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Creating tuples based on sets

by waxmop (Beadle)
on Apr 16, 2003 at 14:41 UTC ( #250910=perlquestion: print w/replies, xml ) Need Help??

waxmop has asked for the wisdom of the Perl Monks concerning the following question:

I have a project that I'm not making good headway on. The end product has to be written in C, but I want to make a perl prototype first.

Here's the deal: I need to write a function that gets two arguments:

  • A set, sort of like {A, B, C}
  • an integer, that specifies the size of the tuples to create.
This function will return a reference to a 2-d array. Each row in the array will be a tuple. For example,
my @set = qw(A B C); my $output = mk_tuples(@set, 2);
And $output would be a ref to a 2-d array that looks like this (each row is an array):
The number of tuples to be created is equal to the number of elements in the set raised to the power of the size of the tuples. In this case, that turns out to be 3**2, or 9.

The stumbling block for me is how to create the tuples. I can hear a voice in my head that says recursion might be useful, but I'm just not seeing it right now. Any help would be appreciated.

Replies are listed 'Best First'.
Re: Creating tuples based on sets
by Thelonius (Priest) on Apr 16, 2003 at 15:36 UTC
    #!perl -w use strict; my @set = qw(a b c); my @tuples = mk_tuples(3, \@set); for (@tuples) { print "(", join(", ", @$_), ")\n"; } sub mk_tuples { my $n = shift; my $setr = shift; my $len = 0; my @result = (); my @hold = (); mk_tuples_r($n, $setr, \@hold, \@result); return @result; } sub mk_tuples_r { my ($n, $setr, $holdr, $resultr) = @_; my $len = 1 + @$holdr; for my $e (@$setr) { push @$holdr, $e; if ($len == $n) { push @$resultr, [ @$holdr ]; } else { mk_tuples_r($n, $setr, $holdr, $resultr); } pop @$holdr; } }
      Your response was the best combination of easy to understand and successful. I love this website.
Re: Creating tuples based on sets
by mojotoad (Monsignor) on Apr 16, 2003 at 15:23 UTC
    Update: I missed the part about needing arbitrarily sized tuples. This only groups in sets of bad. :(

    Original message follows...

    Ah, the joys of dot products. No need for recursion:

    #!/usr/bin/perl -w @set = qw(A B C); print join("\n", map(join(',', @$_), @{&make_tuples(\@set, \@set)})); sub make_tuples { my($set1, $set2) = @_; my @product; foreach my $x (0 .. $#$set1) { foreach my $y (0 .. $#$set2) { push(@product, [$set1->[$x], $set2->[$y]]); } } \@product; }

    I daresay I just answered a homework question for you, but so be it.


      At the end of the program, you just do

      Does this do the same thing as
      return \@product;
      There is a definite possibility that I am missing something, but isn't waxmop asking for something that will solve for an arbitrary number of elements (in the solution), not just 2 necessarily?

        Yeah, I need the function to fit any of these situations:

        my @set1 = qw(A B C D); my @set2 = qw(X Y Z); mk_tuples(@set1, 2); mk_tuples(@set2, 5);

        Here's a side question. How come this doesnt' work in perl?

        mk_tuples(["A", "B"], 3);
        I can't replace an array with just a list of elements.
Re: Creating tuples based on sets
by The Mad Hatter (Priest) on Apr 16, 2003 at 14:55 UTC
    You'll have to have multiple foreach loops that go through each item in the set. For tuples of size 2, you'll need two. For tuples of size 3, you'll need three. You get the idea. You might be able to use Algorithm::Loops to help you with this.

      Exactly what I was thinking. (:

      sub tuples { my( $count, @set )= @_; my @out; nestedLoops( [ ([0..$#set]) x $count ], { Code => sub { push @out, [@set[@_]] } } ); return \@out; } my $tuples= tuples( 2, qw(A B C) ); for my $tuple ( @$tuples ) { print "@$tuple\n"; }
      or, if you want to take advantage of iterators:
      sub tupleMaker { my( $count, @set )= @_; my $iter= nestedLoops( [ ([0..$#set]) x $count ] ); return sub { @set[ $iter->() ] }; } my $gen= tupleMaker( 2, qw(A B C) ); while( my @tuple= $gen->() ) { print "@tuple\n"; }
      which means you can deal with tons of tuples without needing tons of RAM.

                      - tye
Re: Creating tuples based on sets
by perlguy (Deacon) on Apr 16, 2003 at 15:15 UTC

    Here is my solution. This is not a great example, as I didn't have time to really think this out, but...

    use strict; my @set = qw(A B C); mk_tuples(2, 0, \@set, []); # put the number of tuples first print join "\n", map join(' ', @$_), get_tuples(); { my @full_builds; sub mk_tuples { my ($depth, $previous_depth, $set, $built) = @_; push(@full_builds, $built) if $depth == $previous_depth; for my $current_depth ($previous_depth..$depth) { for (@$set) { mk_tuples($depth, $previous_depth+1, $set, [@$built, $_]); } } } sub get_tuples { die "none made\n" unless @full_builds; return @full_builds; } }

    Hope that helps.

    Update: As you can see, mine requires 4 arguments: 1) the depth to stop at, 2) the depth we are at currently, 3) the full set of possibles to iterate over, and 4) the partial_builds to be passed recursively. I know this isn't exactly what you want... Sorry.

Re: Creating tuples based on sets
by adrianh (Chancellor) on Apr 16, 2003 at 21:50 UTC

    One more variation ;-)

    CPAN provides us with Set::CrossProduct, which allows us to write your mk_tuples like this:

    use Set::CrossProduct; sub mk_tuples(\@$) { my ($arrayref, $n) = @_; [ Set::CrossProduct->new( [($arrayref) x $n] )->combinations ]; }; my @set = qw(A B C); my $output = mk_tuples(@set, 2);
Re: Creating tuples based on sets
by shotgunefx (Parson) on Apr 17, 2003 at 04:46 UTC
    You could use my code at Non- recursive permutation of arrays. like so.
    my @elements = qw(a b c); my $tupler = make_permutator(([@elements]) x 2 ); while (my @tuple = $tupler->() ){ print @tuple,"\n"; }
    If a ref return value is wanted, just change "return @els" to "return \@els" in the sub that make_permutator returns.


    "To be civilized is to deny one's nature."

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