#### 8.3.11 Example programs for vectors

This program shows how to allocate, initialize and read from a vector using the functions `gsl_vector_alloc`, `gsl_vector_set` and `gsl_vector_get`.

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

    int
    main (void)
    {
      int i;
      gsl_vector * v = gsl_vector_alloc (3);

      for (i = 0; i < 3; i++)
        {
          gsl_vector_set (v, i, 1.23 + i);
        }

      for (i = 0; i < 100; i++) /* OUT OF RANGE ERROR */
        {
          printf ("v_%d = %g\n", i, gsl_vector_get (v, i));
        }

      gsl_vector_free (v);
      return 0;
    }

Here is the output from the program. The final loop attempts to read outside the range of the vector `v`, and the error is trapped by the range-checking code in `gsl_vector_get`.

    $ ./a.out
    v_0 = 1.23
    v_1 = 2.23
    v_2 = 3.23
    gsl: vector_source.c:12: ERROR: index out of range
    Default GSL error handler invoked.
    Aborted (core dumped)

The next program shows how to write a vector to a file.

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

    int
    main (void)
    {
      int i;
      gsl_vector * v = gsl_vector_alloc (100);

      for (i = 0; i < 100; i++)
        {
          gsl_vector_set (v, i, 1.23 + i);
        }

      {
         FILE * f = fopen ("test.dat", "w");
         gsl_vector_fprintf (f, v, "%.5g");
         fclose (f);
      }

      gsl_vector_free (v);
      return 0;
    }

After running this program the file `test.dat` should contain the elements of `v`, written using the format specifier `%.5g`. The vector could then be read back in using the function `gsl_vector_fscanf (f, v)` as follows:

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

    int
    main (void)
    {
      int i;
      gsl_vector * v = gsl_vector_alloc (10);

      {
         FILE * f = fopen ("test.dat", "r");
         gsl_vector_fscanf (f, v);
         fclose (f);
      }

      for (i = 0; i < 10; i++)
        {
          printf ("%g\n", gsl_vector_get(v, i));
        }

      gsl_vector_free (v);
      return 0;
    }