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Re: multidimensional heap arrays

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On 04/25/2011 03:36 PM, Jack Boot wrote:
> hi
> 
> we have been discussing methods for multidimensional heap arrays, between 
> us we have 3 preferred approaches
> 
> 1: array of pointers each mallocated, ie
> int ** A = malloc(nrow * sizeof(int*));
> for(i=0;i<nrow;i++) A[i]=malloc(ncol * sizeof(int));

I've used this approach before. It's particularly useful when the rows
of the array have different lengths, and in a triangular matrix. With
careful attention to how you access such a matrix, you can avoid storing
all of the useless zero elements.

> 2: array of pointers set into one big mallocated array, ie
> int ** A = malloc(nrow * sizeof(int*));
> int * Astorage = malloc(nrow * ncol * sizeof(int));
> for(i=0;i<nrow;i++) A[i]=Astorage+i*ncol;

I've used something like this approach before, but I don't like it. I
used it because I was trying to reverse-engineer some routines from a
commercial third-party library (I told my boss I wanted to get advice
from a corporate lawyer before I tackled this project - he told me to
just get on with it, and not worry about such issues. I filled the code
with comments documenting which library my code was emulating, as well
as exactly how I determine what the functions should do without
disassembling it or stealing the original source code. I hope that was
sufficient to avoid legal problems.)

One of these routines created a dynamically-allocated structure filled
with pointers to arrays of several different dimensions, whose length in
each dimension was variable, and stored in that structure. My black-box
testing determined that it was using something like the following
method: it allocated a single block of memory big enough to store the
structure and all of the arrays it contained pointers to. The arrays
were not merely of several different sizes, but even of several
different dimensions. The routine filled in the pointers in that
structure with values pointing at various parts of that single big
allocation.

I perforce followed the same approach in my own reverse-engineered code.
I can see the advantages of such an approach, but it strikes me as a
delicate system, easily broken if a single pointer gets set incorrectly
during an update.

> 3: one big mallocated array, no separate row pointers but an accessor 
> macro instead, ie
> int * Astorage = malloc(nrow * ncol * sizeof(int));
> #define A(i,j) Astorage[i*ncol+j];
> 
> what is your opinion of these methods? what situations is one is better 
> than another? should some of them never be used? what is standard/best 
> practice?

I'd replace the accessor macro with an inline function, if only for type
safety. Otherwise, this is another reasonable approach.
-- 
James Kuyper

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multidimensional heap arrays Jack Boot <jackboot@mailinator.com> - 2011-04-25 19:36 +0000
  Re: multidimensional heap arrays Francois Grieu <fgrieu@gmail.com> - 2011-04-25 23:16 +0200
  Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-25 22:35 +0100
    Re: multidimensional heap arrays Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2011-04-25 21:19 -0700
      Re: multidimensional heap arrays Francois Grieu <fgrieu@gmail.com> - 2011-04-26 08:19 +0200
        Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 12:19 +0100
          Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-26 23:25 +1200
            Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 21:06 +0100
              Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 08:59 +1200
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-26 14:39 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 10:03 +1200
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 23:41 +0100
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-26 16:33 -0700
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-27 04:05 +0100
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-26 22:49 +0100
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 10:19 +1200
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-27 00:38 +0100
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 11:48 +1200
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-26 17:38 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 13:08 +1200
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-26 18:31 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 14:01 +1200
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-27 00:04 -0700
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 19:58 +1200
                Re: multidimensional heap arrays Ian Collins <ian-news@hotmail.com> - 2011-04-27 20:03 +1200
                Re: multidimensional heap arrays Keith Thompson <kst-u@mib.org> - 2011-04-27 00:03 -0700
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-26 18:20 -0700
                Re: multidimensional heap arrays Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-04-27 03:59 +0100
                Re: multidimensional heap arrays Tim Rentsch <txr@alumni.caltech.edu> - 2011-04-27 00:24 -0700
    Re: multidimensional heap arrays "io_x" <a@b.c.invalid> - 2011-04-26 09:42 +0200
  Re: multidimensional heap arrays Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-04-25 20:50 -0400
  Re: multidimensional heap arrays James Kuyper <jameskuyper@verizon.net> - 2011-04-25 22:57 -0400

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