NAME

Tie::Pick - Randomly pick (and remove) an element from a set.

SYNOPSIS

    use Tie::Pick;

    tie my $beatle => Tie::Pick => qw /Paul Ringo George John/;

    print "My favourite beatles are $beatle and $beatle.\n";
    # Prints: My favourite beatles are John and Ringo.

DESCRIPTION

Tie::Pick lets you randomly pick an element from a set, and have that element removed from the set.

The set to pick from is given as an list of extra parameters on tieing the scalar. If the set is exhausted, the scalar will have the undefined value. A new set to pick from can be given by assigning a reference to an array of the values of the set to the scalar.

The algorithm used for picking values of the set is a variant of the Fisher-Yates algorithm, as discussed in Knuth [3]. It was first published by Fisher and Yates [2], and later by Durstenfeld [1]. The difference is that we only perform one iteration on each look up.

If you want to pick elements from a set, without removing the element after picking it, see the Tie::Select module.

CAVEAT

Salfi [4] points to a big caveat. If the outcome of a random generator is solely based on the value of the previous outcome, like a linear congruential method, then the outcome of a shuffle depends on exactly three things: the shuffling algorithm, the input and the seed of the random generator. Hence, for a given list and a given algorithm, the outcome of the shuffle is purely based on the seed. Many modern computers have 32 bit random numbers, hence a 32 bit seed. Hence, there are at most 2^32 possible shuffles of a list, foreach of the possible algorithms. But for a list of n elements, there are n! possible permutations. Which means that a shuffle of a list of 13 elements will not generate certain permutations, as 13! > 2^32.

REFERENCES

[1]

R. Durstenfeld: CACM, 7, 1964. pp 420.

[2]

R. A. Fisher and F. Yates: Statistical Tables. London, 1938. Example 12.

[3]

D. E. Knuth: The Art of Computer Programming, Volume 2, Third edition. Section 3.4.2, Algorithm P, pp 145. Reading: Addison-Wesley, 1997. ISBN: 0-201-89684-2.

[4]

R. Salfi: COMPSTAT 1974. Vienna: 1974, pp 28 - 35.

REVISION HISTORY

    $Log: Pick.pm,v $
    Revision 1.1  1999/07/20 06:55:25  abigail
    Initial revision

AUTHOR

This package was written by Abigail.

COPYRIGHT and LICENSE

This package is copyright 1999 by Abigail.

This program is free and open software. You may use, copy, modify, distribute and sell this program (and any modified variants) in any way you wish, provided you do not restrict others to do the same.