Calling a Function by its Name - GNU Octave
Description: GNU Octave

Next: Evaluation in a Different Context, Up: Evaluation

### 9.1 Calling a Function by its Name

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. See Predicates for Numeric Objects, for example, for a list of predicates for numeric objects, and see Status of Variables, for a description of the `exist` function.

— Built-in Function: feval (name, ...)

> Evaluate the function named name. Any arguments after the first are passed on to the named function. For example,

>               feval ("acos", -1)
>                    ⇒ 3.1416

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

> The function `feval` is necessary in order to be able to write functions that call user-supplied functions, because Octave does not have a way to declare a pointer to a function (like C) or to declare a special kind of variable that can be used to hold the name of a function (like `EXTERNAL` in Fortran). Instead, you must refer to functions by name, and use `feval` to call them.

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

— Command: run script
— Function File: run (script)

> Run scripts in the current workspace that are not necessarily on the path. If script is the script to run, including its path, then `run` changes the directory to the directory where script is found. `run` then executes the script, and returns to the original directory.

> See also: system.