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Re: Randomize an array

by PipTigger (Friar)
on Sep 11, 2000 at 19:13 UTC ( #31893=note: print w/ replies, xml ) Need Help??


in reply to Randomize an array

Hey... I may be a silly goose but I'd linearly cast it into an associative array and then use the hash to provide O(n) performance at the expense of memory and relatively poor randomness (if there is a such thing as good randomness) for nearly sorted data (based on the internal hashing mechanism) ... like this:

#!/usr/bin/perl -w my @unrandom = ("red", "orange", "yellow", "green", "blue", "indigo", +"violet"); my @ununrand = (); my %temphash = (); my $indX = 0; for ($indX=0;$indX<@unrandom;$indX++) { $temphash{$unrandom[$indX]} = $indX; } @ununrand = keys(%temphash); print "Unn:@unrandom\nRnd:@ununrand\n";
Which prints:
Unn:red orange yellow green blue indigo violet Rnd:blue orange green violet yellow red indigo
Not great but fast anyway. Hope this is cute and werks for someone. TTFN.

-PipTigger

p.s. Initiate Nail Removal Immediately!


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Hashing the order out of an array (Re: Randomize an array)
by tye (Cardinal) on Sep 11, 2000 at 19:37 UTC

    That gave me an idea...

    values %{ map { rand() => $_ } @list };

    And for those of you who haven't applied Tilly's Patch® by hand and rebuilt your copy of perl and for whom performance is important:

    sub { my %hash; for(@list){ $hash{rand()}= $_ }; return values %hash }
            - tye (but my friends call me "Tye")
      Sick, slick...and imperfect. :-)

      When hashing random elements you expect to see a certain number of collisions in buckets. The second one in the bucket will be ordered after the first element. Therefore your algorithm is not a perfect shuffle, and the rising sequences test that I described in RE (tilly) 2: Randomize an array should be able to detect that.

      But even so, I like it. :-)

        Okay, here is some code to demonstrate the test that tilly described. I'm posting this because it gave me a couple of more chances to abuse the language.

        #!/usr/bin/perl -w use strict; exit main(); # This is not tilly's recommended test for how random an # ordering is. Repeatedly give a sorted list of numbers # (if not using numbers, swap the < line for the "lt" # line) to your randomizer and pass the results to # cntincseq(). If your randomizer is good, then # cntincseq() will return values that are distributed # around something close to ( list size / 2 ). sub cntincseq (\@) { my( $aList )= @_; my %seq; @seq{ 0..$#{$aList} }= @$aList; my( $idx, $cnt )= 0..1; while( ++$idx < @$aList ) { $cnt++ if $seq{$idx} < $seq{$idx-1} # $cnt++ if $seq{$idx} lt $seq{$idx-1} } return $cnt; } sub validate ($&$) { my( $cnt, $munge, $size )= @_; my @counts; for( 1..$cnt ) { my @list= 1..$size; &$munge( \@list ); push @counts, cntincseq( @list ); print " ",$counts[-1]; } return @counts; } sub fy { my( $aList )= @_; for( reverse 2..@$aList ) { my $i= rand($_); @$aList[$_-1,$i]= @$aList[$i,$_-1]; } } sub hash { my %hash; for( @{$_[0]} ) { $hash{rand()}= $_; } @{$_[0]}= values %hash; } sub main { $|= 1; my $tot; my @counts= validate( 10, \&fy, 10000 ); # And here we have a particularly nasty abuse # of C<map> for entertainment purposes only. # Do not use in production code! print "\nFisher-Yates: ", ($tot=0,map{$tot+=$_}@counts)[-1]/@counts, "\n"; @counts= validate( 10, \&hash, 10000 ); print "\nTye hash: ", ($tot=0,map{$tot+=$_}@counts)[-1]/@counts, "\n"; return 0; }

        Here are the results of comparing Fisher-Yates to my hash randomizer:

        4987 4974 5022 5012 5078 4967 4996 5007 4998 4990 Fisher-Yates: 5003.1 4368 4542 4558 4529 4586 4601 4581 4566 4549 4535 Tye hash: 4541.5

        Grumble... Back to that work on my bucketless hash table...

        Update: Having just barely posted this, I think perhaps my test is similar but not quite the same as the test tilly described. I started writing the described test then mistakenly thought I could make it work on a wider selection of inputs, fixed what looked like a typo, and got a reasonable but different test out of it.

        Update 2: And it isn't nearly as good of a test. I'll follow-up with some discussion in another node in a bit.

                - tye (but my friends call me "Tye")

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