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;
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 { $current = sort::current; }
print "$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 { $current = sort::current; }
print "$current\n";
@c = sort @d;
}