Manual Reference Pages - ALGORITHM::NUMERICAL::SHUFFLE (3)
Algorithm::Numerical::Shuffle - Shuffle a list.
use Algorithm::Numerical::Shuffle qw /shuffle/;
@shuffled = shuffle (1, 2, 3, 4, 5, 6, 7);
$in_situ = [qw /one two three four five six/];
shuffle performs a one pass, fair shuffle on a list. If the list is
passed as a reference to an array, the shuffle is done in situ.
The subroutine returns the list in list context, and a reference to
the list in scalar context.
The running time of the algorithm is linear in the size of the list.
For an in situ shuffle, the memory overhead is constant; otherwise,
linear extra memory is used.
The algorithm used is discussed by Knuth . It was first published
by Fisher and Yates , and later by Durstenfeld .
Salfi  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, <B>7B>, 1964. pp 420.
R. A. Fisher and F. Yates: Statistical Tables. London, 1938.
D. E. Knuth: The Art of Computer Programming, Volume 2, Third edition.
Section 3.4.2, Algorithm P, pp 145. Reading: Addison-Wesley, 1997.
R. Salfi: COMPSTAT 1974. Vienna: 1974, pp 28 - 35.
List::Util also has a shuffle function which uses a similar
algorithm. But since its written in C, its much faster. For all practical
purposes, List::Util supersedes this module. Unless you really need
in situ sorting.
The current sources of this module are found on github,
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