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Perl: the Markov chain saw

Re^2: A bad shuffle

by tlm (Prior)
on Mar 20, 2005 at 23:04 UTC ( #441071=note: print w/replies, xml ) Need Help??

in reply to Re: A bad shuffle
in thread A bad shuffle

Your algorithm looks perfectly fine to me. random_perm is optimal in the limited sense that it achieves a random permutation in place with the miniminum number of swaps. (You can't do this problem in place without swapping.) It is also nice in that its implementation varies little from language to language, and in that respect it is pretty general. In contrast, to implement the algorithm like the one you posted in C is not straightforward; Perl does a lot behind the scenes to allow one to write splice(@in, rand @in, 1) while @in.

That said, I confess that I am quite surprised by the benchmarks. Then again, this is not by any means the first time that intuitions from C programming as to the efficiencies of various operations turn out to be waaaay off when the problem is translated to Perl. Your post is a good reminder to benchmark, benchmark, benchmark. And just to act on this reminder, I decided to benchmark against your original a version of shuffle that omits the last call to splice:

sub shuffle_2 { my @in = @_; my @out; push @out, splice(@in, rand @in, 1) while @in > 1; push @out, @in; @out; }
And once again, my intuition is way off:
Rate shuffle_2 shuffle shuffle_2 5856/s -- -5% shuffle 6140/s 5% --

BTW, I should clarify that I don't recall the real name of the algorithm used by random_perm, if it actually has a name.

Update: As pointed out by several others, the algorithm used by random_perm goes by the name of Fisher-Yates. I've also found it referred to as "Knuth's shuffle."

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[marto]: neat :)
[Corion]: marto: Yeah - more involved will be the ways to automatically trigger money transfers etc. , but I didn't implement that in the old world either, so it's not really a priority for the new world :)
[Corion]: I think the next step should be to take a good look at the old API and either break it, or reimplement it using the new facilities. Maybe I should also talk to the users I know about the change if I decide to do a change

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