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<h4 class="subsection">A.1.5 Structures in Oct-Files</h4>
<p>A structure in Octave is map between a number of fields represented and
their values. The Standard Template Library <code>map</code> class is used,
with the pair consisting of a <code>std::string</code> and an octave
<code>Cell</code> variable.
<p>A simple example demonstrating the use of structures within oct-files is
<pre class="example"><pre class="verbatim"> #include <octave/oct.h>
#include <octave/ov-struct.h>
DEFUN_DLD (structdemo, args, , "Struct demo.")
{
int nargin = args.length ();
octave_value retval;
if (nargin != 2)
print_usage ();
else
{
Octave_map arg0 = args(0).map_value ();
std::string arg1 = args(1).string_value ();
if (! error_state && arg0.contains (arg1))
{
// The following two lines might be written as
// octave_value tmp;
// for (Octave_map::iterator p0 =
// arg0.begin();
// p0 != arg0.end(); p0++ )
// if (arg0.key (p0) == arg1)
// {
// tmp = arg0.contents (p0) (0);
// break;
// }
// though using seek is more concise.
Octave_map::const_iterator p1 = arg0.seek (arg1);
octave_value tmp = arg0.contents(p1)(0);
Octave_map st;
st.assign ("selected", tmp);
retval = octave_value (st);
}
}
return retval;
}
</pre></pre>
<p>An example of its use is
<pre class="example"> x.a = 1; x.b = "test"; x.c = [1, 2];
structdemo (x, "b")
selected = test
</pre>
<p>The commented code above demonstrates how to iterate over all of the
fields of the structure, where as the following code demonstrates finding
a particular field in a more concise manner.
<p>As can be seen the <code>contents</code> method of the <code>Octave_map</code> class
returns a <code>Cell</code> which allows structure arrays to be represented.
Therefore, to obtain the underlying <code>octave_value</code> we write
<pre class="example"> octave_value tmp = arg0.contents (p1) (0);
</pre>
<p>where the trailing (0) is the () operator on the <code>Cell</code> object. We
can equally iterate of the elements of the Cell array to address the
elements of the structure array.
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