And here I thought IEEE floats had distinct values to represent overflows and
underflows that were distinct from both the zeros and the infinities. -- Darren
Duncan
On 2016-11-22 8:19 PM, Zefram wrote:
Zoffix Znet via RT wrote:
The reason we have a negative floating point zero at all is more due to
underlying implementations at whose level such zeros are used to signal
various exceptions.
No, that's not what negative zero is about in floating point. (Maybe
you're thinking of ones-complement integer formats.) In floating point,
zero doesn't only represent exact zero quantities, it also represents
underflow, and it's useful to know from which side of zero a quantity
underflowed. Generally, IEEE 754 provides well defined semantics
for signed zeroes throughout, which put negative zero on a par with
positive zero.
Are you able to describe any usecases where the string cast resulting
in a positive zero as opposed to negative zero creates a problem?
Getting the right zero particularly matters in trigonometry and in complex
arithmetic, where the zeroes are on opposite sides of many branch cuts.
For example:
atan2(0e0, -1)
3.14159265358979
atan2(-0e0, -1)
-3.14159265358979
The above behaviour is correct and desirable. In a situation where the
arguments come from string input, getting this correct behaviour depends
on the Str->Num conversion properly supporting signed zeroes.
-zefram
