Normally, you evaluate expressions simply by typing them at the Octave prompt, or by asking Octave to interpret commands that you have saved in a file.
Sometimes, you may find it necessary to evaluate an expression that has
been computed and stored in a string, or use a string as the name of a
function to call. The
feval functions allow you
to do just that, and are necessary in order to evaluate commands that
are not known until run time, or to write functions that will need to
call user-supplied functions.
evalreturns. For example,
eval ("a = 13") -| a = 13 => 13
In this case, the value of the evaluated expression is printed and it is
also returned returned from
eval. Just as with any other
expression, you can turn printing off by ending the expression in a
semicolon. For example,
eval ("a = 13;") => 13
In this example, the variable
a has been given the value 13, but
the value of the expression is not printed. You can also turn off
automatic printing for all expressions executed by
eval using the
evalthat do not end with semicolons. If it is zero, automatic printing is suppressed. The default value is 1.
feval ("acos", -1) => 3.1416
calls the function
acos with the argument `-1'.
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
EXTERNAL in Fortran). Instead, you must refer to functions
by name, and use
feval to call them.
Here 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 See section Predicates for Numeric Objects, for example,
for a list of predicates for numeric objects, and See section Status of Variables, for a description of the
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