#### A.1.5 Structures in Oct-Files

A structure in Octave is map between a number of fields represented and their values. The Standard Template Library `map` class is used, with the pair consisting of a `std::string` and an octave `Cell` variable.

A simple example demonstrating the use of structures within oct-files is

         #include <octave/oct.h>
         #include <octave/ov-struct.h>

         DEFUN_DLD (structdemo, args, , "Struct demo.")
         {
           int nargin = args.length ();
           octave_value retval;

           if (args.length () == 2)
             {
               octave_scalar_map arg0 = args(0).scalar_map_value ();

               if (! error_state)
                 {
                   std::string arg1 = args(1).string_value ();

                   if (! error_state)
                     {
                       octave_value tmp = arg0.contents (arg1);

                       if (tmp.is_defined ())
                         {
                           octave_scalar_map st;

                           st.assign ("selected", tmp);

                           retval = octave_value (st);
                         }
                       else
                         error ("sruct does not contain field named '%s'\n",
                                arg1.c_str ());
                     }
                   else
                     error ("expecting character string as second argument");
                 }
               else
                 error ("expecting struct as first argument");
             }
           else
             print_usage ();

           return retval;
         }

An example of its use is

         x.a = 1; x.b = "test"; x.c = [1, 2];
         structdemo (x, "b")
         ⇒ selected = test

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.

As can be seen the `contents` method of the `Octave_map` class returns a `Cell` which allows structure arrays to be represented. Therefore, to obtain the underlying `octave_value` we write

         octave_value tmp = arg0.contents (p1) (0);

where the trailing (0) is the () operator on the `Cell` object. We can equally iterate of the elements of the Cell array to address the elements of the structure array.