sort(3pm) Perl Programmers Reference Guide sort(3pm)[top]NAMEsort- perl pragma to control sort() behaviourSYNOPSISusesort'stable'; # guarantee stability usesort'_quicksort'; # use a quicksort algorithm usesort'_mergesort'; # use a mergesort algorithm usesort'defaults'; # revert to default behavior nosort'stable'; # stability not important usesort'_qsort'; # alias for quicksort my $current; BEGIN { $current = sort::current(); # identify prevailing algorithm }DESCRIPTIONWith the "sort" pragma you can control the behaviour of the builtin "sort()" function. In Perl versions 5.6 and earlier the quicksort algorithm was used to implement "sort()", but in Perl 5.8 a mergesort algorithm was also made available, mainly to guarantee worst case O(N log N) behaviour: the worst case of quicksort is O(N**2). In Perl 5.8 and later, quicksort defends against quadratic behaviour by shuffling large arrays before sorting. A stablesortmeans that for records that compare equal, the original input ordering is preserved. Mergesort is stable, quicksort is not. Stability will matter only if elements that compare equal can be distinguished in some other way. That means that simple numerical and lexical sorts do not profit from stability, since equal elements are indistinguishable. However, with a comparison such as { substr($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 defaultsortis a side-effect that could change in later versions. If stability is important, be sure to say so with a usesort'stable'; The "no sort" pragma doesn't forbid what follows, it just leaves the choice open. Thus, after nosortqw(_mergesort stable); a mergesort, which happens to be stable, will be employed anyway. Note that nosort"_quicksort"; nosort"_mergesort"; have exactly the same effect, leaving the choice ofsortalgorithm open.CAVEATSAs 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 'usesortqw(defaults _quicksort)'; # force quicksort eval 'nosort"stable"'; # stability not wanted print sort::current . "\n"; @a =sort@b; eval 'usesort"defaults"'; # clean up, for others } { eval 'usesortqw(defaults stable)'; # force stability print sort::current . "\n"; @c =sort@d; eval 'usesort"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: { usesortqw(defaults _quicksort); # force quicksort nosort"stable"; # stability not wanted my $current; BEGIN { $current = print sort::current; } print "$current\n"; @a =sort@b; # Pragmas go out of scope at the end of the block } { usesortqw(defaults stable); # force stability my $current; BEGIN { $current = print sort::current; } print "$current\n"; @c =sort@d; }perl v5.10.12009-02-12 sort(3pm)

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