VB Forum / General 2 / November 2004

Recursion is normally overkill, but here is a case where it solves a real
problem, that of generating n permutations of a string's characters (e.g.:
for ABC generate: ABC, ACB, BCA, BAC, CAB, CBA). Note that it will work for
most any number of characters (until you run out of memory or crash
something in that category), but it takes forever after 9 or 10.

------- cut here ---------------------------------------------

'To test, make a form and define everything not defined by the
' following form code:

Option Explicit

' Description:Takes in a string and writes out all possible
' permutations of the inputted characters to a file
' using a simple recursive routine.

Private Sub Form_Load()
   txtArg.Text = ""         'clear input text
   lblStatusMessage.Caption = "Enter Letters to Permutate"
End Sub

Private Sub cmdDoIt_Click()

   Dim s As String, s1 As String, i As Integer
   
   s = UCase$(txtArg.Text)      'check the input
   
   If Len(s) > 10 Then
       MsgBox "Only 10 letters allowed as input for this test", , "FIX
LETTERS"
       Exit Sub
   End If
   
   For i = 1 To Len(s)
       s1 = Mid$(s, i, 1)
       If s1 < "A" Or s1 > "Z" Then
           MsgBox "All input text for this test must be letters", , "INPUT
ERROR"
           Exit Sub
       End If
   Next i
   
   lblStatusMessage.Caption = "Working... Please wait..."
   
   Open App.Path & "\words.txt" For Output As #1
   PermutateToFile s, ""    'get all permutations of given letter group
   Close #1
   
   lblStatusMessage.Caption = "DONE"
   
   'Display the results (3,628,800 lines and 42 megs for 10 letters)
   Shell "notepad " & App.Path & "\words.txt", vbNormalFocus

End Sub

Private Sub cmdExit_Click()
   Unload Me
End Sub

'*********** PROCEDURES **************************************

'Recursive Procedure to generate all permutations of a string
'  and write them to a file
Private Sub PermutateToFile(ByVal LETTERS_IN As String, _
                       ByVal TEXTBUILT_IN As String)
   Dim i As Integer
   
   If Len(LETTERS_IN) = 1 Then
       Print #1, TEXTBUILT_IN & LETTERS_IN
       Exit Sub
   End If
   
   For i = 1 To Len(LETTERS_IN)
       PermutateToFile Mid(LETTERS_IN, 1, i - 1) & Mid(LETTERS_IN, i + 1),
_
                       TEXTBUILT_IN & Mid(LETTERS_IN, i, 1)
   Next i

End Sub

<snip>

Funny that, I quite often find that recursive solutions are the
simplest.

Obviously only where they are appropriate
- to use rucursion when it is inappropriate is just plain daft

No you didn't.

<snipped/>

"good ol' days" and "C" just reminded me of an expression from way back
when...

"To Iterate is Human, to Recurse Divine..."

;-)