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Re: How to apply qsort() with a parameter?

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On 10/22/2012 2:41 PM, Eric Sosman wrote:
> On 10/22/2012 1:51 PM, none Rouben Rostamian wrote:
>> How does one apply qsort() if the comparison criterion
>> depends on a parameter?

     A further observation: Don't go overboard in parameterizing
your comparators.  A comparator may be called a large number of
times during a sort, so it's "in the inner loop" and should be
kept short and swift.  A comparator that examines parameters
P1,P2,P3,... in addition to the items it's comparing starts to
stray from the "short and swift" ideal.

     A particular abuse that always seems to crop up is trying
to use one comparator to sort on multiple keys:

	struct s {
	    int k1;
	    char *k2;
	    double k3;
	};

	int which_key;

	int compare(const void *aa, const void *bb) {
	    const struct s *a = aa;
	    const struct s *b = bb;
	    switch (which_key) {
	    case 1:
	        return (a->k1 > b->k1) - (a->k1 < b->k1);
	    case 2:
	        return strcmp(a->k2, b->k2);
	    case 3:
	        return (a->k3 > b->k3) - (a->k3 < b->k3);
	    default:
	        assert(false);
	    }
	}

	...
	which_key = 2;
	qsort(array, count, sizeof array[0], compare);

     This setup repeats the switch decision -- which is "just
overhead" -- for every comparison, and there may be a lot of
them.  It would almost surely be better to have three short
comparators than one omnibus comparator.

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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Thread

How to apply qsort() with a parameter? rouben@shadow.(none) (Rouben Rostamian) - 2012-10-22 17:51 +0000
  Re: How to apply qsort() with a parameter? ImpalerCore <jadill33@gmail.com> - 2012-10-22 11:18 -0700
    Re: How to apply qsort() with a parameter? ImpalerCore <jadill33@gmail.com> - 2012-10-22 11:26 -0700
    Re: How to apply qsort() with a parameter? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 20:27 +0100
  Re: How to apply qsort() with a parameter? tom st denis <tom@iahu.ca> - 2012-10-22 11:38 -0700
  Re: How to apply qsort() with a parameter? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-22 14:41 -0400
    Re: How to apply qsort() with a parameter? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-22 14:59 -0400
      Re: How to apply qsort() with a parameter? tom st denis <tom@iahu.ca> - 2012-10-22 13:25 -0700
        Re: How to apply qsort() with a parameter? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-22 16:54 -0400
          Re: How to apply qsort() with a parameter? Robert Wessel <robertwessel2@yahoo.com> - 2012-10-22 23:25 -0500
            Re: How to apply qsort() with a parameter? James Kuyper <jameskuyper@verizon.net> - 2012-10-23 07:42 -0400
            Re: How to apply qsort() with a parameter? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-23 08:14 -0400
          Re: How to apply qsort() with a parameter? tom st denis <tom@iahu.ca> - 2012-10-23 07:05 -0700
            Re: How to apply qsort() with a parameter? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-23 10:13 -0400
    Re: How to apply qsort() with a parameter? "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2012-10-23 10:28 -0400
      Re: How to apply qsort() with a parameter? Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-23 10:56 -0400
  Re: How to apply qsort() with a parameter? Öö Tiib <ootiib@hot.ee> - 2012-10-22 15:30 -0700
  Re: How to apply qsort() with a parameter? rouben@shady.(none) (Rouben Rostamian) - 2012-10-23 03:52 +0000
    Re: How to apply qsort() with a parameter? Keith Thompson <kst-u@mib.org> - 2012-10-22 22:46 -0700
      Re: How to apply qsort() with a parameter? pete <pfiland@mindspring.com> - 2012-10-23 16:03 -0400
    Re: How to apply qsort() with a parameter? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-10-23 06:43 -0700

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