How do I sort a collection?

Question

Does anyone know how to sort a collection in VBA?

Answer 1

This is a VBA implementation of the QuickSort algorithm, which is often a better alternative to MergeSort:

Public Sub QuickSortSortableObjects(colSortable As collection, Optional bSortAscending As Boolean = True, Optional iLow1, Optional iHigh1)
    Dim obj1 As Object
    Dim obj2 As Object
    Dim clsSortable As ISortableObject, clsSortable2 As ISortableObject
    Dim iLow2 As Long, iHigh2 As Long
    Dim vKey As Variant
    On Error GoTo PtrExit

    'If not provided, sort the entire collection
    If IsMissing(iLow1) Then iLow1 = 1
    If IsMissing(iHigh1) Then iHigh1 = colSortable.Count

    'Set new extremes to old extremes
    iLow2 = iLow1
    iHigh2 = iHigh1

    'Get the item in middle of new extremes
    Set clsSortable = colSortable.Item((iLow1 + iHigh1) \ 2)
    vKey = clsSortable.vSortKey

    'Loop for all the items in the collection between the extremes
    Do While iLow2 < iHigh2

        If bSortAscending Then
            'Find the first item that is greater than the mid-Contract item
            Set clsSortable = colSortable.Item(iLow2)
            Do While clsSortable.vSortKey < vKey And iLow2 < iHigh1
                iLow2 = iLow2 + 1
                Set clsSortable = colSortable.Item(iLow2)
            Loop

            'Find the last item that is less than the mid-Contract item
            Set clsSortable2 = colSortable.Item(iHigh2)
            Do While clsSortable2.vSortKey > vKey And iHigh2 > iLow1
                iHigh2 = iHigh2 - 1
                Set clsSortable2 = colSortable.Item(iHigh2)
            Loop
        Else
            'Find the first item that is less than the mid-Contract item
            Set clsSortable = colSortable.Item(iLow2)
            Do While clsSortable.vSortKey > vKey And iLow2 < iHigh1
                iLow2 = iLow2 + 1
                Set clsSortable = colSortable.Item(iLow2)
            Loop

            'Find the last item that is greater than the mid-Contract item
            Set clsSortable2 = colSortable.Item(iHigh2)
            Do While clsSortable2.vSortKey < vKey And iHigh2 > iLow1
                iHigh2 = iHigh2 - 1
                Set clsSortable2 = colSortable.Item(iHigh2)
            Loop
        End If

        'If the two items are in the wrong order, swap the rows
        If iLow2 < iHigh2 And clsSortable.vSortKey <> clsSortable2.vSortKey Then
            Set obj1 = colSortable.Item(iLow2)
            Set obj2 = colSortable.Item(iHigh2)
            colSortable.Remove iHigh2
            If iHigh2 <= colSortable.Count Then _
                colSortable.Add obj1, Before:=iHigh2 Else colSortable.Add obj1
            colSortable.Remove iLow2
            If iLow2 <= colSortable.Count Then _
                colSortable.Add obj2, Before:=iLow2 Else colSortable.Add obj2
        End If

        'If the Contracters are not together, advance to the next item
        If iLow2 <= iHigh2 Then
            iLow2 = iLow2 + 1
            iHigh2 = iHigh2 - 1
        End If
    Loop

    'Recurse to sort the lower half of the extremes
    If iHigh2 > iLow1 Then QuickSortSortableObjects colSortable, bSortAscending, iLow1, iHigh2

    'Recurse to sort the upper half of the extremes
    If iLow2 < iHigh1 Then QuickSortSortableObjects colSortable, bSortAscending, iLow2, iHigh1

PtrExit:
End Sub

The objects stored in the collection must implement the ISortableObject interface, which must be defined in your VBA project. To do that, add a class module called ISortableObject with the following code:

Public Property Get vSortKey() As Variant
End Property