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
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