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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, 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.
Using For Each...Next
A For Each...Next loop is similar to a For...Next
loop. Instead of repeating the statements a specified number
of times, a For Each...Next loop repeats a group of
statements for each item in a collection of objects or for
each element of an array. This is especially helpful if you
don't know how many elements are in a collection.
In the following HTML code example, the contents of a Dictionary
object is used to place text in several text boxes.
<HTML>
<HEAD><TITLE>Forms and
Elements</TITLE></HEAD>
<SCRIPT LANGUAGE="VBScript">
<!--
Sub cmdChange_OnClick
Dim d 'Create a variable
Set d = CreateObject("Scripting.Dictionary")
d.Add "0",
"Athens" 'Add some keys and items
d.Add "1", "Belgrade"
d.Add "2", "Cairo"
For Each I in d
Document.frmForm.Elements(I).Value = D.Item(I)
Next
End Sub
-->
</SCRIPT>
<BODY>
<CENTER>
<FORM NAME="frmForm"
<Input Type = "Text"><p>
<Input Type = "Text"><p>
<Input Type = "Text"><p>
<Input Type = "Text"><p>
<Input Type = "Button" NAME="cmdChange"
VALUE="Click Here"><p>
</FORM>
</CENTER>
</BODY>
</HTML>
Try Yourself
1. Open a text editor type following code.
<HTML>
<HEAD><TITLE>Hello World</TITLE>
<SCRIPT LANGUAGE="VBScript">
<!--
Sub Button1_OnClick
MsgBox "Hello to VBScript"
End Sub
-->
</SCRIPT>
</HEAD>
<BODY>
<H3>Welcome to VBScript</H3><HR>
<FORM><INPUT NAME="Button1"
TYPE="BUTTON" VALUE="Click
Here"></FORM>
</BODY>
</HTML>
2. Save the file "Hello.HTM", open the file in
browser.
3. Click the button it shows the message "Hello to
VBScript"
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