Being a mathematician by training, I chose to reduce this to two problems I have already solved (Permuting with duplicates and no memory and mapcar -- map for more than one list):

use mapcar;

sub nCr {
    my( $r, @n )= @_;
    die "Usage: nCr($r,@n)"   if  @n < $r;
    my @pick= ( (0)x(@n-$r), (1)x$r );
    my @c;
    do {
        push @c, [mapcar{$_[0]?$_[1]:()}\@pick,\@n];
    } while(  nextpermute( @pick )  );
    return \@c;
}

for(  @{nCr(@ARGV)}  ) {
    print "[ ",join(", ",@$_)," ]\n";
}
__END__
> nCr 2 a b c
[ b, c ]
[ a, c ]
[ a, b ]
[download]

Generally useful for algorithms and computer science questions.

Writing recursion is 2 tricks together. The first is to take a problem and break it into one or more simpler problems. The other is to figure out the simple problems that you will come to and make them special cases.

So in this case what do we want? We want a function that takes a number (m) and an array of n things, and returns all of the possible choices of m things from the list. Well we can make this simpler by deciding whether or not to choose the first element. That will reduce n, and possibly m as well, which has to be simpler.

Now what simple problems can we come down to? Well we could get down to needing no elements. In which case there is one list of no elements. Or we could get down to a situation where our list is shorter than the number we want, in which case we have no answers.

So we wind up with this:

# Takes a number and a list of elements.  Returns all possible ways to
# select that number out of the list.
sub choose_m {
  my $m = shift;
  # Special cases first
  if (0 == $m) {
    return [];
  }
  elsif (@_ < $m) {
    return ();
  }
  else {
    # This is our recursion step
    my $first = shift;
    return (
      # Things without the first element
      choose_m($m, @_),
      # Things with the first element
      map {[$first, @$_]} choose_m($m - 1, @_)
    );
  }
}
[download]

Try to write an iterative and a recursive solution to calculating Fibonacci numbers, see which is faster. Likewise Perl's RE engine uses recursion, if you look around you will find some discussions of the possible problems that result. And in this case, well try to list 30 choose 15 things, see what happens to your memory... :-)

To help you on the road: have a look at this. To whack up a first solution, take only the first r items of the permutations, and filter out the duplicates.

Hope this help,

Jeroen
"We are not alone"(FZ)

jeroenes as a side thread, if I had said that I was trying to implement a CGI parsing routine, someone would undoubtedly point me to use CGI;. I admit that I didn't search for permutations, because I didn't think of it (however, I did do some searching on similar topics as my little project). I did recall some discussion from the CB on similar subjects and, for lack of a useful search term, I figured this SoPW would get me a good answer (with either a link or code such as tye provided). So, instead of poorly reinventing the wheel, I asked my fellow wheelmakers for an appropriate wheel.

So while I can fully see your point of view (as I CBed), I think that it would be remiss of me to not request an answer, considering the purpose of SoPW and PerlMonks itself. Or has the purpose of PerlMonks been perverted from this style of question? Not to be snide, but can you point me to that change in the Need Help?? or Perl Monks FAQ?

My goal isn't to solve all of this toy project myself, but to simply finish it. If that involves requests for code (noting that this is my first such request, and happens to be the last thing I need to finish the current instantiation), then I think such questions are well within the nature of PerlMonks and SoPW.

Note: This is not an attack on your reply, I found your link to be useful. It is simply a response to the homework statement and my own interpretation of what SoPW is supposed to be for. I just wanted to address an issue that I think I tried to avoid initially.

ALL HAIL BRAK!!!

PsychoSpunk I can see your point, and thank you for softening your statement in CB ;-). Of course you are free to request some code in a SoPW, naturellement. And I think we are here to help others, at least, that's my opinion. But too often there are people here who ask and expect others to solve their problems, and they clearly do so trying to minimize the effort from their side. Well, that is not polite.

I don't think you mean to be rude, (neither do I) but simply a statement like 'this is no homework' doesn't help much (IMHO). You better state something like: I've had some discussion about this in CB, but I wouldn't know where to search for in Supersearch, etc etc...

If I had such a question, I would simply ask it in CB, and very soon I would have got some links were to look for. But that's me.

So, feel free to post anything you want here. SoPW is meant for such questions. It's merely the attitude of the posting I responded to, and I think just a little difference in the style/approach could have made your post somewhat politer. But again, that's just me. I know it is fairly easy to be misinterpreted when it comes to the intention of a posting.... I made some errors in that direction myself.

Cheers,

Jeroen
"We are not alone"(FZ)