### 13.2 Examples

The following program computes the product of two matrices using the Level-3 BLAS function DGEMM,

    [ 0.11 0.12 0.13 ]  [ 1011 1012 ]     [ 367.76 368.12 ]
    [ 0.21 0.22 0.23 ]  [ 1021 1022 ]  =  [ 674.06 674.72 ]
                        [ 1031 1032 ]

The matrices are stored in row major order, according to the C convention for arrays.

    #include <stdio.h>
    #include <gsl/gsl_blas.h>

    int
    main (void)
    {
      double a[] = { 0.11, 0.12, 0.13,
                     0.21, 0.22, 0.23 };

      double b[] = { 1011, 1012,
                     1021, 1022,
                     1031, 1032 };

      double c[] = { 0.00, 0.00,
                     0.00, 0.00 };

      gsl_matrix_view A = gsl_matrix_view_array(a, 2, 3);
      gsl_matrix_view B = gsl_matrix_view_array(b, 3, 2);
      gsl_matrix_view C = gsl_matrix_view_array(c, 2, 2);

      /* Compute C = A B */

      gsl_blas_dgemm (CblasNoTrans, CblasNoTrans,
                      1.0, &A.matrix, &B.matrix,
                      0.0, &C.matrix);

      printf ("[ %g, %g\n", c[0], c[1]);
      printf ("  %g, %g ]\n", c[2], c[3]);

      return 0;
    }

Here is the output from the program,

    $ ./a.out

    [ 367.76, 368.12
      674.06, 674.72 ]