Just for the record, since everyone's talking about it but noone has shown any code:

use List::Util qw( shuffle );

sub distinct_random_int {
    my ( $num, $start, $end ) = @_;
    return if $start > $end or $num > $end - $start + 1;
    ( shuffle $start .. $end )[ 0 .. $num - 1 ];
}

[download]

Update: indeed, so long as the size of the range and the number of elements requested is within an order of magnitude, this is faster than the hash&retry solutions, in my tests by about 20%. If the discrepancy grows any larger, the hash&retry approach becomes much cheaper very quickly.

use strict;
use warnings;

use Benchmark qw( cmpthese );
use List::Util qw( shuffle );

sub randseq_shuffle {
    my ( $num, $start, $end ) = @_;
    return if $start > $end or $num > $end - $start + 1;
    ( shuffle $start .. $end )[ 0 .. $num - 1 ];
}

sub randseq_retry {
    my ( $num, $start, $end ) = @_;
    return if $start > $end or $num > $end - $start + 1;
    my %seen;
    my $range = $end - $start;
    my $base = $start;
    undef $seen{ $base + int rand $range } until $num == keys %seen;
    keys %seen;
}

for (
    [ 5, 10, 30 ],
    [ 50, 100, 300 ],
    [ 500, 1000, 3000 ],
    [ 5000, 10000, 30000 ],
    [ 50000, 100000, 300000 ],
    [ 5000, 10000, 30000 ],
    [ 500, 10000, 30000 ],
    [ 50, 10000, 30000 ],
    [ 5, 10000, 30000 ],
) {
    my ( $n, $s, $e ) = @$_;
    cmpthese -1 => {
        "shuffle($n,$s,$e)" => sub { randseq_shuffle $n, $s, $e },
        "retry($n,$s,$e)"   => sub { randseq_retry   $n, $s, $e },
    };
}

[download]

Makeshifts last the longest.

In reply to Re: How to generate distinct random numbers? by Aristotle
in thread How to generate distinct random numbers? by johnnywang

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