### 32.4 Examples

The following program demonstrates the use of the one-dimensional wavelet transform functions. It computes an approximation to an input signal (of length 256) using the 20 largest components of the wavelet transform, while setting the others to zero.

    #include <stdio.h>
    #include <math.h>
    #include <gsl/gsl_sort.h>
    #include <gsl/gsl_wavelet.h>

    int
    main (int argc, char **argv)
    {
      (void)(argc); /* avoid unused parameter warning */
      int i, n = 256, nc = 20;
      double *data = malloc (n * sizeof (double));
      double *abscoeff = malloc (n * sizeof (double));
      size_t *p = malloc (n * sizeof (size_t));

      FILE * f;
      gsl_wavelet *w;
      gsl_wavelet_workspace *work;

      w = gsl_wavelet_alloc (gsl_wavelet_daubechies, 4);
      work = gsl_wavelet_workspace_alloc (n);

      f = fopen (argv[1], "r");
      for (i = 0; i < n; i++)
        {
          fscanf (f, "%lg", &data[i]);
        }
      fclose (f);

      gsl_wavelet_transform_forward (w, data, 1, n, work);

      for (i = 0; i < n; i++)
        {
          abscoeff[i] = fabs (data[i]);
        }

      gsl_sort_index (p, abscoeff, 1, n);

      for (i = 0; (i + nc) < n; i++)
        data[p[i]] = 0;

      gsl_wavelet_transform_inverse (w, data, 1, n, work);

      for (i = 0; i < n; i++)
        {
          printf ("%g\n", data[i]);
        }

      gsl_wavelet_free (w);
      gsl_wavelet_workspace_free (work);

      free (data);
      free (abscoeff);
      free (p);
      return 0;
    }

The output can be used with the GNU plotutils `graph` program,

    $ ./a.out ecg.dat > dwt.txt
    $ graph -T ps -x 0 256 32 -h 0.3 -a dwt.txt > dwt.ps