I encountered negative zero in output from python; it's created for example as follows:
k = 0.0 print(-k)
The output will be
However, when I compare the
-k to 0.0 for equality, it yields True. Is there any difference between
-0.0 (I don't care that they presumably have different internal representation; I only care about their behavior in a program.) Is there any hidden traps I should be aware of?
Check out : −0 (number) in Wikipedia
Basically IEEE does actually define a negative zero
And by this definition for all purposes :
-0.0 == +0.0 == 0
I agree with aaronasterling that -0.0 and +0.0 are different objects. Making them equal (equality operator) makes sure that subtle bugs are not introduced in the code. Think of a * b == c * d
>>> a = 3.4 >>> b =4.4 >>> c = -0.0 >>> d = +0.0 >>> a*c -0.0 >>> b*d 0.0 >>> a*c == b*d True >>>
[Edit: More info based on comments]
When i said for all practical purposes, I had chosen the word rather hastily. I meant standard equality comparison.
I would add more information and references in this regard:
(1) As the reference says, the IEEE standard defines comparison so that +0 = -0, rather than -0 < +0. Although it would be possible always to ignore the sign of zero, the IEEE standard does not do so. When a multiplication or division involves a signed zero, the usual sign rules apply in computing the sign of the answer.
Operations like divmod, atan2 exhibits this behavior. In fact, atan2 complies with the IEEE definition as does the underlying "C" lib. See reference #2 for definition.
>>> divmod(-0.0,100) (-0.0, 0.0) >>> divmod(+0.0,100) (0.0, 0.0) >>> math.atan2(0.0, 0.0) == math.atan2(-0.0, 0.0) True >>> math.atan2(0.0, -0.0) == math.atan2(-0.0, -0.0) False
One way is to find out through the documentation, if the implementation complies with IEEE behavior . It also seems from the discussion that there are subtle platform variations too.
How ever this aspect(IEEE definition compliance) has not been respected every where. See the rejection of PEP 754 (#3) due to disinterest! I am not sure if this was picked up later.