How To Change QuickSort Ascending To Descending

Question

I am using a quicksort algorithm for listing information in ascending order.  How would I change it to list the items in descending order.  The site I was at said to switch the comparison operators but that doesn't seem to work.  Here is the code for what I have :

    'This function sorts the particular list box using the Quick Sort algorithm [ASCENDING]
    Private Sub Sort_List_Boxes_Ascending(ByRef vArray() As String, ByVal inLow As Integer, ByVal inHi As Integer)
        Dim pivot As Object
        Dim tmpSwap As Object
        Dim tmpLow As Long
        Dim tmpHi As Long

        tmpLow = inLow
        tmpHi = inHi

        pivot = vArray((inLow + inHi) \ 2)

        While (tmpLow <= tmpHi)

            While (vArray(tmpLow) < pivot And tmpLow < inHi)
                tmpLow = tmpLow + 1
            End While

            While (pivot < vArray(tmpHi) And tmpHi > inLow)
                tmpHi = tmpHi - 1
            End While

            If (tmpLow <= tmpHi) Then
                tmpSwap = vArray(tmpLow)
                vArray(tmpLow) = vArray(tmpHi)
                vArray(tmpHi) = tmpSwap
                tmpLow = tmpLow + 1
                tmpHi = tmpHi - 1
            End If

        End While

        If (inLow < tmpHi) Then Sort_List_Boxes_Ascending(vArray, inLow, tmpHi)
        If (tmpLow < inHi) Then Sort_List_Boxes_Ascending(vArray, tmpLow, inHi)

    End Sub

Thanks!

Answer 1

Well, first thing - Are you aware that Array.Sort uses the QuickSort to do it's sorting?  So, there is no real reason to implement QuickSort at all for your particular operation.

Now assuming your in VB2008, you can quite simply do as you ask:

Option Explicit On
Option Strict On

Module Module1

    Sub Main()
        Dim a() As String = {"D", "A", "C", "B"}

        ' assending
        Array.Sort(a)
        PrintArray(a)

        'descending
        Array.Sort(a, Function(x, y) x.CompareTo(y) * -1)
        PrintArray(a)
    End Sub

    Private Sub PrintArray(ByVal a() As String)
        Console.WriteLine(String.Join(", ", a))
    End Sub
End Module

HTH

---

Tom Shelton

Answer 2

I am having issues with your solution as I keep receiving "Item cannot be null when I try adding my array back to the listbox" .

Could you/someone direct me on reversing the sort order of the quicksort

*CORRECTION*  You were right Tom, my apologies.  An error in my code on my part.  Thank you very much!

But if someone could still provide the descending info that would be appreciated