From: "John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Dynamically declaring 2-d arrays
Date: Mon, 09 Feb 1998 19:22:05 -0500
Organization: Two pounds of chaos and a pinch of salt.
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Jason Alexander wrote:
> 
>   Can someone explain to me how to dynamically declare a 2-d array?
>   I know the following code works for dynamically declaring a 1-d array
> of integers, but I don't see how to extend it to the 2-d case.

Again, this is a basic C question and is not strictly relevant to
DJGPP.  However, in the interests of amity among the Usenet community,
I'll answer anyway.  :-)

Arrays are stored contiguously in memory, starting from the first
element, and going to the last.  Multiple-dimensionality is an illusion
that makes it easier for programmers to handle different types of
arrays, but really the data is still stored the same way.

In order to create a dynamically allocated multi-dimensional array, you
can simply declare a pointer, allocate space for the # of columns x the
# of rows, and then reference it in standard array notation.  The only
difference is that you cannot use multi-dimensional notation with a
pointer.  Instead, you must substitute the calculation that the compiler
actually uses:

    /* int array[max_x][max_y] */
    int *array;
    array = (int *) malloc( max_x * max_y * sizeof(int) );

    /* array[x][y] = 10 */
    *(array + x * max_y + y) = 10;
    /* or */
    array[x * max_y + y] = 10;

If you really want to access the array in [][] notation, then you would
have to create an array of pointers-to-int (or a
pointer-to-pointer-to-int), and allocate first the space for the array,
and second the space for each subarray.  Such a pointer cannot be
treated like a normal array in terms of memory, but can be accessed
using the [][] notation (I think).  After this it gets kind of complex,
but can still be done.

-- 
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|      John M. Aldrich       | "Always listen to experts.  They'll  |
|       aka Fighteer I       | tell you what can't be done, and why.|
|   mailto:fighteer AT cs DOT com   | Then do it."                         |
| http://www.cs.com/fighteer/|                - Lazarus Long        |
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