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Having 0 in that list is a mistake since the qsort algorithm takes that to mean that they are equal and hence it only needs to use one of those to compare with others.

I personally believe that before someone else points out that the node I'm replying to is some eight years old, I'd better do so myself: the reason why I'm doing it that I've seen this very thread referenced quite recently; which means that some people may still be looking here for info they will try to actually use. And here a terribly wrong technique has been advertised, so I'm writing this post for the benefit of potential readers to the effect of warning them not to use it! Namely, even if amended of the "0 in that list" problem, the general idea of shuffling a list by sorting it with a routine returning random values e.g.:

my @shuffled = sort {.5<rand} @input;

is flawed! Several reasons why it is are explained in a thread which came two years later. To put it briefly, if you really want to use sort for this, at the very least you must generate the list of random values in advance, whatever actual technique you actually choose in the end. E.g.:

my %r; my @shuffled = sort { ($r{$a}||=rand) <=> ($r{$b}||=rand) } @input;

or

my @r = map rand, @input; my @shuffled = @input[sort {$r[$a] <=> $r[$b]} 0..$#input];

Update: as a novice, back in my old days at the Monastery whenever I happened to see a post of mine which -exactly like this one and its companion- turned out to have a negative reputation, I would edit it to the effect of inserting an update and whine about the downvotes or more precisely asking why it earned them. Of course, I've learnt how to do better in the meantime, and generally refrain to. Except this time: since I would like to "thank" all those geniuses who downvoted this node on part of the poor newbie who will stumble upon it one day, and judging from the reputation will think that my complaints are moot, thus possibly following the advice of the broken shuffling technique... only to be bitten in the neck, but hopefully not later than having spread the word, because "it is so cool..."

--
If you can't understand the incipit, then please check the IPB Campaign.

In reply to Re: RE (tilly) 2: Randomize an array by blazar
in thread Randomize an array by Zebu

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