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Here is a recent article describing a simpler algorithm for sampling derangements. I also found slides for the presentation. Since the paper is so recent, I guess this means that a small modification of Fisher-Yates is unlikely to generate derangements, since someone would have already come up with it by now. Still, their algorithm is in-place and has better expected running time than retrying Fisher-Yates until you get a derangement.

Here is a Perl implementation I whipped up. It is slightly odd because I followed their lead and used array indexing from 1.

sub rand_derangement { my $n = shift; return if $n == 1; ## no derangements of size 1 ## precompute $D[n] == number of derangements of size n my @D = (1,0); push @D, $#D * ($D[-1] + $D[-2]) while $#D < $n; my @A = (undef, 1 .. $n); my @mark = (1, (0) x $n); my ($i, $u) = ($n, $n); while ($u > 1) { if (! $mark[$i]) { my $j = 0; $j = 1 + int rand($i-2) while $mark[$j]; @A[$i,$j] = @A[$j,$i]; if ( rand(1) < ($u-1) * $D[$u-2] / $D[$u] ) { $mark[$j] = 1; $u--; } $u--; } $i--; } return @A[1..$n]; }

blokhead


In reply to Re^3: Random Derangement Of An Array by blokhead
in thread Random Derangement Of An Array by Limbic~Region

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    [vedagiri89]: how to fix: Attempt to reload DynaLoader.pm aborted
    [hippo]: Ah, presumably 7 then.
    [vedagiri89]: CentOS Linux release 7.0.1406 (Core)
    [hippo]: How did you install libapreq2?
    [vedagiri89]: that one is done by system team from the tar i guess
    [hippo]: Really? Not using the one from the CentOS repository? Perhaps that's the problem.
    [vedagiri89]: i need when this issue will come usually
    [hippo]: How about mod_perl? Was that installed from the CentOS packge or from source?
    [vedagiri89]: from the source
    [hippo]: But APR.pm is under vendor_perl in your error message so you are mixing up modules installed from the packager and by hand. That's rarely a good plan.

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