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Re^2: mem usage

by halfcountplus (Hermit)
on May 26, 2010 at 18:43 UTC ( #841785=note: print w/ replies, xml ) Need Help??


in reply to Re: mem usage
in thread mem usage

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.)


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Re^3: mem usage
by BrowserUk (Pope) on May 26, 2010 at 19:17 UTC

    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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