Looping Through Code

Looping allows you to run a group of statements repeatedly. Some loops repeat statements until a condition is False; others repeat statements until a condition is True. There are also loops that repeat statements a specific number of times.

The following looping statements are available in VBScript:
  • Do…Loop: Loops while or until a condition is True
  • While…Wend: Loops while a condition is True
  • For…Next: Uses a counter to run statements a specified number of times

Using Do Loops

You can use Do…Loop statements to run a block of statements an indefinite number of times. The statements are repeated either while a condition is True or until a condition becomes True.

Repeating Statements While a Condition is True

Use the While keyword to check a condition in a Do…Loop statement. You can check the condition before you enter the loop (as shown in the following ChkFirstWhile example), or you can check it after the loop has run at least once (as shown in the ChkLastWhile example). In the ChkFirstWhile procedure, if myNum is set to 9 instead of 20, the statements inside the loop will never run. In the ChkLastWhile procedure, the statements inside the loop run only once because the condition is already False. 
  Sub ChkFirstWhile()    Dim counter, myNum    counter = 0    myNum = 20    Do While myNum > 10      myNum = myNum - 1      counter = counter + 1    Loop    MsgBox "The loop made " & counter & " repetitions."  End Sub    Sub ChkLastWhile()    Dim counter, myNum    counter = 0    myNum = 9    Do      myNum = myNum - 1      counter = counter + 1    Loop While myNum > 10    MsgBox "The loop made " & counter & " repetitions."  End Sub

Repeating a Statement Until a Condition Becomes True

There are two ways to use the Until keyword to check a condition in a Do…Loop statement. You can check the condition before you enter the loop (as shown in the following ChkFirstUntil example), or you can check it after the loop has run at least once (as shown in the ChkLastUntil example). As long as the condition is False, the looping occurs. 
  Sub ChkFirstUntil()    Dim counter, myNum    counter = 0    myNum = 20    Do Until myNum = 10      myNum = myNum - 1      counter = counter + 1    Loop    MsgBox "The loop made " & counter & " repetitions."  End Sub    Sub ChkLastUntil()    Dim counter, myNum    counter = 0    myNum = 1    Do      myNum = myNum + 1      counter = counter + 1    Loop Until myNum = 10    MsgBox "The loop made " & counter & " repetitions."  End Sub

Exiting a Do…Loop Statement from Inside the Loop

You can exit a Do…Loop by using the Exit Do statement. Because you usually want to exit only in certain situations, such as to avoid an endless loop, you should use the Exit Do statement in the True statement block of an If…Then…Else statement. If the condition is False, the loop runs as usual.

In the following example, myNum is assigned a value that creates an endless loop. The If…Then…Else statement checks for this condition, preventing the endless repetition. 
  Sub ExitExample()    Dim counter, myNum    counter = 0    myNum = 9    Do Until myNum = 10      myNum = myNum - 1      counter = counter + 1      If myNum < 10 Then Exit Do    Loop    MsgBox "The loop made " & counter & " repetitions."  End Sub

Using While…Wend

The While…Wend statement is provided in VBScript for those who are familiar with its usage. However, because of the lack of flexibility in While…Wend, it is recommended that you use Do…Loop instead.

Using For…Next

You can use For…Next statements to run a block of statements a specific number of times. For loops, use a counter variable whose value increases or decreases with each repetition of the loop.

The following example causes a procedure called MyProc to execute 50 times. The For statement specifies the counter variable x and its start and end values. The Next statement increments the counter variable by 1. 
  Sub DoMyProc50Times()    Dim x    For x = 1 To 50      MyProc    Next  End Sub
Using the Step keyword, you can increase or decrease the counter variable by the value you specify. In the following example, the counter variable j is incremented by 2 each time the loop repeats. When the loop is finished, the total is the sum of 2, 4, 6, 8, and 10. 
  Sub TwosTotal()    Dim j, total    For j = 2 To 10 Step 2      total = total + j    Next    MsgBox "The total is " & total  End Sub
To decrease the counter variable, use a negative Step value. You must specify an end value that is less than the start value. In the following example, the counter variable myNum is decreased by 2 each time the loop repeats. When the loop is finished, the total is the sum of 16, 14, 12, 10, 8, 6, 4, and 2. 
  Sub NewTotal()    Dim myNum, total    For myNum = 16 To 2 Step -2      total = total + myNum    Next    MsgBox "The total is " & total  End Sub

You can exit any For…Next statement before the counter reaches its end value by using the Exit For statement. Because you usually want to exit only in certain situations, such as when an error occurs, you should use the Exit For statement in the True statement block of an If…Then…Else statement. If the condition is False, the loop runs as usual.

Parent topic: Overview of VBScript
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Looping Through Code