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VB.NET Language in a Nutshell, Second Edition by Steven Roman PhD, Paul Lomax, Ron Petrusha

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Name

Do...Loop Statement

Syntax

Do [{While | Until} condition]
   [statements]
[Exit Do]
   [statements]
Loop

or:

Do
   [statements]
[Exit Do]
   [statements]
Loop [{While | Until} condition]
condition (optional; Boolean expression)

An expression that evaluates to True or False

statements (optional)

Program statements that are repeatedly executed while, or until, condition is True

Description

Repeatedly executes a block of code while or until a condition becomes True

Rules at a Glance

  • On its own, Do...Loop infinitely executes the code that is contained within its boundaries. You therefore need to specify within the code under what conditions the loop is to stop repeating. In addition, if the loop executes more than once, the variable controlling loop execution must be modified inside of the loop. For example:

    Do
       intCtr = intCtr + 1   ' Modify loop control variable
       MsgBox("Iteration " & intCtr & " of the Do loop...")
       ' Compare to upper limit
       If intCtr = 10  Then Exit Sub   
    Loop

    Failure to do this results in the creation of an endless loop.

  • Adding the Until keyword after Do instructs your program to Do something Until the condition is True. Its syntax is:

    Do Until condition
       'code to execute
    Loop

    If condition is True before your code gets to the Do statement, the code within the Do...Loop is ignored.

  • Adding the While keyword after Do repeats the code while a particular condition is True. When the condition becomes False, the loop is automatically exited. The syntax of the Do While statement is:

    Do While condition ...

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