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From: marwk@levels.sait.edu.au
Newsgroups: comp.lang.c
Subject: Re: How to pass arbitrary 2-d array argument: done!
Message-ID: <15805.2787625f@levels.sait.edu.au>
Date: 6 Jan 91 17:46:07 GMT
References: <1991Jan6.044056.23028@noose.ecn.purdue.edu> <1991Jan6.052238.16398@zoo.toronto.edu>
Organization: Sth Australian Inst of Technology
Lines: 40

In article <1991Jan6.052238.16398@zoo.toronto.edu>, henry@zoo.toronto.edu (Henry Spencer) writes:
> In article <1991Jan6.044056.23028@noose.ecn.purdue.edu> luj@delta.ecn.purdue.edu (Jun Lu) writes:
>>It would be nice if I can have a "subroutine" which takes arbitrary matrices as
>>arguments and performs some operations on them. The dimensions of the 2-d
>>array are not known a priori...
>
> Can't be done in C.  The dimensions of an array, with the exception of the
> first, must be known at compile time.
>
> (Well, "can't be done" is an exaggeration, but what you end up doing is
> cheating, declaring the arguments as pointers and doing the subscript
> arithmetic yourself rather than using [] notation.)
>
> This is generally considered a deficiency, but it is harder to solve than
> it looks.  Some compilers have extensions to do it, although most of those
> extensions have problems of one kind or another.  There is work being done
> on finding a good solution.
> --


The method is quite straight forward and I have done it but I cannot find the
code at the moment, but I describe its method here.

#define M 4  /* number of rows */
#define N 7  /* number of columns */

int size = sizeof(int) * M * N   /* number of bytes required /*

int *a;  /* a 2-D array */

a = (int *) malloc(size);

for (i = 0; i < M; ++i)  /* set up the matrix */
for (j = 0; j < N; ++j)
    a[(i - 1) * M + j) = 10 * (i + 1) + j + 1;  /* 11, 12, 13, ...*/

Now the array can be passed to a function and M and (in particular) N are
not known before the call.  Simple!

Ray