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sort(3pm)			 Perl Programmers Reference Guide			sort(3pm)

       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 numeri-
       cal 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 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.  Merge-
       sort 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 impor-
       tant, 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,

	 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 cour-
       tesy 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 v5.8.0				    2002-06-01					sort(3pm)
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