Visual Basic Code of the Week (COTW)
Issue #25
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Requirements for this Issue

The source code in this issue is designed for Visual Basic 4.0 16-bit and higher. It will probably work in VB3 with some minor changes (such as the removal of the vbNullString constant).

In this Issue

This issue introduces a subroutine that eliminates duplicate values in listboxes, variant arrays and string arrays. It provides a framework to expand the subroutine to use other with objects.



This routine works fastest when you have lots of duplicates in your objects. The approach used for listboxes is a little different than the approach used for arrays.

When removing duplicates from a listbox the routine starts at the beginning of the list and compares it with each element in the list (starting with the last element first). If it finds a duplicate it will remove that element and continue until all items have been checked. It will then move to the second element and repeat the process until it is completed.

When removing duplicates from arrays the routine starts at the beginning of the array and compares it with each element in the array. If it finds a duplicate it assigns that element the value vbNullString. It will continue this process until it is complete. Once it has removed all the duplicates it will then copy all items that are not vbNullString to a new array. When this process is complete it will copy the new array back into the original array.

Depending on your application you might need to change the value used when erasing the values in the arrays from vbNullString to something else.


Public Sub RemoveDuplicates(vList As Variant)



vList will be returned with all unique values. The type of vList depends on how it is called.

Sample Usage

Below shows several examples. There is one for each type RemoveDuplicates supports (ListBox, String(), Variant())

    Dim x As Integer
    Dim v As Variant
    Dim sData() As String


    ' Test with a ListBox
    For x = 1 To 500
        List1.AddItem "abc" & Int(Rnd * 200)
    RemoveDuplicates List1

    ' Test with a Variant Array
    ReDim v(1 To 500)
    For x = 1 To 500
        v(x) = "abc" & Int(Rnd * 200)
    RemoveDuplicates v

    ' Test with a String Array
    ReDim sData(1 To 500)
    For x = 1 To 500
        sData(x) = "abc" & Int(Rnd * 200)
    RemoveDuplicates sData

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