VB.Net - Nested Loops

VB.Net allows using one loop inside another loop. Following section shows few examples to illustrate the concept.

Syntax

The syntax for a nested For loop statement in VB.Net is as follows −

For counter1 [ As datatype1 ] = start1 To end1 [ Step step1 ]
   For counter2 [ As datatype2 ] = start2 To end2 [ Step step2 ]
      ...
   Next [ counter2 ]
Next [ counter 1]

The syntax for a nested While loop statement in VB.Net is as follows −

While condition1
   While condition2
      ...
   End While
End While

The syntax for a nested Do...While loop statement in VB.Net is as follows −

Do { While | Until } condition1
   Do { While | Until } condition2
      ...
   Loop
Loop

A final note on loop nesting is that you can put any type of loop inside of any other type of loop. For example, a for loop can be inside a while loop or vice versa.

Example

The following program uses a nested for loop to find the prime numbers from 2 to 100 −

Module loops
   Sub Main()
      ' local variable definition 
      Dim i, j As Integer
      For i = 2 To 100
         For j = 2 To i
            ' if factor found, not prime
            If ((i Mod j) = 0) Then
               Exit For
            End If
         Next j
            If (j > (i \ j)) Then
               Console.WriteLine("{0} is prime", i)
            End If
         Next i
      Console.ReadLine()
   End Sub
End Module

When the above code is compiled and executed, it produces the following result −

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime