int    i, j, k;
double A[N][N], B[N][N], C[N][N];
/* C = A*B */
/* code for matrix product */
for ( i = 0; i < N; i++ )
  for ( j = 0; j < N; j++ )
  {
    C[i][j] = 0;
    for ( k = 0; k < N; k++ )
      C[i][j] += + A[i][k]*B[k][j];
  }