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A structure in Octave is a 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")
{
  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;
}
An example of its use is
x.a = 1; x.b = "test"; x.c = [1, 2]; structdemo (x, "b") ⇒ selected = test
The example above specifically uses the octave_scalar_map class which
is for representing a single struct.  For structure arrays the
octave_map class is used instead.  The commented code shows how the
demo could be modified to handle a structure array.  In that case the
contents method returns a Cell which may have more than one
element.  Therefore, to obtain the underlying octave_value in
this single-struct example we write
octave_value tmp = arg0.contents (arg1)(0);
where the trailing (0) is the () operator on the Cell object.  If
this were a true structure array with multiple elements we could iterate
over the elements using the () operator.
Structures are a relatively complex data container and there are more
functions available in oct-map.h which make coding with them easier
than relying on just contents.
Next: Sparse Matrices in Oct-Files, Previous: Cell Arrays in Oct-Files, Up: Oct-Files [Contents][Index]