in reply to Re: Random Derangement Of An Array
in thread Random Derangement Of An Array
Here's the approach I wanted to code up, but I've been sufficiently distracted.
After much brainracking I have been unable to come up with a less silly approach than simply encoding the combinatorial proof that d_(n+1) = n (d_n + d_(n1)) in Perl, which was essentially your idea.
All of the other ideas in this thread so far produce a biased distribution; the 'obvious' modification to the FisherYates shuffle algorithm looked promising but misses some derangements entirely.
It might be straightforward to do it purely iteratively too, but my brain works recursively so my programs do too!
#!/usr/bin/perl use strict; sub d_l_rec { # Calculates the pair (d_n, d_{n1}) # # Why calculate the pair? It's O(n) instead of O(2^n) # for the 'obvious' recursive algorithm. # # Why recursive? No need to do it iteratively. my ($n) = @_; return 1 if $n < 1; return (0, 1) if $n == 1; my ($d1, $d2) = d_l_rec($n1); my $d = ($n1) * ($d1 + $d2); return ($d, $d1); } sub random_local_derangement { # Returns a randomlychosen local derangement of # (0..($n1)). A local derangement is a derangement # *except* that the last place may be a fixed point. # # It's 'local' in the sense that, given $n people and a # hat of $n tickets you can generate a local derangement # by each person in turn pulling tickets out of the hat # until they get one that isn't theirs, ensuring that # (local to their draw) the permutation looks like it's # going to be a derangement. This is how 'Secret Santa' # derangements often seem to be organised, and this # process sometimes leaves the last person with their # own ticket. # # Unfortunately the 'Secret Santa' approach definitely # does not give uniform probabilities to each outcome. # This function, on the other hand, does. [At least, # it's definitely uniform for $n <= 12 and merely a very # good approximation for larger $n.] my ($n) = @_; if ($n == 0) { return []; } my ($i, $threshold); # A local $nderangement is either a full 'total' # $nderangement or else it is a ($n1)derangement with # a fixed point added at the end. We must choose between # these options with appropriate probability weighting. # Note that this means that l_k = d_k + d_{k1} where # l_k is the number of local kderangements and d_k is # the number of total kderangements. # # Note that d_{12} < 2^31 < d_{13} so we have to be # careful of overflow for $n > 13. Fortunately there's a # good approximation that can be used. if ($n <= 12) { # Calculate d_{$n} and d_{$n1} and a random value # $i in [0, d_{$n} + d_{$n1}) to decide which of # the two options to use. my ($dn, $dn1) = d_l_rec($n); $i = int(rand($dn+$dn1)); $threshold = $dn1; } else { # If $n is large then d_$n/d_{$n1} is very close to # $n. Therefore it'll do to pick a random $i in the # range [0, $n] and see if it is 0 or not. $i = int(rand($n+1)); $threshold = 1; # I think this is ok, but it just might contain an # offbyone error. The upshot of such an error # would be a degree of bias in the results that is # going to be hard to detect  you may have to run # it literally trillions of times to pick up a # statistically significant result. } if ($i < $threshold) { # Case 1  pick a properly local derangement my $d = random_derangement($n1); push @$d, $n1; return $d; } else { # Case 2  pick a total derangement my $d = random_derangement($n); return $d; } } sub random_derangement { # Returns a randomlychosen (total) derangement of # (0..($n1)), uniformlychosen amongst all possible # derangements. my ($n) = @_; if ($n == 0) { return []; } # There are (n1) l_{n1} of them, so pick a (uniformly) # random local ($n1)derangement and a random $m in the # range [0, $n1). my $ld = random_local_derangement($n1); my $m = int(rand($n1)); # If L_k is the set of all local kderangements and D_k # is the set of all total kderangements then the code # below encodes the proof that (n1) l_{n1} = d_n in a # bijection between [0, $n1) x L_{n1} and D_{n}. # # Since the pair ($m, $ld) are chosen uniformly, this # shows that the resulting derangement is also uniformly # chosen. if ($n2 == $ld>[$n2]) { # $ld is properly local. Therefore the desired # derangement swaps the $m'th and last places and # uses $ld to derange the other places. my $j = $n1; while ($j) { my $k = $j < $m ? $j : $j1; $ld>[$j] = $ld>[$k] < $m ? $ld>[$k] : $ld>[$k]+1; } $ld>[$n1] = $m; $ld>[$m] = $n1; return $ld; } else { # $ld is total. Therefore put the $m'th entry at the # end and put $n1 in the $m'th place. $ld>[$n1] = $ld>[$m]; $ld>[$m] = $n1; return $ld; } } sub check_derangement { # Check that we have really generated a derangement my ($n, $d) = @_; my $s = join ', ', @$d; die "Wrong length: $s ($n)" unless ($n == @$d); for (my $i = 0; $i < $n; $i++) { die "Not a derangement: $s" if ($i == $d>[$i]); die "Illegal value: $d>[$i] in $s" if ($d>[$i] < 0  $d>[$i] >= $n); } eval { my @check_unique = sort { $a <=> $b  undef } @$d; }; die "Uniqueness check failed: $s" if $@; } my $n = $ARGV[0]; my %f = (); my $c = 0; for (my $i = 0; $i < 1e6; $i++) { my $d = random_derangement($n); check_derangement($n, $d); my $s = join ',', @$d; $f{$s} += 1; $c += 1; } for my $key (sort {$f{$a} <=> $f{$b}} keys %f) { printf "%s: %0.2f%% (expected %+0.2f%%)\n", $key, 100.0*($f{$key}/ +$c), 100.0*(($f{$key}/$c)(1.0/(scalar keys %f))); } printf "Total %d (%d runs)\n", (scalar keys %f), $c;
This produces output like the following
$ ./derangements.pl 4 2,3,0,1: 11.08% (expected 0.03%) 1,0,3,2: 11.09% (expected 0.02%) 2,3,1,0: 11.10% (expected 0.01%) 1,3,0,2: 11.11% (expected 0.01%) 2,0,3,1: 11.11% (expected 0.00%) 1,2,3,0: 11.12% (expected +0.01%) 3,0,1,2: 11.12% (expected +0.01%) 3,2,1,0: 11.13% (expected +0.02%) 3,2,0,1: 11.14% (expected +0.03%) Total 9 (1000000 runs)
Clearly that's not far off uniform!


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Re^3: Random Derangement Of An Array
by blokhead (Monsignor) on Dec 07, 2008 at 20:38 UTC  
by dcturner (Initiate) on Dec 08, 2008 at 19:11 UTC 
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