Matrix Multiplication

Copyright © 2002 - Angelo Bertolli

A little bit of math goes a long way when dealing with matrices. I have found it much easier and less confusing to do matrix multiplication using long single-dimensional arrays. This article will guide you through how to multiply such matrices and provides C code.

Let's say we declare 4 x 4 matrices as A[16], B[16], and D[16] (some floating point type) as they would use in OpenGL. Therefore:

[A][B] = [D]

Where the matrices are structured (as in OpenGL):

[A] = | a0  a4  a8 a12 |
      | a1  a5  a9 a13 |
      | a2  a6 a10 a14 |
      | a3  a7 a11 a15 |

Let s be the size of the square matrix. Let t be the number of elements in a row or column.

We want to find d_k for each k from 0 to s-1

Let r be the row where we find d_k as numbered from 0 to t-1. Let c be the column where we find d_k as numbered from 0 to t-1

For each d_k we want to take the dot-product of the corresponding row in A and the corresponding column in B.

Therefore, d_k = the sum over i from 0 to t-1 of [a_(r+it) * b_(ct+i)], where r = k mod t and c = k div t (integer modulo and division)

Code

double A[16];
double B[16];
double D[16];

int s = 16
int t = 4
int r;
int c;

for (int k=0;k<s;k++) {
   r = k % t;
   c = (int)(k / t);
   D[k] = 0.0;
   for (int i=0;i<t;i++)
      D[k] += A[r+(i*t)] * B[(c*t)+i];
}

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