October 19, 2009

Assigning and returning arrays in Visual Basic 6

Visual Basic 6 adds two important features to arrays. First, you can perform assignments between arrays. Second, you can write procedures that return arrays. You can assign arrays only of the same type and only if the target is a dynamic array. (The latter condition is necessary because Visual Basic might need to resize the target array.)
ReDim a(10, 10) As Integer
Dim b() As Integer
' Fill the a array with data (omitted).
b() = a() ' This works!
It's no surprise that native assignment commands are always faster than the corresponding For…Next loops that copy one item at a time. The actual increment in speed heavily depends on the data type of the arrays and can vary from 20 percent to 10 times faster. A native assignment between arrays also works if the source array is held in a Variant. Under Visual Basic 4 and 5, you could store an array in a Variant, but you couldn't do the opposite—that is, retrieve an array stored in a Variant variable and store it back in an array of a specific type. This flaw has been fixed in Visual Basic 6:
Dim v As Variant, s(100) As String, t() As String
' Fill the s() array (omitted).
v = s() ' Assign to a Variant.
t() = v ' Assign from a Variant to a dynamic string array.
You often use the capacity to assign arrays to build functions that return arrays. Notice that pair of brackets at the end of the first line in the following procedure:
Function InitArray(first As Long, Last As Long) As Long()
ReDim result(first To Last) As Long
Dim i As Long
For i = first To Last
result(i) = i
InitArray = result
End Function
The new capability of returning arrays lets you write highly versatile array routines. Visual Basic 6 itself includes a few new string functions—namely Join, Split, and Filter—that rely on it. (You'll find more about these new string functions in Chapter 5). Here are two examples of what you can do with this intriguing feature:
' Returns a portion of a Long array
' Note: fails if FIRST or LAST are not valid
Function SubArray(arr() As Long, first As Long, last As Long, _
newFirstIndex As Long) As Long()
Dim i As Long
ReDim result(newFirstIndex To last _ first + newFirstIndex) As Long
For i = first To last
result(newFirstIndex + i - first) = arr(i)
SubArray = result
End Function
' Returns an array with all the selected items in a ListBox
Function SelectedListItems(lst As ListBox) As String()
Dim i As Long, j As Long
ReDim result(0 To lst.SelCount) As String
For i = 0 To lst.ListCount - 1
If lst.Selected(i) Then
j = j + 1
result(j) = lst.List(i)
End If
SelectedListItems = result
End Function

The above article doesn't give you any guarantee and the sole purpose of this article is to share my learning in the way I understood. Any comments to refine this article are welcome with great pleasure. Please report any breaking link by commenting below.

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