Shuffling an array and then picking the first k elements would do what I want, but that's still linear in n. The "sample" module seems to be efficient, though.

If you're worried about efficiency, you should measure first.

#!/usr/bin/env perl
use warnings;
use strict;
use Math::Prime::Util qw/randperm/;
use List::Util qw/shuffle/;
use Algorithm::Numerical::Sample qw/sample/;
use Benchmark qw/cmpthese/;

die "Usage: $0 N K\n" unless @ARGV==2;
my ($N,$K) = @ARGV;

cmpthese(-2, {
    mpu => sub {
        my @range = randperm($N, $K);
    },
    shuf => sub {
        my @range = (shuffle 0 .. $N-1)[0 .. $K - 1];
    },
    samp => sub {
        my @range = sample (-set => [0 .. $N-1], -sample_size => $K);
    },
});
__END__

$ perl bench.pl 10 5
          Rate samp shuf  mpu
samp  326934/s   -- -86% -91%
shuf 2388541/s 631%   -- -31%
mpu  3456068/s 957%  45%   --

$ perl bench.pl 100 5
          Rate  samp  shuf   mpu
samp   76377/s    --  -86%  -98%
shuf  539280/s  606%    --  -86%
mpu  3744910/s 4803%  594%    --

$ perl bench.pl 10000 5
          Rate    samp    shuf     mpu
samp     835/s      --    -86%   -100%
shuf    5885/s    605%      --   -100%
mpu  3719610/s 445489%  63101%      --

$ perl bench.pl 100000 5000
       Rate  samp  shuf   mpu
samp 72.2/s    --  -87%  -99%
shuf  562/s  678%    --  -91%
mpu  6544/s 8958% 1064%    --

[download]

Not surprising, since Math::Prime::Util's randperm is implemented in C, and has a bunch of different methods for picking K of N depending on set sizes. See the source.

In reply to Re^2: Pick k numbers at random by haukex
in thread Pick k numbers at random by Chuma

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