Nested 循环

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2023-12-01

VB.Net允许在另一个循环中使用一个循环。以下部分显示了几个例子来说明这个概念。

语法 (Syntax)

VB.Net中nested For loop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nested While loop语句的语法如下 -

While condition1
   While condition2
      ...
   End While
End While

VB.Net中nested Do...While loop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是你可以将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。

例子 (Example)

以下程序使用嵌套for循环来查找从2到100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

编译并执行上述代码时，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