|GNU Octave Manual Version 3|
by John W. Eaton, David Bateman, Søren Hauberg
Paperback (6"x9"), 568 pages
RRP £24.95 ($39.95)
9.1 Calling a Function by its Name
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
that finds the root of a user-supplied function of one variable using
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 4.6 Predicates for Numeric Objects, for example,
for a list of predicates for numeric objects, and see section 7.3 Status of Variables, for a description of the
- 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
acoswith the argument ‘-1’.
fevalis 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
EXTERNALin Fortran). Instead, you must refer to functions by name, and use
fevalto 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
runchange the directory to the directory where f is found.
runthen executes the script, and returns to the original directory.
See also system
|ISBN 095461206X||GNU Octave Manual Version 3||See the print edition|