sort(3p) Perl Programmers Reference Guide sort(3p)
sort - perl pragma to control sort() behaviour
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 $current = sort::current(); # identify prevailing algorithm
With the "sort" pragma you can control the behaviour of the
builtin "sort()" function.
In Perl versions 5.6 and earlier the quicksort algorithm was
used to implement "sort()", but in Perl 5.8 a mergesort
algorithm was also made available, mainly to guarantee worst
case O(N log N) behaviour: the worst case of quicksort is
O(N**2). In Perl 5.8 and later, quicksort defends against
quadratic behaviour by shuffling large arrays before sort-
ing.
A stable sort means that for records that compare equal, the
original input ordering is preserved. Mergesort is stable,
quicksort is not. Stability will matter only if elements
that compare equal can be distinguished in some other way.
That means that simple numerical and lexical sorts do not
profit from stability, since equal elements are indistin-
guishable. However, with a comparison such as
{ substr($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 sub-
sequent 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, mer-
gesort does fewer comparisons than quicksort, so it may be
better when complicated comparison routines are used. Mer-
gesort 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 sub-
pragmas beginning with a "_" may not persist beyond Perl
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sort(3p) Perl Programmers Reference Guide sort(3p)
5.8. The default algorithm is mergesort, which will be
stable even if you do not explicitly demand it. But the sta-
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.
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
courtesy to others.
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sort(3p) Perl Programmers Reference Guide sort(3p)
{ 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.
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