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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 need 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.
Evaluate the function named name. Any arguments after the first are passed on to the named function. For example,
feval ("acos", -1) 3.1416calls the function
acoswith the argument ‘-1’.The function
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 (likeEXTERNALin Fortran). Instead, you must refer to functions by name, and usefevalto call them.
A similar function run exists for calling user script files, that
are not necessarily on the user path
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.