sort

sort(3)		       Perl Programmers Reference Guide		      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 ’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, quick-
       sort 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 dis-
       tinguished 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 charac-
       ters.  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 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.

CAVEATS
       This pragma is not lexically scoped: its effect is global to the pro-
       gram 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.



perl v5.8.8			  2001-09-21			      sort(3)