Calling a Function by its Name (GNU Octave (version 6.2.0))

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### 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 a list of predicates for numeric objects, and see 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 (see 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: path, addpath, source.

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