Ask HN: How is it possible to get -0.0 in a sum?

9 points by gus_massa a day ago

I'm looking for corner cases where he result is -0.0. As far as I know, the only way to get -0.0 in a sum is

  (-0.0) + (-0.0)
Does someone know any other case in IEEE 754?

Bonus question: What happens in subtractions? I only know

  (-0.0) - (+0.0)
Is there any other case?
sparkie 19 hours ago

It depends on the FP rounding mode. If rounding mode is FE_TOWARDZERO/FE_UPWARD/FE_TONEAREST then the case you gave is the only one I'm aware of. If rounding mode is FE_DOWNWARD (towards negative infinity) then other calculations that result in a zero will give a -0.0.

Here's an example of -1.0f + 1.0f resulting in -0.0: https://godbolt.org/z/5qvqsdh9P

gethly 5 hours ago

i would guess that because of how *** * floats are in binary computers, you have something like -0.0000000000000000000000000000000000001 and when you round it you end up with -0.0. Same goes for positive value, you're just not used to write the + sign before every number, so seeing the minus feels strange.

kazinator 17 hours ago

What happens if we take the smallest (as in closest to zero) negative subnormal and add it to itself?