First, just don't call srand, it isn't useful here. At least you didn't pass an argument to it. :)

I find it unfortunate that List::Util's shuffle() doesn't support stopping the shuffle process after the first N new items have been selected. This makes using it for this situation somewhat inefficient. But to really make it inefficient, you should pick from a really huge list. But, having a really huge list means this method becomes inefficient in memory as well as in wasting a lot of time doing shuffling that doesn't matter.

So here is a solution that scales very well as far as memory and CPU use is concerned. It isn't as flexible (it only lets you select from a range of integers) and it probably isn't even faster for such small cases as originally requested, but it was an interesting challenge. Maybe I should add it to Algorithm::Loops.

It uses the same algorithm as the Fisher-Yates shuffle, but using a sparse array. 

use strict;
use warnings;

# My solution:
sub pickInts {
    my( $count, $min, $max )= @_;
    my( @pick, %pick );
    while(  0 < $count--  ) {
        my $i= int( $min + rand($max-$min+1) );
        for(  $pick{$i}  ) {
            $i= $_   if  defined;
            $_= $min++;
        }
        push @pick, $i;
    }
    return wantarray ? @pick : \@pick;
}

# For comparison:
#use List::Util qw( shuffle );
# I don't have this module, so I stole its code:
sub shuffle (@) {
  my @a=\(@_);
  my $n;
  my $i=@_;
  map {
    $n = rand($i--);
    (${$a[$n]}, $a[$n] = $a[$i])[0];
  } @_;
}

# Sample uses:

# Pick 3 integers from the range 1..10:
my @list= pickInts( 3, 1, 10 );
print "@list\n";
print join " ", (shuffle(1..10))[0..2], $/;

$|= 1;
# Pick 10 integers from the range 100000..999999:
@list= pickInts( 10, 100000, 999999 );
print "@list\n";
print join " ", (shuffle(100000..999999))[0..9], $/;

[download]

which produces the output like: 

> perl pick.pl
2 1 9
4 10 9
676232 765332 337745 257079 672444 306794 410807 382463 952100 532838
^C
>

[download]

Sorry, I didn't have time to wait for the full output. (:

- tye        

(Trivial updates applied)

In reply to Re: Select three random numbers between 1..10 (efficiency) by tye
in thread Select three random numbers between 1..10 by Perl_User

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