matrix mult.

[Ken Turkowski]

In article <890 at ncoast.UUCP> simpsong at ncoast.UUCP (Gregory R. Simpson @ The North Coast) writes:
>Below is a kludge version of a matrix multiplier in C. The primary
>reason for posting this is in hope that someone out there knows the
>proper method of doing it.

Anyone who's diddled with two dimensional arrays, and has tried to think
about how a compiler would do it has come up with something like the
following:

-----------------------------------------------------------------

# This is a shell archive.  Remove anything before this line, then
# unpack it by saving it in a file and typing "sh file".  (Files
# unpacked will be owned by you and have default permissions.)
#
# This archive contains:
# matcat.c

echo x - matcat.c
cat > "matcat.c" << '//E*O*F matcat.c//'
/* Matcat multiplies two (n x n) matrices together.  The source matrices
 * are given in A and B, and the result is returned in C.
 */

# ifndef ARRAYS

matcat(C, A, B, n)
register double *C;
double *A, *B;
register int n;
{
	register double *ap, *bp;
	register int k, j, i;

	for (i = n; i-- > 0; A += n) {		/* Each row in A */
	    for (j = 0; j < n; j++) {		/* Each column in B */
		ap = A;		/* Left of ith row of A */
		bp = B + j;	/* Top of jth column of B */
		*C = 0.0;
		for (k = n; --k >= 0; bp += n)
		    *C += *ap++ * (*bp);   /* *C += A[i'][k'] * B[k'][j]; */
	    }
	}
}

# else

matcat(C, A, B, n)
register double *A, *B, *C;
register int n;
{
	register int i, j, k;
	double sum;

	for (i = 0; i < n; i++) {
	    for (j = 0; j < n; j++) {
		sum = 0.0;
		for (k = 0; k < n; k++)
		    sum += A[n*i+k] * B[n*k+j];
		C[n*i+j] = sum;
	    }
	}
}

# endif
//E*O*F matcat.c//

exit 0
-- 
Ken Turkowski @ CIMLINC (formerly CADLINC), Menlo Park, CA
UUCP: {amd,decwrl,hplabs,seismo,spar}!turtlevax!ken
ARPA: turtlevax!ken at DECWRL.DEC.COM