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<title>GNU Scientific Library – Reference Manual: Histogramming ntuple values</title>
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<a name="Histogramming-ntuple-values"></a>
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<p>
Next: <a href="Example-ntuple-programs.html#Example-ntuple-programs" accesskey="n" rel="next">Example ntuple programs</a>, Previous: <a href="Closing-an-ntuple-file.html#Closing-an-ntuple-file" accesskey="p" rel="previous">Closing an ntuple file</a>, Up: <a href="N_002dtuples.html#N_002dtuples" accesskey="u" rel="up">N-tuples</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Histogramming-ntuple-values-1"></a>
<h3 class="section">24.7 Histogramming ntuple values</h3>
<p>Once an ntuple has been created its contents can be histogrammed in
various ways using the function <code>gsl_ntuple_project</code>. Two
user-defined functions must be provided, a function to select events and
a function to compute scalar values. The selection function and the
value function both accept the ntuple row as a first argument and other
parameters as a second argument.
</p>
<a name="index-selection-function_002c-ntuples"></a>
<a name="index-gsl_005fntuple_005fselect_005ffn"></a>
<p>The <em>selection function</em> determines which ntuple rows are selected
for histogramming. It is defined by the following struct,
</p>
<div class="smallexample">
<pre class="smallexample">typedef struct {
int (* function) (void * ntuple_data, void * params);
void * params;
} gsl_ntuple_select_fn;
</pre></div>
<p>The struct component <var>function</var> should return a non-zero value for
each ntuple row that is to be included in the histogram.
</p>
<a name="index-value-function_002c-ntuples"></a>
<a name="index-gsl_005fntuple_005fvalue_005ffn"></a>
<p>The <em>value function</em> computes scalar values for those ntuple rows
selected by the selection function,
</p>
<div class="smallexample">
<pre class="smallexample">typedef struct {
double (* function) (void * ntuple_data, void * params);
void * params;
} gsl_ntuple_value_fn;
</pre></div>
<p>In this case the struct component <var>function</var> should return the value
to be added to the histogram for the ntuple row.
</p>
<a name="index-histogram_002c-from-ntuple"></a>
<a name="index-projection-of-ntuples"></a>
<dl>
<dt><a name="index-gsl_005fntuple_005fproject"></a>Function: <em>int</em> <strong>gsl_ntuple_project</strong> <em>(gsl_histogram * <var>h</var>, gsl_ntuple * <var>ntuple</var>, gsl_ntuple_value_fn * <var>value_func</var>, gsl_ntuple_select_fn * <var>select_func</var>)</em></dt>
<dd><p>This function updates the histogram <var>h</var> from the ntuple <var>ntuple</var>
using the functions <var>value_func</var> and <var>select_func</var>. For each
ntuple row where the selection function <var>select_func</var> is non-zero the
corresponding value of that row is computed using the function
<var>value_func</var> and added to the histogram. Those ntuple rows where
<var>select_func</var> returns zero are ignored. New entries are added to
the histogram, so subsequent calls can be used to accumulate further
data in the same histogram.
</p></dd></dl>
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Next: <a href="Example-ntuple-programs.html#Example-ntuple-programs" accesskey="n" rel="next">Example ntuple programs</a>, Previous: <a href="Closing-an-ntuple-file.html#Closing-an-ntuple-file" accesskey="p" rel="previous">Closing an ntuple file</a>, Up: <a href="N_002dtuples.html#N_002dtuples" accesskey="u" rel="up">N-tuples</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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