Re: Need help with structures and "eval" function

[A Scottedward Hodel]

Date: Mon, 20 Mar 2000 08:23:57 -0600
To: Gerrit Visser <address@hidden>
From: A Scottedward Hodel <address@hidden>
Subject: Re: Need help with structures and "eval" function
Cc:
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> Hi All,
>
> I hope I'm sending this to the correct octave help mailing list.
>
> I'm still using 2.0.13 (I think), and if upgrading would help solve my
> problems I'll gladly do it. On to my problems:
>
> Is there some way I can either execute the "eval" command from a c++
> (.oct) environment or to ask the octave session to evaluate it and return
> a result? I've seen something like this in the Matlab .mex method, but
> couldn't find anything usefull in the octave help. The source files for
> octave isn't much help either, as I do not have a month to trace all the
> classes / functions (probably a optimistic estimate of the time!)


I think you're looking for the feval function; look in the help-octave
archives at http://www.che.wisc.edu/octave.

> The second problem I have is with passing structures to C++. Is there
> somewhere in the help files I can find out how to handle them? The
> reason I need it is the following: For a certain optimisation problem, the
> cost function is a generic one. The cost function needs to call a function
> specified in one of it's argument lists, so that it can call the correct
> function for evaluation:


I haven't used this .oct feature in awhile, so the syntax may be out of
date, but here's a snippet of some code I used with the Octave controls
toolbox a few years ago:

Structures are created and manipulated as "Octave_map" types (such as the
variable mp below):
//
// p a c k : TF form
// pack transfer function coefficients into an octave structure
//

Octave_map pack(const RowVector& num, const RowVector& den,
        const string_vector& inname, const string_vector& outname, double tsam)
{
  #ifdef DEBUG
  cout << "pack(TF form)" << endl;
  #endif
  Octave_map mp;

  // construct sys vector
  RowVector sys(4);
  for( int ii = 0; ii < 4 ; sys(ii++) = 0);
  sys(1) = 1;

  mp["sys"] = octave_value(sys,0);   // make it a row vector
  mp["inname"] = inname;
  mp["outname"] = outname;
  mp["num"] = octave_value(num,0);
  mp["den"] = octave_value(den,0);
  mp["tsam"] = tsam;
  int n, nz;
  if( tsam > 0)
  {
    mp["yd"] = (double) 1;
    n = 0;
    nz = den.length() - 1;
  }
  else
  {
    nz = 0;
    n = den.length() - 1;
    mp["yd"] = (double) 0;
  }
  mp["n"] = (double) n;
  mp["nz"] = (double) nz;
  string_vector stname = sysdefstname(n,nz);
  mp["stname"] = stname;

  return mp;
}


=======================================
Values may be extracted from Octave_map variables as shown below:
=======================================
// unpack system structure into its constituent elements
DEFUN_DLD(sysout, args, ,
" function sysout(sys[,opt])\n\
  print out a packed system in desired format\n\
 \n\
  sys: packed system\n\
  opt: []: primary system form (default)\n\
      \"ss\": state space form\n\
      \"tf\": transfer function form\n\
      \"zp\": zero-pole form\n\
      \"all\": all of the above")
{
  return sysout(args);
}

octave_value_list sysout(const octave_value_list& args)
{
  octave_value_list retval;

  // This stream is normally connected to the pager.

  int nargin = args.length ();
  if( (nargin < 1) || (nargin > 2) )
    {
      print_usage("sysout:");
      return retval;
    }

  octave_value sysval = args(0);
  if(sysval.is_map())
  {
    cout << "Yes, this is an octave structure." << endl;

    Octave_map map = sysval.map_value();

    octave_value sys_ov = map["sys"];
    if(sys_ov.is_numeric_type() && sys_ov.is_matrix_type() )
    {
      cout << "Yes, sys is a numeric type and a matrix:"
        << sys_ov.rows() << " x " << sys_ov.columns()
        << endl;

      RowVector sys_vec = sys_ov.vector_value();

      for ( int ii = 0 ; ii < sys_vec.length() ; ii++)
        cout << "sys(" << ii << ")=" << sys_vec(ii) << endl;

    }
    else cout << "Nope, sys is not a numeric type, nor a matrix." << endl;
  }
  else
  {
    cout << "No, this is not an octave structure." << endl;
  }

  return retval;

}
--
A S Hodel Assoc. Prof. Dept Elect and Computer Eng, Auburn Univ,AL 36849-5201
On leave at NASA Marshall Space Flight Center (256) 544-1426
Address until 31 July 2000:Mail Code TD-55, MSFC, Alabama, 35812
http://www.eng.auburn.edu/~scotte



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