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

sparkie a day 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

gus_massa 35 minutes ago
Thanks! [Sorry for the delay.]
---
FYI: For more context, I'm trying to send a PR to Chez Scheme (and indirectly to Racket) https://github.com/cisco/ChezScheme/pull/959 to reduce expressions like
```
  (+ 1.0 (length L))  ;  ==>  (+ 1.0 (fixnum->flonum (length L)))
```
where the "fixnums" are small integers and "flonums" are double.
It's fine, unless you have the case
```
  (+ -0.0 (length L))  ;  =wrong=>  (+ -0.0 (fixnum->flonum (length L)))
```
because if the length is 0, it get's transformed into 0.0 instead of -0.0
There are a few corner cases, in particular because it's possible to have
```
   (+ 1.0 x (length L))
```
and I really want to avoid the runtime check of (length L) == 0 if possible.
So I took a look, asked there, and now your opinion confirms what I got so far. My C is not very good, so it's nice to have a example of how the rounding directions are used. Luckily Chez Scheme only uses the default rounding and it's probably correct to cut a few corners. I'll take a looks for a few days in case there is some surprise.

kazinator a day ago

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

gus_massa a day ago

Copying the example by sparkie, something like this? https://godbolt.org/z/xhdnb9ax3 I get +0.0 if I comment the round to negative option.

gethly 12 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.

dcminter an hour ago

You're answering a question that OP did not ask.