On 4/11/10, Sheldon Newhouse <newhouse at math.msu.edu> wrote:
> Suppose I have a real function f(x) of a a real variable x and a list,
> say list_a of 1000 real numbers.
> I know that on some numbers , x_0, in 'list_a' the evaluation f(x_0)
> produces numerical overflow, but on others it does not.
> I would like to produce a sublist, list_b, of list of the numbers, y,
> on which f(y) is less than 10^6.
>
> I want to know how to find list_b.
>
> When I try to test each value in list_a, my program stops as soon as an
> overflow is reached. How do I tell maxima to record that an error was
> produced and keep on going?
Enclose the function evaluation within errcatch, which returns
[foo] where foo is the result of a successful evaluation or []
(empty list) when there is an error, but without stopping for
the error.
You could also evaluate the function as a bigfloat which won't
overflow. Then you could compare the results to some limit.
Be advised that Lisp implementations handle
floating point overflow in various ways. Some (SBCL, CMUCL,
maybe others) can handle non-numeric floating point values
(i.e. positive and negative infinity, not-a-number) by setting
a flag. That is a nonstandard extension: the CL spec appears
to rule out the existence of non-numeric floats.
Some other Lisps (Clisp, Allegro, maybe others) follow the
spec to the letter and do not recognize non-numeric floats.
Operations which would create such a float always trigger
an error.
Finally there is also GCL which handles non-numeric floats
inconsistently; such floats can be created but not displayed.
I guess that's a bug.
Which Lisp implementation are you using?
best
Robert Dodier