Monday, July 4, 2011

VBScript Looping



Looping Statements

Looping statements are used to run the same block of code a specified number of times.
In VBScript we have four looping statements:
  • For...Next statement - runs code a specified number of times
  • For Each...Next statement - runs code for each item in a collection or each element of an array
  • Do...Loop statement - loops while or until a condition is true
  • While...Wend statement - Do not use it - use the Do...Loop statement instead

For...Next Loop

Use the For...Next statement to run a block of code a specified number of times.
The For statement specifies the counter variable (i), and its start and end values. The Next statement increases the counter variable (i) by one.

Example

<html>
<body>

<script type="text/vbscript">
For i = 0 To 5
  document.write("The number is " & i & "<br />")
Next
</script>

</body>
</html>

The Step Keyword

With the Step keyword, you can increase or decrease the counter variable by the value you specify.
In the example below, the counter variable (i) is INCREASED by two, each time the loop repeats.
For i=2 To 10 Step 2
  some code
Next
To decrease the counter variable, you must use a negative Step value. You must specify an end value that is less than the start value.
In the example below, the counter variable (i) is DECREASED by two, each time the loop repeats.
For i=10 To 2 Step -2
  some code
Next

Exit a For...Next

You can exit a For...Next statement with the Exit For keyword.
For i=1 To 10
  If i=5 Then Exit For
  some code
Next

For Each...Next Loop

A For Each...Next loop repeats a block of code for each item in a collection, or for each element of an array.

Example

<html>
<body>

<script type="text/vbscript">
Dim cars(2)
cars(0)="Volvo"
cars(1)="Saab"
cars(2)="BMW"

For Each x In cars
  document.write(x & "<br />")
Next
</script>

</body>
</html>

Do...Loop

If you don't know how many repetitions you want, use a Do...Loop statement.
The Do...Loop statement repeats a block of code while a condition is true, or until a condition becomes true.

Repeat Code While a Condition is True

You use the While keyword to check a condition in a Do...Loop statement.
Do While i>10
  some code
Loop
If i equals 9, the code inside the loop above will never be executed.
Do
  some code
Loop While i>10
The code inside this loop will be executed at least one time, even if i is less than 10.

Repeat Code Until a Condition Becomes True

You use the Until keyword to check a condition in a Do...Loop statement.
Do Until i=10
  some code
Loop
If i equals 10, the code inside the loop will never be executed.
Do
  some code
Loop Until i=10
The code inside this loop will be executed at least one time, even if i is equal to 10.

Exit a Do...Loop

You can exit a Do...Loop statement with the Exit Do keyword.
Do Until i=10
  i=i-1
  If i<10 Then Exit Do
Loop
The code inside this loop will be executed as long as i is different from 10, and as long as i is greater than 10.
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