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Tip of the Day
Language: VB4,VB5,VB6
Expertise: Intermediate
Sep 4, 1999

QuickSort - Sort Arrays using the QuickSort Algorithm

' QuickSort an array of any type
' QuickSort is especially convenient with large arrays (>1,000
' items) that contains items in random order. Its performance
' quickly degrades if the array is already almost sorted. (There are
' variations of the QuickSort algorithm that work good with 
' nearly-sorted arrays, though, but this routine doesn't use them.)
'
' NUMELS is the index of the last item to be sorted, and is
' useful if the array is only partially filled.
'
' Works with any kind of array, except UDTs and fixed-length
' strings, and including objects if your are sorting on their
' default property. String are sorted in case-sensitive mode.
'
' You can write faster procedures if you modify the first two lines
' to account for a specific data type, eg.
' Sub QuickSortS(arr() As Single, Optional numEls As Variant,
'  '     Optional descending As Boolean)
'   Dim value As Single, temp As Single

Sub QuickSort(arr As Variant, Optional numEls As Variant, _
    Optional descending As Boolean)

    Dim value As Variant, temp As Variant
    Dim sp As Integer
    Dim leftStk(32) As Long, rightStk(32) As Long
    Dim leftNdx As Long, rightNdx As Long
    Dim i As Long, j As Long

    ' account for optional arguments
    If IsMissing(numEls) Then numEls = UBound(arr)
    ' init pointers
    leftNdx = LBound(arr)
    rightNdx = numEls
    ' init stack
    sp = 1
    leftStk(sp) = leftNdx
    rightStk(sp) = rightNdx

    Do
        If rightNdx > leftNdx Then
            value = arr(rightNdx)
            i = leftNdx - 1
            j = rightNdx
            ' find the pivot item
            If descending Then
                Do
                    Do: i = i + 1: Loop Until arr(i) <= value
                    Do: j = j - 1: Loop Until j = leftNdx Or arr(j) >= value
                    temp = arr(i)
                    arr(i) = arr(j)
                    arr(j) = temp
                Loop Until j <= i
            Else
                Do
                    Do: i = i + 1: Loop Until arr(i) >= value
                    Do: j = j - 1: Loop Until j = leftNdx Or arr(j) <= value
                    temp = arr(i)
                    arr(i) = arr(j)
                    arr(j) = temp
                Loop Until j <= i
            End If
            ' swap found items
            temp = arr(j)
            arr(j) = arr(i)
            arr(i) = arr(rightNdx)
            arr(rightNdx) = temp
            ' push on the stack the pair of pointers that differ most
            sp = sp + 1
            If (i - leftNdx) > (rightNdx - i) Then
                leftStk(sp) = leftNdx
                rightStk(sp) = i - 1
                leftNdx = i + 1
            Else
                leftStk(sp) = i + 1
                rightStk(sp) = rightNdx
                rightNdx = i - 1
            End If
        Else
            ' pop a new pair of pointers off the stacks
            leftNdx = leftStk(sp)
            rightNdx = rightStk(sp)
            sp = sp - 1
            If sp = 0 Then Exit Do
        End If
    Loop
End Sub


Francesco Balena
 
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