http://www.perlmonks.org?node_id=1027135


in reply to Non-Repetitive Random Numbers

The problem with most of the solutions offered is that they create a list the size of the range of numbers required (100,000) in order to obtain the sample of 30 values. Many compound that by additionally creating an array and a hash.

Not so bad whilst the range is in the 100s of thousands but once you need a range in the 10s of millions or more, your going to run out of memory pretty quick in order to create those 30 values.

This will produce as many unique values (in a random order) as you wish to fetch for any range upto 4 billion, whilst never consuming more than 512MB. (and considerably less for smaller ranges. eg. 0-100,000 will use < 13kB.) 

sub uniqRands {
    state $vec = '';
    $vec = '' if @_ == 2;
    my $n;
    1 while vec( $vec, $n = int( rand $_[0] ), 1 );
    vec( $vec, $n, 1 ) = 1;
    return $n;
}

my @sample = map uniqRands( 100_000 ), 1 .. 30;

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Re^2: Non-Repetitive Random Numbers
by FloydATC (Deacon) on Apr 05, 2013 at 13:34 UTC
    What, you're limited to 0..4294967295, this sucks!

    Just kidding, this solution is awesome :-)

    -- Time flies when you don't know what you're doing
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