使用 Do 循环

可以使用 Do...Loop 语句无限次地运行语句块。这些语句会在条件为 True 时重复执行，或者会一直重复执行直至条件变为 True。

条件为 True 时重复执行语句

使用 While 关键字检查 Do...Loop 语句中的条件。您可以在进入循环之前检查条件（如以下 ChkFirstWhile 示例中所示），也可以在循环至少运行一次后检查条件（如 ChkLastWhile 示例中所示）。在 ChkFirstWhile 过程中，如果将 myNum 设置为 9 而不是 20，则循环中的语句将永远不会运行。在 ChkLastWhile 过程中，循环中的语句仅运行一次，因为条件已经为 False。

以下示例说明了 while 的用法：

示例 1：

Sub ChkFirstWhile()
        Dim counter, myNum
        counter = 0
         myNum = 20
        Do While myNum > 10
           myNum = myNum - 1
           counter = counter + 1
        Loop
        'This loop made 10 repetitions."
End Sub

示例 2：

Sub ChkFirstWhile()
    Dim counter, myNum
    counter = 0
    myNum = 9
    Do While myNum > 10
        myNum = myNum - 1
        counter = counter + 1
    Loop
    'This loop made 0 repetitions.
End Sub

示例 3：

Sub ChkLastWhile()
        Dim counter, myNum
        counter = 0
        myNum = 9
        Do
                myNum = myNum - 1
                counter = counter + 1
        Loop While myNum > 10
        'This loop made 1 repetition.
End Sub

示例 4：

Sub ExampleWithDoWhile()
        Dim counter, myNum
        counter = 0
        myNum = 15
        Do While myNum > 10
                        myNum = myNum - 2
                        counter = counter + 1
        Loop
' This loop reduced myNum to 9 and made 3 repetitions
End Sub

示例 5：

Sub ExampleWithDoLoopWhile()
         Dim counter, myNum
        counter = 0
        myNum = 5
        Do
                        myNum = myNum + 1
                        counter = counter + 1
        Loop While myNum < 10
' This loop increased myNum to 10 and made 5 repetitions.
End Sub

示例 6：

Sub ChkLastWhile()
   Dim counter, myNum
   counter = 0
   myNum = 9
   Do
      myNum = myNum - 1
      counter = counter + 1
   Loop While myNum > 10
   'This loop made 1 repetition.
End Sub

重复执行语句直至条件变为 True

可以通过两种方法来使用 Until 关键字检查 Do...Loop 语句中的条件。您可以在进入循环之前检查条件（如以下 ChkFirstUntil 示例中所示），也可以在循环至少运行一次后检查条件（如 ChkLastUntil 示例中所示）。只要条件为 False，就会发生循环。

以下示例说明了 until 的用法：

示例 1：

Sub ChkFirstUntil()
   Dim counter, myNum
   counter = 0
   myNum = 20
   Do Until myNum = 10
      myNum = myNum - 1
      counter = counter + 1
   Loop
   'This loop made 10 repetitions.
End Sub

示例 2：

Sub ChkLastUntil()
   Dim counter, myNum
   counter = 0
   myNum = 1
   Do
      myNum = myNum + 1
      counter = counter + 1
   Loop Until myNum = 10
'This loop made 9 repetitions.
End Sub

示例 3：

Sub ExampleWithDoUntil()
        Dim counter, myNum
        counter = 0
        myNum = 20
        Do Until myNum < 15
                        myNum = myNum - 2
                        counter = counter + 1
        Loop
'This loop reduced myNum to 14 and made 3 repetitions.
End Sub

示例 4：

Sub ExampleWithDoLoopUntil()
    Dim counter, myNum
    counter = 0
    myNum = 5
    Do
        myNum = myNum + 2
        counter = counter + 1
    Loop Until myNum >= 15
    'This loop increased myNum to 15 and made 5 repetitions.
End Sub

从循环内部退出 Do...Loop 语句

可以使用 Exit Do 语句退出 Do...Loop。由于通常只希望在某些情况下退出（例如，为了避免无限循环），因此应在 If...Then...Else 语句的 True 语句块中使用 Exit Do 语句。如果条件为 False，则循环将照常运行。

在以下示例中，为 myNum 赋予了一个用于创建无限循环的值。If...Then...Else 语句会检查此条件，从而防止无休止的重复。

以下示例说明了 Do 循环语句的用法：

示例 1：

Sub ExitExample()
   Dim counter, myNum
      counter = 0
      myNum = 9
      Do Until myNum = 10
         myNum = myNum - 1
         counter = counter + 1
         If myNum < 10 Then Exit Do
      Loop
      'This loop made 1 repetition.
End Sub

示例 2：

Sub PreventEndlessLoop()
        Dim counter, myNum
        counter = 1
        myNum = 0
        Do
                        myNum = myNum + 1
                        counter = counter + 1
                        If counter > 10 Then 
                                Exit Do   ' Avoids endless loop by exiting after 10 iterations.
                        End If
        Loop
        'This loop made 11 repetitions.
End Sub

示例 3：

Sub ExampleWithDoWhile()
    Dim counter, myNum
    counter = 0
    myNum = 20
    Do While myNum > 10
        myNum = myNum - 2
        counter = counter + 1
        If myNum < 15 Then Exit Do   ' Exit the loop early if myNum is less than 15.
    Loop
    'This loop reduced myNum to 14 and made 3 repetitions.
End Sub

示例 4：

Sub NestedLoopsExample()
    Dim i, j
    For i = 1 To 3
        j = 0
        Do
            j = j + 1
            If j > 2 Then Exit Do   ' Exit inner loop after 2 iterations.
        Loop
    Next
End Sub