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<a name="Structures-in-Oct_002dFiles"></a>
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<p>
Next: <a href="Sparse-Matrices-in-Oct_002dFiles.html#Sparse-Matrices-in-Oct_002dFiles" accesskey="n" rel="next">Sparse Matrices in Oct-Files</a>, Previous: <a href="Cell-Arrays-in-Oct_002dFiles.html#Cell-Arrays-in-Oct_002dFiles" accesskey="p" rel="prev">Cell Arrays in Oct-Files</a>, Up: <a href="Oct_002dFiles.html#Oct_002dFiles" accesskey="u" rel="up">Oct-Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Structures-in-Oct_002dFiles-1"></a>
<h4 class="subsection">A.1.5 Structures in Oct-Files</h4>
<p>A structure in Octave is a 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>
<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")
{
octave_value retval;
int nargin = args.length ();
if (args.length () == 2)
{
octave_scalar_map arg0 = args(0).scalar_map_value ();
//octave_map arg0 = args(0).map_value ();
if (! error_state)
{
std::string arg1 = args(1).string_value ();
if (! error_state)
{
octave_value tmp = arg0.contents (arg1);
//octave_value tmp = arg0.contents (arg1)(0);
if (tmp.is_defined ())
{
octave_scalar_map st;
st.assign ("selected", tmp);
retval = octave_value (st);
}
else
error ("structdemo: struct does not have a field named '%s'\n",
arg1.c_str ());
}
else
error ("structdemo: ARG2 must be a character string");
}
else
error ("structdemo: ARG1 must be a struct");
}
else
print_usage ();
return retval;
}
</pre><pre class="example">
</pre></div>
<p>An example of its use is
</p>
<div class="example">
<pre class="example">x.a = 1; x.b = "test"; x.c = [1, 2];
structdemo (x, "b")
⇒ selected = test
</pre></div>
<p>The example above specifically uses the <code>octave_scalar_map</code> class which
is for representing a single struct. For structure arrays the
<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>contents</code> method returns a <code>Cell</code> which may have more than one
element. Therefore, to obtain the underlying <code>octave_value</code> in
this single-struct example we write
</p>
<div class="example">
<pre class="example">octave_value tmp = arg0.contents (arg1)(0);
</pre></div>
<p>where the trailing (0) is the () operator on the <code>Cell</code> object. If
this were a true structure array with multiple elements we could iterate
over the elements using the () operator.
</p>
<p>Structures are a relatively complex data container and there are more
functions available in <samp>oct-map.h</samp> which make coding with them easier
than relying on just <code>contents</code>.
</p>
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Next: <a href="Sparse-Matrices-in-Oct_002dFiles.html#Sparse-Matrices-in-Oct_002dFiles" accesskey="n" rel="next">Sparse Matrices in Oct-Files</a>, Previous: <a href="Cell-Arrays-in-Oct_002dFiles.html#Cell-Arrays-in-Oct_002dFiles" accesskey="p" rel="prev">Cell Arrays in Oct-Files</a>, Up: <a href="Oct_002dFiles.html#Oct_002dFiles" accesskey="u" rel="up">Oct-Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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