VB.Net - 嵌套循环

VB.Net 允许在一个循环内使用另一个循环。以下部分显示了一些示例来说明这个概念。

语法

VB.Net 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.Net 中嵌套 While 循环语句的语法如下：

While condition1
   While condition2
      ...
   End While
End While

VB.Net 中嵌套 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点说明是，您可以在任何类型的循环内放入任何其他类型的循环。例如，For 循环可以在 While 循环内，反之亦然。

示例

以下程序使用嵌套 For 循环查找 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

当以上代码编译并执行时，它会产生以下结果：

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