VBA array sort function?

Question

I\'m looking for a decent sort implementation for arrays in VBA. A Quicksort would be preferred. Or any other sort algorithm other than bubble or merge would suffice.

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Answer 1

I posted some code in answer to a related question on StackOverflow:

Sorting a multidimensionnal array in VBA

The code samples in that thread include:

1. A vector array Quicksort;
2. A multi-column array QuickSort;
3. A BubbleSort.

Alain's optimised Quicksort is very shiny: I just did a basic split-and-recurse, but the code sample above has a 'gating' function that cuts down on redundant comparisons of duplicated values. On the other hand, I code for Excel, and there's a bit more in the way of defensive coding - be warned, you'll need it if your array contains the pernicious 'Empty()' variant, which will break your While... Wend comparison operators and trap your code in an infinite loop.

Note that quicksort algorthms - and any recursive algorithm - can fill the stack and crash Excel. If your array has fewer than 1024 members, I'd use a rudimentary BubbleSort.

Public Sub QuickSortArray(ByRef SortArray As Variant, _
                                Optional lngMin As Long = -1, _ 
                                Optional lngMax As Long = -1, _ 
                                Optional lngColumn As Long = 0)
On Error Resume Next

'Sort a 2-Dimensional array

' Sample Usage: sort arrData by the contents of column 3
'
'   QuickSortArray arrData, , , 3

'
'Posted by Jim Rech 10/20/98 Excel.Programming

'Modifications, Nigel Heffernan:

'       ' Escape failed comparison with empty variant
'       ' Defensive coding: check inputs

Dim i           As Long
Dim j           As Long
Dim varMid      As Variant
Dim arrRowTemp  As Variant
Dim lngColTemp  As Long


 If IsEmpty(SortArray) Then
     Exit Sub
 End If

 If InStr(TypeName(SortArray), "()") < 1 Then  'IsArray() is somewhat broken:  Look for brackets in the type name
     Exit Sub
 End If

 If lngMin = -1 Then
     lngMin = LBound(SortArray, 1)
 End If
 
 If lngMax = -1 Then
     lngMax = UBound(SortArray, 1)
 End If
 
 If lngMin >= lngMax Then ' no sorting required
     Exit Sub
 End If


 i = lngMin
 j = lngMax

 varMid = Empty
 varMid = SortArray((lngMin + lngMax) \ 2, lngColumn)

 ' We  send 'Empty' and invalid data items to the end of the list:
 If IsObject(varMid) Then  ' note that we don't check isObject(SortArray(n)) - varMid might pick up a valid default member or property
     i = lngMax
     j = lngMin
 ElseIf IsEmpty(varMid) Then
     i = lngMax
     j = lngMin
 ElseIf IsNull(varMid) Then
     i = lngMax
     j = lngMin
 ElseIf varMid = "" Then
     i = lngMax
     j = lngMin
 ElseIf varType(varMid) = vbError Then
     i = lngMax
     j = lngMin
 ElseIf varType(varMid) > 17 Then
     i = lngMax
     j = lngMin
 End If


 While i <= j

     While SortArray(i, lngColumn) < varMid And i < lngMax
         i = i + 1
     Wend

     While varMid < SortArray(j, lngColumn) And j > lngMin
         j = j - 1
     Wend


     If i <= j Then

         ' Swap the rows
         ReDim arrRowTemp(LBound(SortArray, 2) To UBound(SortArray, 2))
         For lngColTemp = LBound(SortArray, 2) To UBound(SortArray, 2)
             arrRowTemp(lngColTemp) = SortArray(i, lngColTemp)
             SortArray(i, lngColTemp) = SortArray(j, lngColTemp)
             SortArray(j, lngColTemp) = arrRowTemp(lngColTemp)
         Next lngColTemp
         Erase arrRowTemp

         i = i + 1
         j = j - 1

     End If


 Wend

 If (lngMin < j) Then Call QuickSortArray(SortArray, lngMin, j, lngColumn)
 If (i < lngMax) Then Call QuickSortArray(SortArray, i, lngMax, lngColumn)

 
End Sub