你如何在 VBA 中重新定义一个数组？
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How do you redimension an array in VBA?
提问by Tony Toews
I'm attempting to rediminsion an array in MS Access VBA. What is the most efficient way to do this?
我正在尝试在 MS Access VBA 中重新定义一个数组。执行此操作的最有效方法是什么？
回答by Curtis Inderwiesche
How about...
怎么样...
This will preserve data already in MyArray
这将保留 MyArray 中已有的数据
Redim Preserve MyArray(15)
This will erase any previous data existing in MyArray
这将删除 MyArray 中存在的任何先前数据
Redim MyArray(15)
回答by Mark Nold
The most efficientway to redimension an array is to limit the number of times you resize that array. Every time you resize the array VB will take the whole array and copy it, wasting time and memory.
重新调整数组大小的最有效方法是限制调整数组大小的次数。每次调整数组大小时，VB 都会获取整个数组并复制它，浪费时间和内存。
If you don't know the size of your array at development time you should make the best guess to the maximum size of the array, then fill the array. Once finished filling the array you can resize it down to the correct size.
如果您在开发时不知道数组的大小，您应该对数组的最大大小做出最佳猜测，然后填充数组。完成填充数组后，您可以将其调整为正确的大小。
In loops it's often best to make this guess by doubling the size of the current array once you run out of space. You can see this in action below with RedimTestA()resizing the array each iteration (1,000,000 times) and RedimTestB()only resizing it occasionally (22 times).
在循环中，一旦空间不足，通常最好通过将当前数组的大小加倍来进行这种猜测。您可以在下面通过RedimTestA()每次迭代调整数组大小（1,000,000 次）和RedimTestB()偶尔调整数组大小（22 次）来看到这一点。
On my laptop RedimTestA()takes 3.93 seconds and RedimTestB()takes 0.41 seconds.
在我的笔记本电脑上RedimTestA()需要 3.93 秒和RedimTestB()0.41 秒。
Option Explicit
Sub RedimTest()
Dim tA, tB As Single
tA = RedimTestA(1000000)
tB = RedimTestB(1000000)
MsgBox "Test A takes : " & tA & ", and Test B takes : " & tB
End Sub
Function RedimTestA(iterations As Long) As Single
Dim t As Single
Dim i As Long
Dim aryString() As String
Dim myString As String
t = Timer
Do While i <= iterations
ReDim Preserve aryString(i) As String
aryString(i) = "ABCEFG123"
i = i + 1
Loop
RedimTestA = Timer - t
End Function
Function RedimTestB(iterations As Long) As Single
Dim t As Single
Dim i As Long
Dim aryString() As String
Dim myString As String
t = Timer
ReDim aryString(0) As String
Do While i <= iterations
If i >= UBound(aryString) Then
ReDim Preserve aryString(i * 2) As String
End If
aryString(i) = "ABCEFG123"
i = i + 1
Loop
ReDim Preserve aryString(i - 1) As String ' i - 1 becuase of the final i = i + 1
RedimTestB = Timer - t
End Function
回答by Tony Toews
Also note that you can only redim the right most dimension of a multi dimensional array.
另请注意，您只能重新调整多维数组最右边的维度。
