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Thursday August 26, 2004
12:32 PM

Good morning kata

[ #20582 ]

Ziggy pointed me towards a kata for today, by Norman Walsh. He wants to count the permutations of the middle letters in "morning".

I decided to use a module by Tom Phoenix:

use List::Permutor;
my $p = List::Permutor->new( split //, 'ornin' );
while( my @s = $p->next )
    $hash{ join '', 'm', @s, 'g' }++;
$, = "\n";
print keys %hash;

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  • I decided to take a more manual approach, just to make sure the recursive lobes of my brain still work:

    sub cycle {
        my @chars = @_;
        my @result;

        ## Simple case -- only one choice
        return $chars[0] if (@chars == 1);

        for (1..@chars) {
            my $char = shift(@chars);
            push @result, "$char$_" for (cycle(@chars));
            push(@chars, $char);

        return @resu

  • If, as stated, you just want the count and don't need to see the actual permutations spelled out, it is just a matter of simple arithmetic. The number of permutations of n unique objects is n!. If there are duplicate objects amongst the n, that number must be divided by k(i)! where k(i) is the number of identical objects for the i'th unique item. (That sounds complicated, but for "ornin" the number is 5!/(1!1!2!1!) since there is 1 o, 1 r, 2 n's, and 1 i. So, 120/(1x1x2x1) = 60. If the original word had