Letter permutations/recursive algorithms

[MisterC]

I'm no good with recursive algorithms, but I know I've seen one that will do this for me. I need to know all permutations of a set of letters. Example: If the input is "ABC", the output would be:

     A    AC     CB     BCA
     B    BA     ABC    CAB
     C    BC     ACB    CBA
     AB   CA     BAC

Does anyone have a clue?

 

[wraygun]

There's probably a better way to do this. This can be scaled up for a larger input.


Dim X
Dim Y
Dim Z
Dim TempStr(100)

Private Sub Command1_Click()
For X = 1 To Len(Text1)
TempStr(X) = Mid$(Text1, X, 1)
Text2 = Text2 & TempStr(X) & Chr$(9)
Next

For Z = 1 To Len(Text1)
For X = 1 To Len(Text1)
If Z <> X Then Text2 = Text2 & TempStr(Z) & TempStr(X) & Chr$(9)
Next
Next

For Y = 1 To Len(Text1)
For Z = 1 To Len(Text1)
For X = 1 To Len(Text1)
If Y <> Z And Y <> X And Z <> Y And Z <> X Then Text2 = Text2 & TempStr(Y) & TempStr(Z) & TempStr(X) & Chr$(9)
Next
Next
Next

End Sub

 

[MisterC]

Thanks for your reply!
Your code works fine for a string with 3 or less characters, but would need to be modified if there were more.
I need something recursive that doesn't require modification for different string lengths.
Any ideas?

 

[MichaelRed]

Dont.

The recursion for permutations and combinations of more than a very few (approx 10?) &quot;tokens&quot; is simply not possible in VB, as the system will run out of resources.

Remember that the permutations is n! so 10! is already a LARGE number (3628800) doing a recursive routine means that you have n! copies of the routine &quot;loaded&quot; simultaneously.

MichaelRed
m.red@att.net

There is never time to do it right but there is always time to do it over

 

[MisterC]

Michael,
I understand, but all I need is 7 (n!=5040). Is there a non-recursive method to do this?
Thanks!

 

[MichaelRed]

I recall 'tossing the baby out with ... '. It was a nested set of loops, with the nesting level and an array dimensioned to the number of results, based on the number of tokens. I just used an index within the array to denote the token to be assigned. I never went the last mile (actually assigning the tokens and outputting the final lists), as once the token index for a position was known the remainder was just a lookup. Each loop iteration provided the assignment of ONE element in the array, so -even w/o recursion- it became very slow for more than a few tokens. MichaelRed
m.red@att.net

There is never time to do it right but there is always time to do it over