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<title>GNU Scientific Library &ndash; Reference Manual: Example ntuple programs</title>

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<a name="Example-ntuple-programs"></a>
<div class="header">
<p>
Next: <a href="Ntuple-References-and-Further-Reading.html#Ntuple-References-and-Further-Reading" accesskey="n" rel="next">Ntuple References and Further Reading</a>, Previous: <a href="Histogramming-ntuple-values.html#Histogramming-ntuple-values" accesskey="p" rel="previous">Histogramming ntuple values</a>, Up: <a href="N_002dtuples.html#N_002dtuples" accesskey="u" rel="up">N-tuples</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Examples-16"></a>
<h3 class="section">24.8 Examples</h3>

<p>The following example programs demonstrate the use of ntuples in
managing a large dataset.  The first program creates a set of 10,000
simulated &ldquo;events&rdquo;, each with 3 associated values <em>(x,y,z)</em>.  These
are generated from a Gaussian distribution with unit variance, for
demonstration purposes, and written to the ntuple file <samp>test.dat</samp>.
</p>
<div class="example">
<pre class="verbatim">#include &lt;gsl/gsl_ntuple.h&gt;
#include &lt;gsl/gsl_rng.h&gt;
#include &lt;gsl/gsl_randist.h&gt;

struct data
{
  double x;
  double y;
  double z;
};

int
main (void)
{
  const gsl_rng_type * T;
  gsl_rng * r;

  struct data ntuple_row;
  int i;

  gsl_ntuple *ntuple 
    = gsl_ntuple_create (&quot;test.dat&quot;, &amp;ntuple_row, 
                         sizeof (ntuple_row));

  gsl_rng_env_setup ();

  T = gsl_rng_default; 
  r = gsl_rng_alloc (T);

  for (i = 0; i &lt; 10000; i++)
    {
      ntuple_row.x = gsl_ran_ugaussian (r);
      ntuple_row.y = gsl_ran_ugaussian (r);
      ntuple_row.z = gsl_ran_ugaussian (r);
      
      gsl_ntuple_write (ntuple);
    }
  
  gsl_ntuple_close (ntuple);
  gsl_rng_free (r);

  return 0;
}
</pre></div>

<p>The next program analyses the ntuple data in the file <samp>test.dat</samp>.
The analysis procedure is to compute the squared-magnitude of each
event, <em>E^2=x^2+y^2+z^2</em>, and select only those which exceed a
lower limit of 1.5.  The selected events are then histogrammed using
their <em>E^2</em> values.
</p>
<div class="example">
<pre class="verbatim">#include &lt;math.h&gt;
#include &lt;gsl/gsl_ntuple.h&gt;
#include &lt;gsl/gsl_histogram.h&gt;

struct data
{
  double x;
  double y;
  double z;
};

int sel_func (void *ntuple_data, void *params);
double val_func (void *ntuple_data, void *params);

int
main (void)
{
  struct data ntuple_row;

  gsl_ntuple *ntuple 
    = gsl_ntuple_open (&quot;test.dat&quot;, &amp;ntuple_row,
                       sizeof (ntuple_row));
  double lower = 1.5;

  gsl_ntuple_select_fn S;
  gsl_ntuple_value_fn V;

  gsl_histogram *h = gsl_histogram_alloc (100);
  gsl_histogram_set_ranges_uniform(h, 0.0, 10.0);

  S.function = &amp;sel_func;
  S.params = &amp;lower;

  V.function = &amp;val_func;
  V.params = 0;

  gsl_ntuple_project (h, ntuple, &amp;V, &amp;S);
  gsl_histogram_fprintf (stdout, h, &quot;%f&quot;, &quot;%f&quot;);
  gsl_histogram_free (h);
  gsl_ntuple_close (ntuple);

  return 0;
}

int
sel_func (void *ntuple_data, void *params)
{
  struct data * data = (struct data *) ntuple_data;  
  double x, y, z, E2, scale;
  scale = *(double *) params;
  
  x = data-&gt;x;
  y = data-&gt;y;
  z = data-&gt;z;

  E2 = x * x + y * y + z * z;

  return E2 &gt; scale;
}

double
val_func (void *ntuple_data, void *params)
{
  (void)(params); /* avoid unused parameter warning */
  struct data * data = (struct data *) ntuple_data;  
  double x, y, z;

  x = data-&gt;x;
  y = data-&gt;y;
  z = data-&gt;z;

  return x * x + y * y + z * z;
}
</pre></div>

<p>The following plot shows the distribution of the selected events.
Note the cut-off at the lower bound.
</p>

<hr>
<div class="header">
<p>
Next: <a href="Ntuple-References-and-Further-Reading.html#Ntuple-References-and-Further-Reading" accesskey="n" rel="next">Ntuple References and Further Reading</a>, Previous: <a href="Histogramming-ntuple-values.html#Histogramming-ntuple-values" accesskey="p" rel="previous">Histogramming ntuple values</a>, Up: <a href="N_002dtuples.html#N_002dtuples" accesskey="u" rel="up">N-tuples</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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