NDXShellsort

Sub NdxShellSort(arr As Variant, ndx() As Long, Optional numEls As Variant, _
Optional descending As Boolean)

Dim value As Variant
Dim index As Long, index2 As Long
Dim firstItem As Long
Dim distance As Long
Dim tempNdx As Long

' account for optional arguments

If IsMissing(numEls) Then numEls = UBound(arr)
firstItem = LBound(arr)
' init index array if necessary

If ndx(firstItem) = 0 And ndx(UBound(ndx)) = 0 Then
For index = firstItem To UBound(ndx)
ndx(index) = index
Next
End If

' find the best value for distance

Do
distance = distance * 3 + 1
Loop Until distance > numEls

Do
distance = distance \ 3
For index = distance + 1 To numEls
tempNdx = ndx(index)
value = arr(tempNdx)
index2 = index
Do While (arr(ndx(index2 - distance)) > value) Xor descending
ndx(index2) = ndx(index2 - distance)
index2 = index2 - distance
If index2 <= distance Then Exit Do
Loop
ndx(index2) = tempNdx
Next
Loop Until distance = 1
End Sub


Indexed ShellSort of an array of any type

Indexed Sorts are sort procedures that sort an index array
instead of the main array. You can then list the items in
sorted member by simply scanning the index, as in
For i = 1 To numEls: Print arr(ndx(i)): Next

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 NdxShellSortS(arr() As Single, ndx() As Long,
' Optional numEls As Variant, Optional descending As Boolean)

' Dim value As Single