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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 &lt;octave/oct.h>
     #include &lt;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 &amp;&amp; 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")
     &rArr; 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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