(octave.info)Calling Octave Functions from Oct-Files

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A.1.8 Calling Octave Functions from Oct-Files
---------------------------------------------

There is often a need to be able to call another Octave function from
within an oct-file, and there are many examples of such within Octave
itself.  For example, the ‘quad’ function is an oct-file that calculates
the definite integral by quadrature over a user-supplied function.

   There are also many ways in which a function could be given as input.
It might be passed as one of

  1. Function Handle

  2. Anonymous Function Handle

  3. Inline Function

  4. String

   The code below demonstrates all four methods of passing a function to
an oct-file.

     #include <octave/oct.h>
     #include <octave/parse.h>
     
     DEFUN_DLD (funcdemo, args, nargout, "Function Demo")
     {
       int nargin = args.length ();
     
       if (nargin < 2)
         print_usage ();
     
       octave_value_list newargs;
     
       for (octave_idx_type i = nargin - 1; i > 0; i--)
         newargs(i-1) = args(i);
     
       octave_value_list retval;
     
       if (args(0).is_function_handle () || args(0).is_inline_function ())
         {
           octave_function *fcn = args(0).function_value ();
     
           retval = feval (fcn, newargs, nargout);
         }
       else if (args(0).is_string ())
         {
           std::string fcn = args(0).string_value ();
     
           retval = feval (fcn, newargs, nargout);
         }
       else
         error ("funcdemo: INPUT must be string, inline, or function handle");
     
       return retval;
     }

   The first input to the demonstration code is a user-supplied function
and the remaining arguments are all passed to the function.

     funcdemo (@sin, 1)
     ⇒ 0.84147
     funcdemo (@(x) sin (x), 1)
     ⇒ 0.84147
     funcdemo (inline ("sin (x)"), 1)
     ⇒ 0.84147
     funcdemo ("sin", 1)
     ⇒ 0.84147
     funcdemo (@atan2, 1, 1)
     ⇒ 0.78540

   When the user function is passed as a string the treatment of the
function is different.  In some cases it is necessary to have the user
supplied function as an ‘octave_function’ object.  In that case the
string argument can be used to create a temporary function as
demonstrated below.

     std::octave fcn_name = unique_symbol_name ("__fcn__");
     std::string fcode = "function y = ";
     fcode.append (fcn_name);
     fcode.append ("(x) y = ");
     fcn = extract_function (args(0), "funcdemo", fcn_name,
                             fcode, "; endfunction");
     ...
     if (fcn_name.length ())
       clear_function (fcn_name);

   There are two important things to know in this case.  First, the
number of input arguments to the user function is fixed, and in the
above example is a single argument.  Second, to avoid leaving the
temporary function in the Octave symbol table it should be cleared after
use.  Also, by convention all internal function names begin and end with
the character sequence ‘__’.