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

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On 10/22/2012 1:51 PM, none Rouben Rostamian wrote:
> How does one apply qsort() if the comparison criterion
> depends on a parameter?
>
> Specifically, suppose I wish to sort an array of int,
> according to distance from a prescribed value `n':
>
> int compar(const void *aa, const void *bb)
> {
>          int a = *(int *)aa;
>          int b = *(int *)bb;
>          return abs(a - n) < abs(b - n);

     Aside: This can't be right, as it doesn't define
a total ordering.

> }
>
> One way to make this work, is to declare n as a global
> (file scope) variable.  Is there a clever way to do this
> without bringing in a global variable?

     No, nothing clever.  You could write multiple comparators,
each hard-wiring a different `n' value:

	int compar10(const void *aa, const void *bb) {
	    ... use n=10 ...
	}

	int compar20(const void *aa, const void *bb) {
	    ... use n=20 ...
	}

     Or, you could subtract n from all the array elements,
use the equivalent of compar0(), and then add n back again.
(Sounds wasteful, but it only adds O(N) time to an operation
that's probably O(N log N) already.)

     Neither alternative strikes me as "clever."  You either
live with the qsort() interface or write your own sort.

-- 
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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