# The solutions presented so far by the other monks all return a
# subset of a shuffle of the whole (except tye's) array of possible
# numbers. Compare the following beautiful algorithm, which calls
# rand() only as many times as there are numbers to pick.  It is
# attributed to Bob Floyd by Jon Bentley, in his "More Programming
# Pearls" (Addison-Wesley, 1988). Here is Bentley's explanation,
# which I've adapted for the displayed Perl code:
#
# "We can appreciate the correctness of [the algorithm]
# anecdotally. When $m is 5 and $n is 10, the algorithm first [...]
# computes in %sample a 4-element sample in the range 0..8. Next it
# assigns to $val a random integer in the range 0..9. Of the 10
# values that $val can assume, exactly 5 result in inserting 9 into
# %sample: the four values already in %sample, and the value 9
# itself. Thus element 9 is inserted into the set with the correct
# probability of 5/10."

use strict;

sub sample {
  # Returns a list of $m different random integers
  # between 0 and $n - 1.
  my ($m, $n) = @_;
  my %sample;
  my $j = $n - $m + 1;
  while ($m-- > 0) {
    my $val = int($j * rand(1));
    $sample{ exists $sample{$val} ? $j - 1 : $val } = 0;
    ++$j;
  }
  keys %sample;
}

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In reply to Re: Creating an array of unique numbers by lucs
in thread Creating an array of unique numbers by TStanley

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