sort(3pm) Perl Programmers Reference Guide sort(3pm)
NAME
sort - 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 $current = sort::current(); # identify prevailing algorithm
DESCRIPTION
With the "sort" pragma you can control the behaviour of the builtin "sort()" function.
In Perl versions 5.6 and earlier the quicksort algorithm was used to implement "sort()", but in Perl
5.8 a mergesort algorithm was also made available, mainly to guarantee worst case O(N log N) behav-iour: behaviour:
iour: the worst case of quicksort is O(N**2). In Perl 5.8 and later, quicksort defends against qua-dratic quadratic
dratic behaviour by shuffling large arrays before sorting.
A stable sort means that for records that compare equal, the original input ordering is preserved.
Mergesort is stable, quicksort is not. Stability will matter only if elements that compare equal can
be distinguished in some other way. That means that simple numerical and lexical sorts do not profit
from stability, since equal elements are indistinguishable. However, with a comparison such as
{ substr($a, 0, 3) cmp substr($b, 0, 3) }
stability might matter because elements that compare equal on the first 3 characters may be distin-guished distinguished
guished 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 quick-sort, quicksort,
sort, so it may be better when complicated comparison routines are used. Mergesort also takes advan-tage advantage
tage 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 val-ues, values,
ues, 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 sta-bility stability
bility 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
This pragma is not lexically scoped: its effect is global to the program it appears in. That means
the following will probably not do what you expect, because both pragmas take effect at compile time,
before either "sort()" happens.
{ use sort "_quicksort";
print sort::current . "\n";
@a = sort @b;
}
{ use sort "stable";
print sort::current . "\n";
@c = sort @d;
}
# prints:
# quicksort stable
# quicksort stable
You can achieve the effect you probably wanted by using "eval()" to defer the pragmas until run time.
Use the quoted argument form of "eval()", not the BLOCK form, as in
eval { use sort "_quicksort" }; # WRONG
or the effect will still be at compile time. Reset to default options before selecting other sub-pragmas subpragmas
pragmas (in case somebody carelessly left them on) and after sorting, as a courtesy to others.
{ 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
}
# prints:
# quicksort
# stable
Scoping for this pragma may change in future versions.
perl v5.8.8 2001-09-21 sort(3pm)
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