From:	"John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Help on array of pointers
Date:	Sat, 02 May 1998 01:17:11 -0400
Organization:	Two pounds of chaos and a pinch of salt.
Lines:	48
Message-ID:	<354AAC57.1948@cs.com>
References:	<3549c0bb DOT 1910797 AT news DOT unisys DOT com DOT br>
NNTP-Posting-Host:	ppp110.cs.net
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Bruno Barberi Gnecco wrote:
> 
>   I'm writing a program that creates combinations. It (should) work this way:
> there's a string containing possible characters, and I need all the possible
> combinations of X characters. What I need is an array of X pointers, so I could do
> this way:

Be careful with your terminology.  What you are describing appears to
describe a permutation, not a combination.  When dealing with these
types of mathematical concepts, there are two formulas that can be
used.  Permutation (nPr) finds the number of possible arrangments of n
items in groups of r.  Combination (nCr) finds all possible _unique_
arrangements of n items in groups of r; i.e., the number of permutations
that do not use the same items as any other.

The formula for permutation can be described as follows:

        n!
nPr = ------
      (n-r)!

The formula for combination can be described as follows:

       nPr
nCr = -----
       rPr

This part is relatively simple, and storing all possible permutations
follows naturally:  create an array of nPr elements and recursively fill
it.  However, this can take a ridiculous amount of memory for large
permutations, so I assume you're trying to just display them.  As for
the code to do that, I'm tired and I don't have the time to work it
out.  However, it should be a fairly easy operation if you take
advantage of recursion.  If you explain in a bit more detail exactly
what you are trying to produce, then I can help further.

Before you do, though, I suggest asking on a more suitable newsgroup. 
While I like these kinds of mental challenges, your post has absolutely
nothing to do with DJGPP and there are better places to ask.  Please
keep this in mind, and feel free to follow up with me via email.

-- 
---------------------------------------------------------------------
|      John M. Aldrich       | "It may be better to be a live jackal|
|       aka Fighteer I       | than a dead lion, but it is better   |
|   mailto:fighteer AT cs DOT com   | still to be a live lion."            |
| http://www.cs.com/fighteer |                - Lazarus Long        |
---------------------------------------------------------------------

- Raw text -