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The feval function allow 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.