/* Example: matrices represented by 2-dimensional arrays */

#include <stdio.h>

main()
{
  int A[3][3] = {{2,  4,  6},
                 {0, -1,  3},
                 {5,  2, -1}
                };
     
  int B[3][3] = {{1,  3, -2},
                 {3,  0,  2},
                 {1, -3,  4}
                };

  int C[3][3];

  int i, j, k;

  printf("A   = [%3i  %3i  %3i ]\n", 
          A[0][0], A[0][1], A[0][2]);
  printf("      [%3i  %3i  %3i ]\n", 
          A[1][0], A[1][1], A[1][2]);
  printf("      [%3i  %3i  %3i ]\n\n", 
          A[2][0], A[2][1], A[2][2]);
  
  printf("B   = [%3i  %3i  %3i ]\n", 
          B[0][0], B[0][1], B[0][2]);
  printf("      [%3i  %3i  %3i ]\n", 
          B[1][0], B[1][1], B[1][2]);
  printf("      [%3i  %3i  %3i ]\n\n", 
          B[2][0], B[2][1], B[2][2]);
  
  /* multiply C = A.B: */
  
  for (i = 0; i < 3; i++)
    for (j = 0; j < 3; j++)
    {
      C[i][j] = 0;
      for (k =0; k < 3; k++)
        C[i][j] += A[i][k] * B[k][j];
    }
  
  printf("A.B = [%3i  %3i  %3i ]\n", 
          C[0][0], C[0][1], C[0][2]);
  printf("      [%3i  %3i  %3i ]\n", 
          C[1][0], C[1][1], C[1][2]);
  printf("      [%3i  %3i  %3i ]\n\n", 
          C[2][0], C[2][1], C[2][2]);
  
  /* multiply C = B.A: */
  
  for (i = 0; i < 3; i++)
    for (j = 0; j < 3; j++)
    {
      C[i][j] = 0;
      for (k =0; k < 3; k++)
        C[i][j] += B[i][k] * A[k][j];
    }
  
  printf("B.A = [%3i  %3i  %3i ]\n", 
          C[0][0], C[0][1], C[0][2]);
  printf("      [%3i  %3i  %3i ]\n", 
          C[1][0], C[1][1], C[1][2]);
  printf("      [%3i  %3i  %3i ]\n\n", 
          C[2][0], C[2][1], C[2][2]);
}