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Permutations and combinations

by merlyn (Sage)
on Jul 25, 2000 at 16:49 UTC ( #24270=snippet: print w/replies, xml ) Need Help??
Description: Here's a couple of code snippets I keep handy when I want to generate all combinations (no replacement) and permutations (all possible ways of combining a list of varying things). Enjoy.

You can probably see that I speak Perl with a lisp sometimes.


print "permute:\n";
print "[", join(", ", @$_), "]\n" for permute([1,2,3], [4,5,6], [7,8,9
+]);
print "combinations:\n";
print  "[", join(", ", @$_), "]\n" for combinations(1..5);

sub permute {
  my $last = pop @_;
  unless (@_) {
    return map [$_], @$last;
  }
  return map { my $left = $_; map [@$left, $_], @$last } permute(@_);
}
sub combinations {
  return [] unless @_;
  my $first = shift;
  my @rest = combinations(@_);
  return @rest, map { [$first, @$_] } @rest;
} 
Replies are listed 'Best First'.
RE: Permutations and combinations
by AltBlue (Chaplain) on Aug 21, 2000 at 13:19 UTC
    interesting approach that reminded me about some old project that needed some permutations generations stuff... so, i've checked up that code and here i come with a reviewed standalone version:
    #!/usr/bin/perl -w # DESCRIPTION: Generate permutations in lexicographic order # USAGE: ./permlex.pl <term1> <term2> <term3> ..... use strict; die "bleah... nothing to permutate\n" if $#ARGV<0; my @terms = @ARGV; my $n = $#ARGV; my @a = (0..$n); genperm(); exit(0); sub genperm { print join(" ",@terms[@a]),"\n"; my ($k,$j) = ($n-1,$n); $k-- while ($k>=0 and $a[$k]>$a[$k+1]); return(0) if ($k<0); $j-- while ($a[$k]>$a[$j]); swap($j,$k++); $j=$n; swap($j--,$k++) while ($j>$k); genperm(); } sub swap { my ($i,$j) = @_; my $t = $a[$i]; ($a[$i],$a[$j]) = ($a[$j],$t); }

    as you may see, it's a pure lexicographic permutations generator algorithm, as in the books ;-)

    oh, not to forget, just checked up on cpan and found out there is a Algorithm::Permute module. here is a lame example for module users ;-)

    #!/usr/bin/perl -w use strict; die "bleah... nothing to permutate\n" unless defined @ARGV; use Algorithm::Permute qw(permute permute_ref); print join(" ", @$_), "\n" for permute(\@ARGV);

    --
    AltBlue.

      your sub swap can be written without $t:
      sub swap { my ($i,$j) = @_; @a[$i,$j] = @a[$j,$i]; }
        lol, you bother to look upon this old piece of code :))

        heh, of course that your snippet is good, but a swap routine could be written even simpler:

        sub swap { reverse @_ }
        ... heh, it's not the case for that code thou, as the array is a global one :) cheers.

        --
        AltBlue.

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[marto]: the ticketing system does not accept calls via email, nor has it a working API. It's tied into Active Directory for authentication and the Solaris boxes aren't on that domain
[Corion]: The one thing I haven't figured out a solution to is how to get an edge-trigger instead of sending an email every 5 minutes if the usage is above 90%. I want one mail when it goes over 90% but no more emails as long as it stays between 90% and 95%.
[Corion]: marto: Clever! ;)
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[marto]: they already have an entire BMC patrol system, which they disabled, because it was sending out spurious messages. So rather than fix the issue, or even find out what it was, they turned it off. No messages, can't be any problems, right?
[Corion]: marto: But having open tickets / incidents increases the pressure on them ;) Of course, likely your contract / SLA specifies an upper limit for the number of incidents :-D
[Corion]: marto: Ow ...

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