Randomizing an array

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Randomizing an array by diotalevi (Canon) on Jun 14, 2003 at 14:31 UTC
Run the following command `perldoc -q shuffle`	[reply] [d/l]
Re: Randomizing an array by cciulla (Friar) on Jun 14, 2003 at 14:32 UTC
A quick glance at the FAQs provides: `use List::Util 'shuffle'; @shuffled = shuffle(@list);` [download] ...and... `sub fisher_yates_shuffle { my $deck = shift; # $deck is a reference to an array my $i = @$deck; while ($i--) { my $j = int rand ($i+1); @$deck[$i,$j] = @$deck[$j,$i]; } } # shuffle my mpeg collection # my @mpeg = <audio//.mp3>; fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place print @mpeg;` [download] C²	[reply] [d/l] [select]
Re: Randomizing an array by Dr. Mu (Hermit) on Jun 14, 2003 at 16:39 UTC
The above comments are spot on, but it may still be instructive to see why your rand method gave such poor results. The reason is that sort wants a comparison result that's -1 (less than), 0 (equal), or 1 (greater than). In your program, only 0 and 1 can result from the comparison. If you substitute the three-way comparison operator <=> for the >=, you'll see a big improvement in the rearranging. (Granted, rand(1) <=> 0.5 won't yield 0 very often, but that doesn't matter. What's important is that -1 and 1 occur with roughly equal preponderance.) Update: Looks like belg4mit beat me to it!	[reply]
Re: Randomizing an array by belg4mit (Prior) on Jun 14, 2003 at 16:37 UTC
FYI `perldoc sort` explains why your solution fails. Sort expects a (consistent) tri-state value be returned by the sorting routine eg; -1, 0, 1. Both `cmp` and `<=>` do, wheras your block does not. `-- I'm not belgian but I play one on TV.`	[reply]
Re: Randomizing an array by The Mad Hatter (Priest) on Jun 14, 2003 at 14:36 UTC
See `perldoc -q shuffle`. Update Seems diotalevi beat me to it. ; )	[reply] [d/l]
Re: Randomizing an array by vek (Prior) on Jun 14, 2003 at 23:29 UTC
You may find these two nodes handy: Fisher-Yates Shuffle and Fisher-Yates shuffle? -- vek --	[reply]
Re: Randomizing an array by svsingh (Priest) on Jun 15, 2003 at 08:14 UTC
This is similar to a shuffle routine I wrote in college. Basically, you swap two elements in the array at random. I used to do this n^2 times, but since you have so many elements, that may not be too effective for you. Instead I took a cue from cciulla's post and perform n swaps. For 170,000 elements, it didn't take too long to run on my end. `my @shuffledList = &shuffle(1..170000); sub shuffle { my @sl = @_; # soon to be Sorted List for my $i ( 1..$#sl ) { my $x = int(rand() * $#sl); my $y = int(rand() * $#sl); ($sl[$x], $sl[$y]) = ($sl[$y], $sl[$x]); } return @sl; }` [download] Hope this helps.	[reply] [d/l]