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.
An 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.
Detail 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
1322882256
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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Summary. 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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Nov 11, 2023 (simplify).
