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.
DEVELOPMENT
The current sources of this module are found on github,
.
AUTHOR
Abigail .
COPYRIGHT and LICENSE
Copyright (C) 1999, 2009 by Abigail
Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.