Tie::Pick - Randomly pick (and remove) an element from a set.
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.
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.
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.
R. Durstenfeld: CACM, 7, 1964. pp 420.
R. A. Fisher and F. Yates: Statistical Tables. London, 1938. Example 12.
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.
R. Salfi: COMPSTAT 1974. Vienna: 1974, pp 28 - 35.
The current sources of this module are found on github, git://github.com/Abigail/tie--pick.git.
Abigail <cpan@abigail.be>.
Copyright (C) 1999, 2009 by Abigail
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