sort(3pm) Perl Programmers Reference Guide sort(3pm)NAMEsort - perl pragma to control sort() behaviour
SYNOPSIS
use sort 'stable'; # guarantee stability
use sort '_quicksort'; # use a quicksort algorithm
use sort '_mergesort'; # use a mergesort algorithm
use sort 'defaults'; # revert to default behavior
no sort 'stable'; # stability not important
use sort '_qsort'; # alias for quicksort
my $current;
BEGIN {
$current = sort::current(); # identify prevailing algorithm
}
DESCRIPTION
With the "sort" pragma you can control the behaviour of the builtin
"sort()" function.
In Perl versions 5.6 and earlier the quicksort algorithm was used to
implement "sort()", but in Perl 5.8 a mergesort algorithm was also made
available, mainly to guarantee worst case O(N log N) behaviour: the
worst case of quicksort is O(N**2). In Perl 5.8 and later, quicksort
defends against quadratic behaviour by shuffling large arrays before
sorting.
A stable sort means that for records that compare equal, the original
input ordering is preserved. Mergesort is stable, quicksort is not.
Stability will matter only if elements that compare equal can be
distinguished in some other way. That means that simple numerical and
lexical sorts do not profit from stability, since equal elements are
indistinguishable. However, with a comparison such as
{ substr($a, 0, 3) cmp substr($b, 0, 3) }
stability might matter because elements that compare equal on the first
3 characters may be distinguished based on subsequent characters. In
Perl 5.8 and later, quicksort can be stabilized, but doing so will add
overhead, so it should only be done if it matters.
The best algorithm depends on many things. On average, mergesort does
fewer comparisons than quicksort, so it may be better when complicated
comparison routines are used. Mergesort also takes advantage of pre-
existing order, so it would be favored for using "sort()" to merge
several sorted arrays. On the other hand, quicksort is often faster
for small arrays, and on arrays of a few distinct values, repeated many
times. You can force the choice of algorithm with this pragma, but
this feels heavy-handed, so the subpragmas beginning with a "_" may not
persist beyond Perl 5.8. The default algorithm is mergesort, which
will be stable even if you do not explicitly demand it. But the
stability of the default sort is a side-effect that could change in
later versions. If stability is important, be sure to say so with a
use sort 'stable';
The "no sort" pragma doesn't forbid what follows, it just leaves the
choice open. Thus, after
no sort qw(_mergesort stable);
a mergesort, which happens to be stable, will be employed anyway. Note
that
no sort "_quicksort";
no sort "_mergesort";
have exactly the same effect, leaving the choice of sort algorithm
open.
CAVEATS
As of Perl 5.10, this pragma is lexically scoped and takes effect at
compile time. In earlier versions its effect was global and took effect
at run-time; the documentation suggested using "eval()" to change the
behaviour:
{ eval 'use sort qw(defaults _quicksort)'; # force quicksort
eval 'no sort "stable"'; # stability not wanted
print sort::current . "\n";
@a = sort @b;
eval 'use sort "defaults"'; # clean up, for others
}
{ eval 'use sort qw(defaults stable)'; # force stability
print sort::current . "\n";
@c = sort @d;
eval 'use sort "defaults"'; # clean up, for others
}
Such code no longer has the desired effect, for two reasons. Firstly,
the use of "eval()" means that the sorting algorithm is not changed
until runtime, by which time it's too late to have any effect.
Secondly, "sort::current" is also called at run-time, when in fact the
compile-time value of "sort::current" is the one that matters.
So now this code would be written:
{ use sort qw(defaults _quicksort); # force quicksort
no sort "stable"; # stability not wanted
my $current;
BEGIN { $current = print sort::current; }
print "$current\n";
@a = sort @b;
# Pragmas go out of scope at the end of the block
}
{ use sort qw(defaults stable); # force stability
my $current;
BEGIN { $current = print sort::current; }
print "$current\n";
@c = sort @d;
}
perl v5.16.3 2013-03-04 sort(3pm)
