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

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"Eric Sosman" <esosman@comcast-dot-net.invalid> wrote in message 
news:k6441n$oa7$1@dont-email.me...
> 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.
>

I believe this point could use some expansion.  What Eric was refering to 
was the fact that a qsort implementation expects a comparison function that 
returns negative if the first element should appear earlier than the second 
element, positive if the second element should appear earlier, and 0 if you 
don't care.  In particular, if compare(a, b) returns positive, compare(b, a) 
must return negative.

If the above isn't true, well, I don't know if there is any requirement on 
what qsort is allowed to do.  The most obvious thing it could do in this 
case is not sort the array as you expect (for example, given the array [1000 
1] and N=0, it may leave the array undisturbed).  Other misbehaviors (such 
as infinite looping, or returning an array which is not a permutation of the 
original, or simply crashing) are less likely, but are still conceivable.

-- 
poncho

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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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