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<h4 class="subsection" id="Structures-in-Oct_002dFiles-1"><span>A.1.5 Structures in Oct-Files<a class="copiable-link" href="#Structures-in-Oct_002dFiles-1"> ¶</a></span></h4>
<p>A structure in Octave is a map between a number of fields represented and their
values. The Standard Template Library <code class="code">map</code> class is used, with the pair
consisting of a <code class="code">std::string</code> and an Octave <code class="code">Cell</code> variable.
</p>
<p>A simple example demonstrating the use of structures within oct-files is
</p>
<div class="example">
<pre class="verbatim">#include <octave/oct.h>
#include <octave/ov-struct.h>
DEFUN_DLD (structdemo, args, , "Struct Demo")
{
if (args.length () != 2)
print_usage ();
if (! args(0).isstruct ())
error ("structdemo: ARG1 must be a struct");
octave_scalar_map arg0 = args(0).scalar_map_value ();
//octave_map arg0 = args(0).map_value ();
if (! args(1).is_string ())
error ("structdemo: ARG2 must be a character string");
std::string arg1 = args(1).string_value ();
octave_value tmp = arg0.contents (arg1);
//octave_value tmp = arg0.contents (arg1)(0);
if (! tmp.is_defined ())
error ("structdemo: struct does not have a field named '%s'\n",
arg1.c_str ());
octave_scalar_map st;
st.assign ("selected", tmp);
return octave_value (st);
}
</pre></div>
<p>An example of its use is
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">x.a = 1; x.b = "test"; x.c = [1, 2];
structdemo (x, "b")
⇒ selected = test
</pre></div></div>
<p>The example above specifically uses the <code class="code">octave_scalar_map</code> class which is
for representing a single struct. For structure arrays, the <code class="code">octave_map</code>
class is used instead. The commented code shows how the demo could be modified
to handle a structure array. In that case, the <code class="code">contents</code> method returns
a <code class="code">Cell</code> which may have more than one element. Therefore, to obtain the
underlying <code class="code">octave_value</code> in the single struct example we would write
</p>
<div class="example">
<pre class="example-preformatted">octave_value tmp = arg0.contents (arg1)(0);
</pre></div>
<p>where the trailing <code class="code">(0)</code> is the <code class="code">()</code> operator on the <code class="code">Cell</code>
object. If this were a true structure array with multiple elements we could
iterate over the elements using the <code class="code">()</code> operator.
</p>
<p>Structures are a relatively complex data container and there are more functions
available in <samp class="file">oct-map.h</samp> which make coding with them easier than relying
on just <code class="code">contents</code>.
</p>
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