
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 $current = sort::current(); # identify prevailing algorithm
DESCRIPTION
With the "sort" pragma you can control the behaviour of the builtin "sort()" function.
In Perl versions 5.6 and earlier the quicksort algorithm was used to implement "sort()",
but in Perl 5.8 a mergesort algorithm was also made available, mainly to guarantee worst
case O(N log N) behaviour: the worst case of quicksort is O(N**2). In Perl 5.8 and later,
quicksort defends against quadratic behaviour by shuffling large arrays before sorting.
A stable sort means that for records that compare equal, the original input ordering is
preserved. Mergesort is stable, quicksort is not. Stability will matter only if elements
that compare equal can be distinguished in some other way. That means that simple numeri
cal and lexical sorts do not profit from stability, since equal elements are indistin
guishable. However, with a comparison such as
{ substr($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. Merge
sort also takes advantage of preexisting 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 heavyhanded, 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 sideeffect that could change in later versions. If stability is impor
tant, 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 subpragmas (in case somebody carelessly left them on) and after sorting, as a cour
tesy 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 v5.8.0 20020601 sort(3pm) 
