Visual Basic 嵌套循环

VB 支持在一个循环中使用另外一个循环，以下部分显示了几个例子来说明这个概念。

语法

VB 中嵌套 For 循环语句的语法如下:

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

VB 中嵌套 While 循环语句的语法如下:

While condition1
    While condition2
    ...
    End While
End While

VB 中嵌套的 Do … While 循环语句的语法如下:

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

关于循环嵌套的最后一个注意事项是，可以把任何类型的循环放在任何其他类型的循环中。 例如，for 循环可以在 while 循环内，反之亦然。

实例

以下程序使用嵌套 for 循环来查找从 2 到 100 之间的素数:

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

结果如下:

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