(octave.info)Calling a Function by its Name

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9.1 Calling a Function by its Name
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The ‘feval’ function allows you to call a function from a string
containing its name.  This is useful when writing a function that needs
to call user-supplied functions.  The ‘feval’ function takes the name of
the function to call as its first argument, and the remaining arguments
are given to the function.

   The following example is a simple-minded function using ‘feval’ that
finds the root of a user-supplied function of one variable using
Newton’s method.

     function result = newtroot (fname, x)

     # usage: newtroot (fname, x)
     #
     #   fname : a string naming a function f(x).
     #   x     : initial guess

       delta = tol = sqrt (eps);
       maxit = 200;
       fx = feval (fname, x);
       for i = 1:maxit
         if (abs (fx) < tol)
           result = x;
           return;
         else
           fx_new = feval (fname, x + delta);
           deriv = (fx_new - fx) / delta;
           x = x - fx / deriv;
           fx = fx_new;
         endif
       endfor

       result = x;

     endfunction

   Note that this is only meant to be an example of calling
user-supplied functions and should not be taken too seriously.  In
addition to using a more robust algorithm, any serious code would check
the number and type of all the arguments, ensure that the supplied
function really was a function, etc.  Note: Predicates for Numeric
Objects, for a list of predicates for numeric objects, and Note:
Status of Variables, for a description of the ‘exist’ function.

 -- : feval (NAME, ...)
     Evaluate the function named NAME.

     Any arguments after the first are passed as inputs to the named
     function.  For example,

          feval ("acos", -1)
               ⇒ 3.1416

     calls the function ‘acos’ with the argument ‘-1’.

     The function ‘feval’ can also be used with function handles of any
     sort (Note: Function Handles).  Historically, ‘feval’ was the
     only way to call user-supplied functions in strings, but function
     handles are now preferred due to the cleaner syntax they offer.
     For example,

          F = @exp;
          feval (F, 1)
              ⇒ 2.7183
          F (1)
              ⇒ 2.7183

     are equivalent ways to call the function referred to by F.  If it
     cannot be predicted beforehand whether F is a function handle,
     function name in a string, or inline function then ‘feval’ can be
     used instead.

   A similar function ‘run’ exists for calling user script files, that
are not necessarily on the user path

 -- : run SCRIPT
 -- : run ("SCRIPT")
     Run SCRIPT in the current workspace.

     Scripts which reside in directories specified in Octave’s load
     path, and which end with the extension ‘".m"’, can be run simply by
     typing their name.  For scripts not located on the load path, use
     ‘run’.

     The filename SCRIPT can be a bare, fully qualified, or relative
     filename and with or without a file extension.  If no extension is
     specified, Octave will first search for a script with the ‘".m"’
     extension before falling back to the script name without an
     extension.

     Implementation Note: If SCRIPT includes a path component, then
     ‘run’ first changes the working directory to the directory where
     SCRIPT is found.  Next, the script is executed.  Finally, ‘run’
     returns to the original working directory unless SCRIPT has
     specifically changed directories.

     See also: Note: path, Note: addpath, Note:
     source.