## Bug #17098

### Float#negative? reports negative zero as not negative

**Description**

Is this intended behaviour?

irb(main):001:0> neg_zero = -0.0 => -0.0 irb(main):002:0> neg_zero.negative? => false irb(main):003:0> neg_zero < 0 => false

It happens because `Numeric#negative?`

uses `< 0`

. My understanding of IEEE floating point is that negative zero is not less than zero, but I think it should still report as negative.

#### Updated by jeremyevans0 (Jeremy Evans) 6 months ago

I disagree. By that logic, positive float zero should be positive (it isn't), and since `-0.0 == 0.0`

both should report as negative and positive, which is a contradiction.

#### Updated by chrisseaton (Chris Seaton) 6 months ago

It's already the case that `neg_zero == pos_zero`

but not all methods on the two returns the same result - for example `neg_zero.inspect != pos_zero.inspect`

.

#### Updated by sawa (Tsuyoshi Sawada) 6 months ago

chrisseaton (Chris Seaton) wrote in #note-2:

not all methods on the two [return] the same result - for example

`neg_zero.inspect != pos_zero.inspect`

.

What is your suggestion? To dispense with `-0.0`

?

#### Updated by chrisseaton (Chris Seaton) 6 months ago

My suggestion is that `Float#negative?(neg_zero) == true`

and `Float#positive?(pos_zero) == false`

.

We can't get rid of negative zero - it's a standard part of floating point numbers as implemented by the hardware. It's also useful in some applications.

#### Updated by chrisseaton (Chris Seaton) 6 months ago

If people aren't keen on changing `Float#negative?`

since it has existing semantics, then another option could be to add a new predicate `Float#negative_zero?`

- that would allow people to differentiate as needed.

#### Updated by Eregon (Benoit Daloze) 6 months ago

chrisseaton (Chris Seaton) wrote in #note-5:

then another option could be to add a new predicate

`Float#negative_zero?`

- that would allow people to differentiate as needed.

Interesting, I thought `equal?`

would work for that (it does on TruffleRuby) but it does not on CRuby 2.6.6:

[19] pry(main)> 0.0.equal?(0.0) => true [20] pry(main)> -0.0.equal?(-0.0) => false

I guess it's a result of implementing Float as tagged (flonum) but only for a subset of Float:

[25] pry(main)> a=-0.0 => -0.0 [26] pry(main)> a.equal?(a) => true

`eql?`

and `==`

do not differentiate 0.0 and -0.0.

One way to test for -0.0 seems `(-f).equal?(0.0)`

but that's kind of brittle as it relies on 0.0 being a flonum on CRuby.

I tend to agree that `0.0.positive? == -0.0.negative?`

and since `0.positive? => false`

then both as `false`

seems to make sense (essentially they are the same as `> 0`

and `< 0`

).

#### Updated by marcandre (Marc-Andre Lafortune) 6 months ago

AFAIK, the only way to check for `-0.0`

is `1.0 / var == -Float::INFINITY`

I find it difficult to discuss what need to be done about any of this as I do not know of the use cases for `-0.0`

; without them it seems very theoretical.

#### Updated by sawa (Tsuyoshi Sawada) 6 months ago

Ruby or IEEE 754 seem to regard `-0.0`

and `0.0`

not as 0 in the mathematical sense, but as (something like) negative and positive infinitesimal. Taking this into account, I started to think `-0.0.negative?`

and `0.0.positive?`

should both be `true`

.

Regarding the fact that `-0.0 == 0.0`

is true, I think it should be understood as '`-0.0`

approaches `0.0`

' rather than '`-0.0`

is `0.0`

'. Then it would not contradict with the above. Floating point numbers are approximated numbers to begin with, so it does not make much sense to talk about their exact identity. Hence it makes sense to regard `Float#==`

to mean 'close enough' rather than 'exactly the same'.