Combination Algorithm in Excel VBA

Question

I need an algorithm which generates all possible combination of a set number and output all of them onto Excel spreadsheet.

For example, with n = 5(1,2,3,4,5) and r = 2(created a small gui for this), it will generate all possible combinations and output them into excel spreadsheet like this...

1,2
1,3
1,4
...

The order in which it prints doesn't matter. It can first print (5,1), then (1,2). Can anyone show me how to do this?

Thank you very much.

Answer 1

How about this code...

Option Explicit

Private c As Integer

Sub test_print_nCr()
  print_nCr 5, 3, Range("A1")
End Sub

Function print_nCr(n As Integer, r As Integer, p As Range)
  c = 1
  internal_print_nCr n, r, p, 1, 1
End Function


Private Function internal_print_nCr(n As Integer, r As Integer, ByVal p As Range, Optional i As Integer, Optional l As Integer) As Integer

  ' n is the number of items we are choosing from
  ' r is the number of items to choose
  ' p is the upper corner of the output range
  ' i is the minimum item we are allowed to pick
  ' l is how many levels we are in to the choosing
  ' c is the complete set we are working on

  If n < 1 Or r > n Or r < 0 Then Err.Raise 1
  If i < 1 Then i = 1
  If l < 1 Then l = 1
  If c < 1 Then c = 1
  If r = 0 then 
    p = 1
    Exit Function
  End If

  Dim x As Integer
  Dim y As Integer

  For x = i To n - r + 1
    If r = 1 Then
      If c > 1 Then
        For y = 0 To l - 2
          If p.Offset(c - 1, y) = "" Then p.Offset(c - 1, y) = p.Offset(c - 2, y)
        Next
      End If
      p.Offset(c - 1, l - 1) = x
      c = c + 1
    Else
      p.Offset(c - 1, l - 1) = x
      internal_print_nCr n, r - 1, p, x + 1, l + 1
    End If
  Next

End Function