Strange result with math.atan2()

[Vedran Furač]

Ben Caradoc-Davies wrote:
> Vedran Furač wrote:
>> I think that this results must be the same:
>> In [3]: math.atan2(-0.0,-1)
>> Out[3]: -3.1415926535897931
>> In [4]: math.atan2(-0,-1)
>> Out[4]: 3.1415926535897931
> 
> -0 is converted to 0, then to 0.0 for calculation, losing the sign. You 
> might as well write 0.0 instead of -0
> 
> The behaviour of atan2 conforms to the ISO C99 standard (Python is 
> implemented in C). Changing the sign of the first argument changes the 
> sign of the output, with no special treatment for zero.
> 
> http://www.ugcs.caltech.edu/manuals/libs/mpfr-2.2.0/mpfr_22.html

Well, here I can read:

Special values are currently handled as described in the ISO C99 standard
for the atan2 function (note this may change in future versions):

        * atan2(+0, -0) returns +Pi.
        * atan2(-0, -0) returns -Pi. /* wrong too */
        * atan2(+0, +0) returns +0.
        * atan2(-0, +0) returns -0. /* wrong too */
        * atan2(+0, x) returns +Pi for x < 0.
        * atan2(-0, x) returns -Pi for x < 0
		^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And the formula (also from that site):
	if x < 0, atan2(y, x) = sign(y)*(PI - atan (abs(y/x)))
				^^^^^^^

So, you can convert -0 to 0, but you must multiply the result with sign of
y, which is '-' (minus).

Also, octave:

octave2.9:1> atan2(-0,-1)
ans = -3.1416

or matlab:

>> atan2(-0,-5)

ans =

   -3.1416