On 11/8/2011 10:41 PM, Raymond Toy wrote:
> On 11/8/11 1:42 PM, Richard Fateman wrote:
>> On 11/8/2011 12:55 PM, Edwin Woollett wrote:
>>> Is it possible to return an error in bessel.lisp
>>> when, for example, the numerical value of
>>> a bessel function is so large that the code/system
>>> can't cope, and the answer returned for a calculation
>>> is completely wrong?
>>>
>> The "right" solution is to roll over to another computer program that
>> uses bigfloats and an appropriate algorithm for the argument values,
>> and gives the right answer. (This is one of the things that
>> Mathematica claims to do.)
>>
> On the other hand, I think there's an expectation that floats are
> reasonably fast. If they automatically overflowed to bfloats, then
> suddenly everything gets slower.
It would be noticeably slower when they DID overflow to bfloats, but
should be just as fast for double-floats.
> That would go against my expectations.
>
> But if we signaled an error, as Ted suggests, then the user could catch
> it and do something about it, including conversion to bfloats.
Not so easy to do, unless the error message was something like ...
argument wxyz too big for bessel_j(pqr,wxyz), but if you really want to
know, its bigfloat value is ab........b1234. I think that your
suggestion -- which is, I think, that the user re-do the whole problem
in bigfloats -- seems unnecessarily painful, and also requires something
that Maxima does not have, which is a complete coverage of all floating
point library routines in bigfloat arithmetic. As you know, we can't
just run the FORTRAN 2 Lisp stuff in bfloat -- most algorithms/
constants are not adequate for anything of higher precision than machine
double-floats.
While I recognize that people might hope for fast computing, they are,
after all, using a computer algebra system and might be willing to wait
for an answer that is "correct" if possible.
But my proposal is more long terml thinking than a proposal to "fix this
bug".
>
> Ray
>
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