使用 Do 迴圈

您可以使用 Do...Loop 陳述式，以無限次數執行陳述式區塊。迴圈會重複執行陳述式，直到條件為 True 或條件變成 True。

條件為 True 時重複執行陳述式

使用 While 關鍵字來檢查 Do...Loop 陳述式中的條件。您可以在進入迴圈之前檢查條件 (如下列 ChkFirstWhile 範例所示)，或者在迴圈至少執行一次之後再檢查 (如 ChkLastWhile 範例所示)。在 ChkFirstWhile 程序中，如果 myNum 設為 9 而不是 20，則迴圈內的陳述式永遠不會執行。在 ChkLastWhile 程序中，迴圈內的陳述式只會執行一次，因為條件已是 False。

下列範例說明 while 的用法：

範例 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

範例 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

範例 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

範例 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

範例 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

範例 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

重複執行陳述式直到條件變成 True

有兩種方法可以使用 Until 關鍵字來檢查 Do...Loop 陳述式中的條件。您可以在進入迴圈之前檢查條件 (如下列 ChkFirstUntil 範例所示)，或者在迴圈至少執行一次之後再檢查 (如 ChkLastUntil 範例所示)。只要條件為 False，迴圈就會持續執行。

下列範例說明 until 的用法：

範例 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

範例 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

範例 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

範例 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

從迴圈內部結束 Do...Loop 陳述式

您可以使用 Exit Do 陳述式來結束 Do...Loop。因為通常只會在特定情況下 (例如避免無限迴圈) 需要結束，所以應在 If...Then...Else 陳述式的 True 陳述式區塊中使用 Exit Do 陳述式。如果條件為 False，則迴圈會照常執行。

在下列範例中，指派給 myNum 的值會建立無限迴圈。If...Then...Else 陳述式會檢查此情況，以防止無限重複。

下列範例說明 Do...Loop 陳述式的用法：

範例 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

範例 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

範例 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

範例 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