12.2 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 ]