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Tip of the Day
Language: VB4/32,VB5,VB6
Expertise: Advanced
Apr 20, 1999



Building the Right Environment to Support AI, Machine Learning and Deep Learning

The number of dimensions of an array

Using "pure" VB, the only way to build a generic routine that returns the number of dimensions of an array passed as an argument is using a loop that repeatedly tests the LBound (o UBound) function until it fails:

Function ArrayDims(arr As Variant) As Integer
    Dim i As Integer, bound As Long
    On Error Resume Next
    For i = 1 To 60
        bound = LBound(arr, i)
        If Err Then
            ArrayDims = i - 1
            Exit Function
        End If
End Function
You can write a faster routine by peeking at the memory location where Visual Basic holds the number of dimensions of any array:

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (dest As _
    Any, source As Any, ByVal bytes As Long)

Function ArrayDims(arr As Variant) As Integer
    Dim ptr As Long
    Dim VType As Integer
    Const VT_BYREF = &H4000&
    ' get the real VarType of the argument
    ' this is similar to VarType(), but returns also the VT_BYREF bit
    CopyMemory VType, arr, 2
    ' exit if not an array
    If (VType And vbArray) = 0 Then Exit Function
    ' get the address of the SAFEARRAY descriptor
    ' this is stored in the second half of the
    ' Variant parameter that has received the array
    CopyMemory ptr, ByVal VarPtr(arr) + 8, 4
    ' see whether the routine was passed a Variant
    ' that contains an array, rather than directly an array
    ' in the former case ptr already points to the SA structure.
    ' Thanks to Monte Hansen for this fix
    If (VType And VT_BYREF) Then
        ' ptr is a pointer to a pointer
        CopyMemory ptr, ByVal ptr, 4
    End If
    ' get the address of the SAFEARRAY structure
    ' this is stored in the descriptor
    ' get the first word of the SAFEARRAY structure
    ' which holds the number of dimensions
    ' ...but first check that saAddr is non-zero, otherwise
    ' this routine bombs when the array is uninitialized
    ' (Thanks to VB2TheMax aficionado Thomas Eyde for
    '  suggesting this edit to the original routine.)
    If ptr Then
        CopyMemory ArrayDims, ByVal ptr, 2
    End If
End Function
Francesco Balena
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