On 3 Oct 2006, at 17:47, James Y Knight wrote:
> On Oct 3, 2006, at 8:30 AM, Martin v. Löwis wrote:
>> As Michael Hudson observed, this is difficult to implement, though:
>> You can't distinguish between -0.0 and +0.0 easily, yet you should.
>
> Of course you can. It's absolutely trivial. The only part that's even
> *the least bit* sketchy in this is assuming that a double is 64 bits.
> Practically speaking, that is true on all architectures I know of,
How about doing 1.0 / x, where x is the number you want to test? On
systems with sane semantics, it should result in an infinity, the
sign of which should depend on the sign of the zero. While I'm sure
there are any number of places where it will break, on those
platforms it seems to me that you're unlikely to care about the
difference between +0.0 and -0.0 anyway, since it's hard to otherwise
distinguish them.
e.g.
double value_to_test;
...
if (value_to_test == 0.0) {
double my_inf = 1.0 / value_to_test;
if (my_inf < 0.0) {
/* We have a -ve zero */
} else if (my_inf > 0.0) {
/* We have a +ve zero */
} else {
/* This platform might not support infinities (though we might
get a
signal or something rather than getting here in that
case...) */
}
}
(I should add that presently I've only tried it on a PowerPC, because
it's late and that's what's in front of me. It seems to work OK here.)
Kind regards,
Alastair
--
http://alastairs-place.net
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