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Re: Is there any way to refer to another array in qsort's compare function?

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Dr Nick於 2011年12月29日星期四UTC+8下午8時37分26秒寫道：
> 88888 Dihedral <dihedr...@googlemail.com> writes:
> 
> > Ben Bacarisse於 2011年12月29日星期四UTC+8下午7時58分14秒寫道：
> >> Stone <sto...@gmail.com> writes:
> >> 
> >> > Here's the problem.
> >> >
> >> > I wrote a function 'int my_compare(const void *, const void *);' and
> >> > passed it to qsort() of stdlib.h, in order to sort an array named
> >> > *ptr* which contains indices of another array *data*. By that I
> >> > intended to sort elements of *ptr* (i.e. ptr[i]) using data[ptr[i]] as
> >> > the key value.
> >> >
> >> > However, the *data* array was defined as static in main(), and seemed
> >> > not easy to access in my_compare(). Is there any way to achieve this?
> >> 
> >> You can either move the array so that it is defined at file scope
> >> (i.e. outside main) or you can arrange for there to be a file-scope
> >> pointer to it:
> >> 
> >>   #include <stdlib.h>
> >>   #include <stdio.h>
> >>    
> >>   int *aux_sort_ptr;
> >>    
> >>   int my_compare(const void *p1, const void *p2)
> >>   {
> >>       const int *ip1 = p1, *ip2 = p2;
> >>       return aux_sort_ptr[*ip1] - aux_sort_ptr[*ip2];
> >>   }
> >>    
> >>   int main(void)
> >>   {
> >>       static int data[] = { 3, 1, 2 };
> >>       int idx[] = { 1, 2, 3 };
> >>       aux_sort_ptr = data;
> >>       qsort(idx, sizeof idx/sizeof *idx, sizeof *idx, my_compare);
> >>       for (int i = 0; i < sizeof idx/sizeof *idx; i++)
> >>            printf("idx[%d] = %d, data[%d] = %d\n",
> >>                   i, idx[i], idx[i], data[idx[i]]);
> >>   }
> >> 
> >> This is ugly (and problematic in threaded code), but there's no elegant
> >> way round it using standard C.  Many systems have other sort functions
> >> whose comparison function can be handed another void * (see Dr Nick's
> >> reply) but I don't think and have become common enough to though of as
> >> "almost standard".
> >
> > This method uses more memory. 
> 
> One pointer's worth.
> 
> > Do you think that just 
> > overloading the comparison function by OOP can make it not choking at all?
> 
> Why do you say that "Do you think that just overloading the comparison
> function by OOP can make it not choking at all?"
> 
> To Stone: No-one has a clue what 88888 Dihedral is on about.  Many of us
> suspect him to be a poor attempt to pass the Turing test.  Whatever he
> is, don't worry about his comments.
> -- 
> Online waterways route planner            | http://canalplan.eu
> Plan trips, see photos, check facilities  | http://canalplan.org.uk

  Just feed a segment of repeated integers, say, 3,4,5,6,3,4, 5, 6 
3,4,5, 6.... for several thousands then the program you posted will choke at the segment. 

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Is there any way to refer to another array in qsort's compare function? Stone <stonekyx@gmail.com> - 2011-12-29 00:27 -0800
  Re: Is there any way to refer to another array in qsort's compare function? Dr Nick <3-nospam@temporary-address.org.uk> - 2011-12-29 08:57 +0000
  Re: Is there any way to refer to another array in qsort's compare function? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-29 11:58 +0000
    Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-29 04:20 -0800
      Re: Is there any way to refer to another array in qsort's compare function? Dr Nick <3-nospam@temporary-address.org.uk> - 2011-12-29 12:37 +0000
        Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-29 04:48 -0800
          Re: Is there any way to refer to another array in qsort's compare function? Dr Nick <3-nospam@temporary-address.org.uk> - 2011-12-29 12:59 +0000
            Re: Is there any way to refer to another array in qsort's compare function? Geoff <geoff@invalid.invalid> - 2011-12-29 10:09 -0800
              Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-29 17:50 -0800
                Re: Is there any way to refer to another array in qsort's compare function? Geoff <geoff@invalid.invalid> - 2011-12-29 19:29 -0800
                Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-30 00:36 -0800
                Re: Is there any way to refer to another array in qsort's compare function? Bull in a China Blue Shop <chine.bleu@yahoo.com> - 2011-12-30 00:52 -0800
                Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-30 02:16 -0800
              Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-29 18:01 -0800
            Re: Is there any way to refer to another array in qsort's compare function? James Kuyper <jameskuyper@verizon.net> - 2011-12-29 13:21 -0500
        Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-30 23:18 -0800
    Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-07 23:40 -0800
      Re: Is there any way to refer to another array in qsort's compare function? pete <pfiland@mindspring.com> - 2012-01-08 07:26 -0500
        Re: Is there any way to refer to another array in qsort's compare function? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-08 12:50 +0000
          Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-08 06:02 -0800
        Re: Is there any way to refer to another array in qsort's compare function? Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-01-17 12:35 +0200
  Re: Is there any way to refer to another array in qsort's compare function? Bull in a China Blue Shop <chine.bleu@yahoo.com> - 2011-12-29 05:07 -0800
    Re: Is there any way to refer to another array in qsort's compare function? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-29 14:16 +0000
      Re: Is there any way to refer to another array in qsort's compare function? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-29 15:29 +0000
        Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-29 09:33 -0800
        Re: Is there any way to refer to another array in qsort's compare function? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-29 15:40 -0800
  Re: Is there any way to refer to another array in qsort's compare function? James Kuyper <jameskuyper@verizon.net> - 2011-12-29 10:50 -0500
  Re: Is there any way to refer to another array in qsort's compare function? pete <pfiland@mindspring.com> - 2012-01-07 20:50 -0500

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