Data Sorting in VB6

This program sorts a list using a standard quicksort algorithm. See the code for how the algorithm works.

Then if you have the Remove Dupes option selected, the program removes duplicates. It scans through the sorted array, comparing each item to the one before it. If the item is different, it copies the item into a new array.

 Public Function RemoveDups(strings() As String) As String()
 Dim old_i As Integer
 Dim last_i As Integer
 Dim result() As String
 
     ' Make the result array.
     ReDim result(1 To UBound(strings))
 
     ' Copy the first item into the result array.
     result(1) = strings(1)
 
     ' Copy the other items
     last_i = 1
     For old_i = 2 To UBound(strings)
         If result(last_i) <> strings(old_i) Then
             last_i = last_i + 1
             result(last_i) = strings(old_i)
         End If
     Next old_i
 
     ' Remove unused entries from the result array.
     ReDim Preserve result(1 To last_i)
 
     ' Return the result array.
     RemoveDups = result
 End Function

Quicksort is a fine general purpose sorting algorithm. For different kinds of data, however, it may not be the fastest or the best. For example, it has trouble sorting lists that contain many duplicate values and the algorithm countingsort is much faster at sorting numeric data.