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<h3 class="section">23.7 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-2078"></a><a name="index-gsl_005fntuple_005fselect_005ffn-2079"></a>The <dfn>selection function</dfn> determines which ntuple rows are selected
for histogramming. It is defined by the following struct,
<pre class="smallexample"> typedef struct {
int (* function) (void * ntuple_data, void * params);
void * params;
} gsl_ntuple_select_fn;
</pre>
<p class="noindent">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-2080"></a><a name="index-gsl_005fntuple_005fvalue_005ffn-2081"></a>The <dfn>value function</dfn> computes scalar values for those ntuple rows
selected by the selection function,
<pre class="smallexample"> typedef struct {
double (* function) (void * ntuple_data, void * params);
void * params;
} gsl_ntuple_value_fn;
</pre>
<p class="noindent">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-2082"></a><a name="index-projection-of-ntuples-2083"></a>
<div class="defun">
— Function: int <b>gsl_ntuple_project</b> (<var>gsl_histogram * h, gsl_ntuple * ntuple, gsl_ntuple_value_fn * value_func, gsl_ntuple_select_fn * select_func</var>)<var><a name="index-gsl_005fntuple_005fproject-2084"></a></var><br>
<blockquote><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></blockquote></div>
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