

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 $current = sort::current(); # identify prevailing algorithm
With the sort
pragma you can control the behaviour of the builtin
sort()
function.
In Perl versions 5.6 and earlier the quicksort algorithm was used to
implement sort()
, but in Perl 5.8 a mergesort algorithm was also made
available, mainly to guarantee worst case O(N log N) behaviour:
the worst case of quicksort is O(N**2). In Perl 5.8 and later,
quicksort defends against quadratic behaviour by shuffling large
arrays before sorting.
A stable sort means that for records that compare equal, the original input ordering is preserved. Mergesort is stable, quicksort is not. Stability will matter only if elements that compare equal can be distinguished in some other way. That means that simple numerical and lexical sorts do not profit from stability, since equal elements are indistinguishable. However, with a comparison such as
{ substr($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 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 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.
{ 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.