How to check whether a variant array is unallocated?

Question

   Dim Result() As Variant

In my watch window, this appears as

Expression | Value | Type
Result     |       | Variant/Variant()
         


        

Answer 1

I recommend a slightly different approach because I think using language artifacts like (Not Array) = -1 to check for initialization is difficult to read and causes maintenance headaches.

If you are needing to check for array allocation, most likely it's because you're trying to make your own "vector" type: an array that grows during runtime to accommodate data as it is being added. VBA makes it fairly easy to implement a vector type, if you take advantage of the type system.

Type Vector
    VectorData() As Variant
    VectorCount As Long
End Type

Dim MyData As Vector

Sub AddData(NewData As Variant)
    With MyData
        ' If .VectorData hasn't been allocated yet, allocate it with an
        ' initial size of 16 elements.
        If .VectorCount = 0 Then ReDim .VectorData(1 To 16)

        .VectorCount = .VectorCount + 1

        ' If there is not enough storage for the new element, double the
        ' storage of the vector.
        If .VectorCount > UBound(.VectorData) Then
            ReDim Preserve .VectorData(1 To UBound(.VectorData) * 2)
        End If

        .VectorData(.VectorCount) = NewData
    End With
End Sub

' Example of looping through the vector:
For I = 1 To MyData.VectorCount
    ' Process MyData.VectorData(I)
Next

Notice how there's no need to check for array allocation in this code, because we can just check the VectorCount variable. If it's 0, we know that nothing has been added to the vector yet and therefore the array is unallocated.

Not only is this code simple and straightforward, vectors also have all the performance advantages of an array, and the amortized cost for adding elements is actually O(1), which is very efficient. The only tradeoff is that, due to how the storage is doubled every time the vector runs out of space, in the worst case 50% of the vector's storage is wasted.