Conformant Arrays in C

[Chris Torek]

In article <676 at cresswell.quintus.UUCP> ok at quintus.UUCP
(Richard A. O'Keefe) writes:
>Background:
>	David Hough sent me his comments on dpANS C.
>	He wants a language which can do at least as much as Fortran could.
>
>	One of his criteria was that you should be able to write
>	subroutines that handle multi-dimensional arrays sensibly.

I would claim that the `best' way to do this is to use the row-vector
approach.  To create a two-dimensional array, e.g., do the following:

	typedef struct dmat2 {
		int	m2_nrows;
		int	m2_ncols;
		double	**m2_m;		/* row vectors */
		double	*m2_data;
	} dmat2;

	dmat2 *
	create(rows, cols)
		register int rows, cols;
	{
		register dmat2 *m;
		register int r;

		m = (dmat2 *)malloc(sizeof(dmat2));
		if (m == NULL)
			return (NULL);
		m->m2_rows = rows;
		m->m2_cols = cols;
		m->m2_m = (double **)malloc(rows * sizeof(double *));
		if (m->m2_m == NULL) {
			free((char *)m);
			return (NULL);
		}
		m->m2_data = (double *)malloc(rows * cols * sizeof(double));
		if (m->m2_data == NULL) {
			free((char *)m->m2_m);
			free((char *)m->m2_data);
		}
		for (r = 0; r < rows; r++)
			m->m2_m[r] = &m->m2_data[r * cols];
		return (m);
	}

These arrays can then be used as

	m->m2_m[r][c] = val;

and parts of the array can be passed to functions with

		f(&m->m2_m[off], rows - off, cols);

Note that a whole matrix can be passed (by reference) with

	f(m);

If you dislike having to specify the range for subarrays, you could
create a `make subarray' function:

	dmat2 *
	makesub(m, roff, coff, rsize, csize)
		dmat2 *m;
		int roff, coff, rsize, csize;
	{

		if (m->m2_rows - roff < rsize || m->m2_rows - coff < csize)
			/* it is not that big! */...
		etc.
	}

This does, of course, require more memory to describe, but row vectors
are generally faster anyway.  Since the whole matrix is still there
as a single block of memory, you can use existing routines as well
(by passing m->m2_data).

So why is this solution insufficient?
-- 
In-Real-Life: Chris Torek, Univ of MD Computer Science, +1 301 454 7163
(hiding out on trantor.umd.edu until mimsy is reassembled in its new home)
Domain: chris at mimsy.umd.edu		Path: not easily reachable