This recursive solution is very interesting, especially because of its
simplicity and relative efficiency. It is the most efficient if you
really just want to generate strings.
In the program I'm working on (a Kakuro generator/solver) I need bit
pattern representations as well and convert bitpatterns to strings and
ranges of numbers and vice versa, and also to know the number of digits
/ items in the pattern as well as the sum of all the digits.
I adapted Andrews recursive routine to accomodate Silvias request and
added my version of the sequential solution, which would be more
efficient than the recursive one if the routines BitCount() and
Bits2String() would be built-in or written in a more efficient
programming language.
Anybody any idea where I can find these? This would speed up my
solver/generator a lot. Especially the BitCount() routine.


Const PermString As String = "abcdefghij", _
MinPerm As Integer = 2, _
MaxPerm As Integer = 5

Private r As Integer

'Adapted recursive version

Sub ShowCombinations(strPrefix As String, strMain As String)
Dim strFirst As String, strRest As String
If Len(strMain) = 0 Then
If Len(strPrefix) = MinPerm And Len(strPrefix) <= MaxPerm Then
Cells(r, 2) = strPrefix
r = r + 1
End If
Exit Sub
End If
strFirst = Left(strMain, 1)
strRest = Mid(strMain, 2)
ShowCombinations strPrefix & strFirst, strRest
ShowCombinations strPrefix, strRest
End Sub

' Sequential version

Sub Combi(Total As Integer, Lo As Integer, Hi As Integer,
Representation As String)
Dim i As Long, cnt As Integer

For i = 1 To 2 ^ Total
cnt = BitCount(i)
If cnt = Lo And cnt <= Hi Then
Cells(r, 1) = Bits2String(i, Representation)
r = r + 1
End If
Next i

End Sub

Function BitCount(ByVal Pat As Long) As Integer

BitCount = 0
While Pat
If Pat And 1 Then BitCount = BitCount + 1
Pat = Int(Pat / 2)
Wend

End Function

Function Bits2String(ByVal BitPat As Long, SourceString As String) As
String
Dim i As Integer

Bits2String = ""
i = 1

While BitPat
If BitPat And 1 Then _
Bits2String = Bits2String & Mid(SourceString, i, 1)
i = i + 1
BitPat = Int(BitPat / 2)
Wend

End Function