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Re: mem usage

by jwkrahn (Monsignor)
on May 26, 2010 at 18:07 UTC ( #841772=note: print w/ replies, xml ) Need Help??


in reply to mem usage

Your shuffle algorithm is not very good.    Better to use the one from the FAQ: How do I shuffle an array randomly? or use the shuffle function from the List::Util module.


Comment on Re: mem usage
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Re^2: mem usage
by ikegami (Pope) on May 26, 2010 at 18:30 UTC

    Your shuffle algorithm is not very good.

    It looks like a Fisher-Yates shuffle, which is what List::Util uses. Or did you spot a bug?

      Sorry, but the way he implemented it threw me off.

Re^2: mem usage
by halfcountplus (Hermit) on May 26, 2010 at 18:43 UTC
    Yeah, that's Fisher-Yates (aka Knuth #12 or something). It works in place and I've never had any problems with it,* dunno why you think it's "not very good". Kind of a wacky place to put i-- but that's pretty trivial, style wise.

    *(eg, just shuffled those 12 millon lines, no errors, since I am feeding that into something else I will notice if something gets duplicated et. al.)

      You can trim your shuffle a little by omitting the line:         next if $i==$j;.

      Swapping an item with itself doesn't affect the algorithm's fairness, and doing it once costs less, than testing n times and avoiding it once.

      And you save a little more by avoiding the list assignment:

      my $tmp = $array[ $i ]; $array[ $i ] = $array[ $j ]; $array[ $j ] = $tmp;

      Doesn't look as nice though.


      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
        Including cleaning up the counter,
        sub shuffle { my $array = shift; my $i = @$array; while ($i) { my $j = int rand($i--); my $t = $array->[$i]; $array->[$i] = $array->[$j]; $array->[$j] = $t; } }
        You can trim your shuffle a little by omitting the line: next if $i==$j;

        Good point!

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