negative zero in python

问题:

[Python 3.1]

I encountered negative zero in output from python; it's created for example as follows:

k = 0.0
print(-k)

The output will be -0.0.

However, when I compare the -k to 0.0 for equality, it yields True. Is there any difference between 0.0 and -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?

回答1:

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.

references :

1. http://docs.sun.com/source/806-3568/ncg_goldberg.html#924
2. FPTAN in http://en.wikipedia.org/wiki/Atan2
3. http://www.python.org/dev/peps/pep-0754/

回答2:

It makes a difference in the atan2() function (at least, in some implementations). In my Python 3.1 and 3.2 on Windows (which is based on the underlying C implementation, according to the note CPython implementation detail near the bottom of the Python math module documentation):

>>> import math
>>> math.atan2(0.0, 0.0)
0.0
>>> math.atan2(-0.0, 0.0)
-0.0
>>> math.atan2(0.0, -0.0)
3.141592653589793
>>> math.atan2(-0.0, -0.0)
-3.141592653589793

回答3:

Yes, there is a difference between 0.0 and -0.0 (though Python won't let me reproduce it :-P). If you divide a positive number by 0.0, you get positive infinity; if you divide that same number by -0.0 you get negative infinity.

Beyond that, though, there is no practical difference between the two values.

回答4:

math.copysign() treats -0.0 and +0.0 differently, unless you are running Python on a weird platform:

math.copysign(x, y)
     Return x with the sign of y. On a platform that supports signed zeros, copysign(1.0, -0.0) returns -1.0.

>>> import math
>>> math.copysign(1, -0.0)
-1.0
>>> math.copysign(1, 0.0)
1.0

回答5:

Same values, yet different numbers

>>> Decimal('0').compare(Decimal('-0'))        # Compare value
Decimal('0')                                   # Represents equality

>>> Decimal('0').compare_total(Decimal('-0'))  # Compare using abstract representation
Decimal('1')                                   # Represents a > b

Reference :
http://docs.python.org/2/library/decimal.html#decimal.Decimal.compare http://docs.python.org/2/library/decimal.html#decimal.Decimal.compare_total