Array Assignment

In various programming languages, including VBA, arrays inhabit a twilight zone between the basic types and the classes.

For example, there is usually a syntax for literal arrays. In VBA this is the function Array(), which returns an array of Variant (since the elements could be of mixed types):

TypeName(Array(1,2,3))     –>  Variant()

However, you have to pass this round as a Variant:

Sub TestLiteral()
    Dim i As Integer
    i = GetNth(2, Array(1, 2, 3, 4, 5))
    MsgBox i
End Sub

Function GetNth(n As Integer, arr As Variant) As Variant
    GetNth = arr(n - 1)      '-1 Because Base 0
End Function

Writing ‘arr() As Variant’ in the function header throws a compiler error.

Here’s another curious feature. Suppose we have a 2-D array: this is effectively an array of 1-D arrays. However, you can’t assign a 1-D array to one of the elements of the 2-D array: compiler error again. Here’s some code:

Sub Test2()

    Dim arr2d(1 To 2, 1 To 3) As Integer
    Dim arr1d(1 To 3) As Integer

    Dim i As Integer
    For i = 1 To 3       'stuff some values into arr1d
        arr1d(i) = i * 2
    Next

'    arr2d(1) = arr1d   'throws a compiler error
End Sub

Here’s the error:

arrayError

However, the left-hand and right-hand sides of the assignment have exactly the same type, Integer(1 To 3), as can be seen in the Locals window:

arrayErrorLocals

MSDN says:

This type of assignment works only for dynamic arrays of the same type. The two arrays must both be dynamic arrays, and they must be declared as the exact same type

So I suppose we must conclude that the type of the second level of a 2-D array is not considered to be the same type as an apparently equivalent 1-D array, even though this seems both reasonable and useful. Anyone with knowledge of the language implementation, please comment. Time for a cup of tea, methinks.

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2 Responses to “Array Assignment”


  1. 1 geoffness July 25, 2009 at 11:47 am

    Nice post Roy. I’ve often thought that this is a weakness of VB/VBA that makes it harder to use. The bottom line in the case you talk about here is that the 1-d array contains the correct number of elements of the correct type – why should you not be able to assign it in the way you have?

    I can’t comment on the language implementation as I don’t know enough about it, but as a guess I’d say this issue might have something to do with static typing…at least in part. A dynamically typed language like Python or Ruby will not either require variable type declaration or perform a compile time check like this.


  1. 1 cv writer exeter Trackback on October 30, 2014 at 12:54 am

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