sort(3)
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 '_qsort'; # alias for quicksort my $current = sort::current(); # identify prevailing algorithm
DESCRIPTION
With the sort pragma you can control the behaviour of the
builtin sort() function.
In Perl versions 5.6 and earlier the quicksort algorithm
was used to implement sort(), but in Perl 5.8 a mergesort
algorithm was also made available, mainly to guarantee
worst case O(N log N) behaviour: the worst case of quick
sort is O(N**2). In Perl 5.8 and later, quicksort defends
against quadratic behaviour by shuffling large arrays
before sorting.
- A stable sort means that for records that compare equal,
the original input ordering is preserved. Mergesort is
stable, quicksort is not. Stability will matter only if
elements that compare equal can be distinguished in some
other way. That means that simple numerical and lexical
sorts do not profit from stability, since equal elements
are indistinguishable. However, with a comparison such as - { substr($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 platforms with small memory caches
that are much faster than main memory. 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.
CAVEATS
- This pragma is not lexically scoped : its effect is global
to the program it appears in. This may change in future
versions.