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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.)

Comment on Re^2: mem usage
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.
      You can trim your shuffle a little by omitting the line: next if $i==$j;

      Good point!
      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; } }

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