While, Do While

Suppose in VB.NET we wish to continue looping until a condition is met. The number of iterations though may be unknown (as of yet).

Loop notes

With Do While, and While, we can loop indefinitely (and stop at a certain point). These loops are ways to continue iterating while one or more conditions are true.

Example

A Do While loop can have one or more conditions in its expression. Here we use 2 conditions. We continue iterating while "i" is positive and "z" is less than or equal to 8.

Then In each iteration, we change the values of these variables: "i" is decremented by 10 and "z" is incremented by 3.

Finally The loop ends because "z" exceeds 8. It is changed to 9 in the final iteration and then the loop terminates.

Also If the "z" condition in the loop expression was removed, the loop would continue until "i" was set to -10.

Module Module1
    Sub Main()
        ' Locals used in Do While loop.
        Dim i As Integer = 100
        Dim z As Integer = 0

        ' Loop.
        Do While i >= 0 And z <= 8
            Console.WriteLine("i = {0}, z = {1}", i, z)
            i = i - 10
            z = z + 3
        Loop
    End Sub
End Modulei = 100, z = 0
i = 90, z = 3
i = 80, z = 6

Do loop

With Do we can loop infinitely (or indefinitely). This loop gets a random number on each iteration. If the number is even, it uses "Exit Do" to stop the loop.

However This style of loop can be used to run until a certain condition is met. This can even be an infinite loop.

Tip We can avoid the "While true" condition for an infinite loop, and just use a Do Loop.

Module Module1

    Sub Main()
        Dim random As New Random()

        ' Enter a loop.
        Do
            ' Get random number.
            Dim number As Integer = random.Next

            ' Print random number.
            Console.WriteLine(number)

            ' Exit loop if we have an even number.
            If number Mod 2 = 0 Then
                Exit Do
            End If
        Loop

    End Sub

End Module1315809053
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While versus Until

In VB.NET we can use a Do Until loop to mean "continue until the condition is matched." A Do While loop can be used in the same way.

Here We use the "not equals" operator with a Do While loop to continue until an array element is equal to 30.

And We show a Do Until loop that has the same effect as the Do While loop. Use whichever is clearest in the program.

Module Module1
    Sub Main()
        Dim index As Integer = 0
        Dim array() As Integer = New Integer() {10, 20, 30, 40}

        ' Use "not equals" operator.
        Do While array(index) <> 30
            Console.WriteLine("[WHILE] NOT 30: {0}", array(index))
            index += 1
        Loop

        ' Use "do until" loop.
        index = 0
        Do Until array(index) = 30
            Console.WriteLine("[UNTIL] NOT 30: {0}", array(index))
            index += 1
        Loop
    End Sub
End Module[WHILE] NOT 30: 10
[WHILE] NOT 30: 20
[UNTIL] NOT 30: 10
[UNTIL] NOT 30: 20

While example

Next, we can use a "While" loop without the "Do" keyword. While-loops are useful for cases where we do not know beforehand how many iterations will run.

Detail The two statements in the While-loop's body are executed repeatedly until that condition evaluates to false.

Tip You can also use the "And" operator to put 2 conditions in the While-loop.

Note This will result in a more complex program, but sometimes multiple conditions are necessary.

Module Module1
    Sub Main()
        Dim i As Integer = 0
        While i < 100
            Console.WriteLine(i)
            i += 10
        End While
    End Sub
End Module0
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Do While in VB.NET is a loop that will continue until the exit condition is met. This can lead to infinite loops unless you are careful.