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Often when looping it is not possible to know in advance how many times the loop needs to be performed. For example, a loop may need to continue until some specific criteria are met. Under such circumstances the Visual Basic ''Do ... Loop'' structure is used. 

== Creating a Visual Basic Do ... Loop ==

The Visual Basic ''Do ... Loop'' structure can be used in a variety of different ways. The first, basic method involves a simple Do loop with an Exit statement called based on an ''If'' statement. The syntax for this type of Do Loop is as follows:

'''Do'''<br>
''[VB Statements]''<br>
'''If''' ''expression'' '''Then Exit Do'''<br>
'''Loop'''<br>

The following code excerpt causes the ''Do ... Loop'' to exit when the ''intTotal'' variable reaches 10:

<pre>
        Dim intTotal As Integer

        Do
            intTotal = intTotal + 1
            If intTotal = 10 Then Exit Do

            MessageBox.Show("intTotal = " + CStr(intTotal))
        Loop
</pre>

== Visual Basic Do While Loops ==
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Another way to use a ''Do Loop'' is to make use of the ''While'' keyword. A ''Do ... Loop'' that uses the ''While'' keyword will loop as long as the ''While'' expression evaluates to ''True''. 

It is important to understand that the ''While'' keyword can be used either at the start or end of the loop. When it is possible that the loop will not have to be executed at all (i.e. the criteria may already be met before the loop executes) then the While should be placed at the start of the loop. Alternatively, when the loop will need to be executed at least once before the expression will evaluate to ''True'' then the ''While'' should be placed at the end of the loop. The syntax for these two uses is as follows:

'''Do While''' ''expression''<br>
''[VB Statements]''<br>
'''Loop'''<br>

'''Do'''<br>
''[VB Statements]''<br>
'''Loop While''' ''expression''<br>

The following code samples demonstrate both uses of the ''While'' keyword. Note that the second loop will never execute because the first loop has already increased the value of ''intTotal'' to 10.

<pre>
Dim intTotal As Integer

        Do
            intTotal = intTotal + 1

            MessageBox.Show("intTotal = " + CStr(intTotal))
        Loop While intTotal < 10

        Do While intTotal < 10
            intTotal = intTotal + 1

            MessageBox.Show("intTotal = " + CStr(intTotal))
        Loop
</pre>

== Visual Basic Do Until Loops ==

The ''While'' loop causes the loop to continue until an expression ceases to evaluate to ''True''. In other words, as soon as the expression returns ''False'' the loop exists. The ''Until'' keyword does the reverse, in other words the loop continues until an expression evaluates to ''true''. As long as the expression returns false, the loop continues.

As with the ''While'' loop, the ''Until'' expression may be placed at the start or end of the loop:

'''Do Until''' ''expression''<br>
''[VB Statement]''<br>
'''Loop'''<br>

'''Do'''<br>
''[VB Statements]''<br>
'''Loop Until''' ''expression''<br>

The following code example shows the ''Do ... Until ... Loop'' in action:

<pre>
Dim intTotal As Integer

        Do
            intTotal = intTotal + 1

            MessageBox.Show("intTotal = " + CStr(intTotal))
        Loop Until intTotal > 10

        Do Until intTotal > 10
            intTotal = intTotal + 1

            MessageBox.Show("intTotal = " + CStr(intTotal))
        Loop
</pre>

As with the previous example, the second loop will not execute because the ''intTotal'' variable already exceeds 10 as a result of the previous loop.

== Summary ==

In summary, the Visual Basic ''Do ... Loop'' structure provides a way to loop a number of times when it is not possible to know in advance how many times the loop may need to iterate. When a loop is required to be performed a pre-determined number of times, then a ''For ... Loop'' may make more sense. For information on Visual Basic ''For'' loops see [[Visual Basic For Loops]].


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