Visual Basic Code of the Week (COTW)
Issue #25
All content and source code is Copyright (c) 1998 by C&D Programming Corp. No part of this issue can be reprinted or distributed in any manner without express written permission of C&D Programming Corp.

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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

Source Code

Just paste this code into any module and change the name of the module to basArrays.

'   Module Name:    basArrays
'   Written By:     C&D Programming Corp.
'   Create Date:    2/21/98
'   Copyright:      Copyright 1998 by C&D Programming Corp.  Source
'                   code may not be reproduced except for use in a
'                   compiled executable.  All rights reserved.  If
'                   you would like to reprint any or all of this
'                   code please email us at
Public Sub RemoveDuplicates(vList As Variant)
    Dim sItem As String
    Dim x As Long
    Dim y As Long

    On Error Goto Handler

    Select Case TypeName(vList)
        Case "ListBox"
            Dim lst As ListBox
            ' For speed.  If we know what type we are
            ' looking at things will run faster.
            Set lst = vList
            ' if we have one or less items in our list
            ' there can not be any duplicates.
            If lst.ListCount <= 1 Then
                Exit Sub
            End If
            x = -1
            While x < lst.ListCount - 1
                x = x + 1
                sItem = lst.List(x)         ' faster this way
                For y = lst.ListCount - 1 To x + 1 Step -1
                    If sItem = lst.List(y) Then
                        lst.RemoveItem y
                    End If

        Case "Variant()", "String()"
            Dim vTemp As Variant
            Dim lLowerBound As Long
            Dim lUpperBound As Long

            ' if we have an empty array there can't be any
            ' duplicates...
            If IsEmpty(vList) Then
                Exit Sub
            End If
            lLowerBound = LBound(vList)
            lUpperBound = UBound(vList)
            ' if we have one or less items in our list
            ' there can not be any duplicates.
            If lUpperBound - lLowerBound <= 0 Then
                Exit Sub
            End If

            ' search through the array and null out the
            ' duplicate values.
            x = lLowerBound - 1
            While x < lUpperBound
                x = x + 1
                If vList(x) <> vbNullString Then
                    sItem = vList(x)         ' faster this way
                    For y = lUpperBound To x + 1 Step -1
                        If sItem = vList(y) Then
                            vList(y) = vbNullString
                        End If
                End If

            ' create temporary array to manage unique values
            ReDim vTemp(lLowerBound To lUpperBound)
            ' copy all the unique values to a new
            ' temporary array
            y = lLowerBound - 1
            For x = lLowerBound To lUpperBound
                If vList(x) <> vbNullString Then
                    y = y + 1
                    vTemp(y) = vList(x)
                End If

            ' y is the new upper bound, so resize the
            ' unique array.  erase the old array and
            ' resize it.  Then assign the temporary
            ' array to the original array.
            ReDim Preserve vTemp(lLowerBound To y)
            Erase vList
            ReDim vList(lLowerBound To y)

            ' If it is a string array we have to copy each
            ' element individually.  If it is a variant
            ' array we can just assign it to the new variant.
            ' this is one of the cool things about variants.
            ' Too bad there is a fair amount of overhead
            ' involved when using variants.
            If TypeName(vList) = "String()" Then
                For x = lLowerBound To y
                    vList(x) = vTemp(x)
                vList = vTemp
            End If

            Erase vTemp
    End Select
    Exit Sub

    Err.Raise Err.Number, Err.Source, Err.Description
End Sub

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