[Numpy-discussion] Behavior of rint?

[Matthew Brett]

Hi,

On Fri, Jan 19, 2018 at 3:24 PM, Charles R Harris
<charlesr.harris at gmail.com> wrote:
>
>
> On Fri, Jan 19, 2018 at 7:48 AM, Matthew Brett <matthew.brett at gmail.com>
> wrote:
>>
>> Hi Chuck,
>>
>> Thanks for the replies, they are very helpful.
>>
>> On Fri, Jan 19, 2018 at 1:51 PM, Charles R Harris
>> <charlesr.harris at gmail.com> wrote:
>> >
>> >
>> > On Fri, Jan 19, 2018 at 6:41 AM, Charles R Harris
>> > <charlesr.harris at gmail.com> wrote:
>> >>
>> >>
>> >>
>> >> On Fri, Jan 19, 2018 at 3:30 AM, Matthew Brett
>> >> <matthew.brett at gmail.com>
>> >> wrote:
>> >>>
>> >>> Hi,
>> >>>
>> >>> Sorry for my confusion, but I noticed (as a result of the discussion
>> >>> here [1]) that np.rint and the fallback C function [2] seem to round
>> >>> to even.  But - my impression was that C rint, by default, rounds down
>> >>> [3].   Is numpy rint not behaving the same way as the GNU C library
>> >>> rint?
>> >>>
>> >>> In [4]: np.rint(np.arange(0.5, 11))
>> >>> Out[4]: array([ 0.,  2.,  2.,  4.,  4.,  6.,  6.,  8.,  8., 10., 10.])
>> >>>
>> >>> In [5]: np.round(np.arange(0.5, 11))
>> >>> Out[5]: array([ 0.,  2.,  2.,  4.,  4.,  6.,  6.,  8.,  8., 10., 10.])
>> >>
>> >>
>> >> The GNU C documentation says that rint "round(s) x to an integer value
>> >> according to the current rounding mode." The rounding mode is
>> >> determined by
>> >> settings in the FPU control word. Numpy runs with it set to round to
>> >> even,
>> >> although, IIRC, there is a bug on windows where the library is not
>> >> setting
>> >> those  bits correctly.
>> >
>> >
>> > Round to even is also the Python default rounding mode.
>>
>> Do you mean that it is Python setting the FPU control word?  Or do we
>> set it?  Do you happen to know where that is in the source?  I did a
>> quick grep just now without anything obvious.
>
>
> I can't find official (PEP) documentation, but googling indicates that in
> Python 3, `round` rounds to even, and in Python 2 it rounds up. See also
> https://docs.python.org/3/whatsnew/3.0.html.

But I guess this could be the Python implementation of round, rather
than rint and the FPU control word?  I'm asking because the question
arose about npy_rint at the C level ...

Cheers,

Matthew