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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 section 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

Function File: run (f)
Command: run f

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

See also: system.


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