhave been moved from the beginning to the end of the loop, thus ensuring the loop executes at least once

While…Wend

The While Wendstatement 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

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 intCountis used to control the loop’s exe-cution strCountListis 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 Ifstatement, 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

Figure 6.11

Counting to

10 using a

While End

loop.

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 intNoGuesseskeeps track

of the number of guesses the player makes

Dim intUserNumber, intRandomNo, strOkToEnd, intNoGuesses

Finally, set the initial value of intNoGuessesto 0, like this:

intNoGuesses = 0

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 strOkToEndvariable 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

.

.

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 OKbutton If the player’s input is not 0characters 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 Ifstatement 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

The second of the three sets of statements provides the player with help if his or her guess

is too low The value of strOkToEndis 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 strOkToEndis 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