[Cython] [cython-users] Integer division

[Robert Bradshaw]

Or perhaps we should respect float division even with cdivision... Thoughts?

On Mon, Sep 7, 2015 at 5:13 PM, Robert Bradshaw <robertwb at gmail.com> wrote:
> Oh, that's another issue. I'll clarify this in the documentation.
>
> On Mon, Sep 7, 2015 at 3:59 PM, Antony Lee <antony.lee at berkeley.edu> wrote:
>> Actually I realized that it's another problem; setting cdivision to True (as
>> I did in the directive of the example I gave, because I basically always set
>> this to True) overrides __future__ division and restores the C division
>> semantic that int / int = int.  The documentation actually says that the
>> cdivision directive should only affect the case of operands with different
>> signs and of division by zero, not whether int / int = int.
>> Antony
>>
>> 2015-09-07 15:50 GMT-07:00 Robert Bradshaw <robertwb at gmail.com>:
>>>
>>> On Mon, Sep 7, 2015 at 3:46 PM, Robert Bradshaw <robertwb at gmail.com>
>>> wrote:
>>> > On Mon, Sep 7, 2015 at 3:29 PM, Antony Lee <antony.lee at berkeley.edu>
>>> > wrote:
>>> >> Sorry, I intended the entire discussion to apply to the case of Python3
>>> >> semantics.  Indeed, even with language_level=3 and from __future__
>>> >> import
>>> >> division, foo.decl(3, 3) prints "1, 2, 0, 1" whereas foo.nodecl(3, 3)
>>> >> prints
>>> >> "1.0, 0.5, 2.0, 1.0".
>>> >
>>> > from __future__ import division behaves as expected; I just added some
>>> > even more explicit tests:
>>> >
>>> > https://github.com/cython/cython/commit/e61da2eb8a292bc34285c895aade523b6d353414
>>> >
>>> > I'm surprised language_level=3 doesn't automatically set this directive.
>>>
>>> Are you sure you're setting it correctly?
>>> https://github.com/cython/cython/blob/master/tests/run/cython3.pyx#L30
>>>
>>> >> 2015-09-07 15:07 GMT-07:00 Robert Bradshaw <robertwb at gmail.com>:
>>> >>>
>>> >>> On Mon, Sep 7, 2015 at 11:59 AM, Antony Lee <antony.lee at berkeley.edu>
>>> >>> wrote:
>>> >>> > I would like to suggest that division of integers with "/" prints a
>>> >>> > warning
>>> >>> > at compile time.  In my opinion, "/" should be restricted to float
>>> >>> > division,
>>> >>> > and "//" should be used when rounding semantics are required.
>>> >>> >
>>> >>> > Otherwise, we get weird edge cases such as:
>>> >>> >
>>> >>> > #cython: cdivision=True, infer_types=True
>>> >>> > def decl(int m, int n):
>>> >>> >     for i in range(1, m):
>>> >>> >         for j in range(1, n):
>>> >>> >             print(i / j) # integer division
>>> >>> >
>>> >>> > def nodecl(m, n):
>>> >>> >     for i in range(1, m):
>>> >>> >         for j in range(1, n):
>>> >>> >             print(i / j) # float division
>>> >>>
>>> >>> Slight correction: int or float division depending on whether you're
>>> >>> using Python 2 or Python 3. The runtime type of i/j isn't something we
>>> >>> can delegate to the C compiler.
>>> >>>
>>> >>> > (whose behavior may even change if type inference rules changes).
>>> >>> >
>>> >>> > A more extreme choice would be to have "/" always behave like float
>>> >>> > (double)
>>> >>> > division even with integer arguments.
>>> >>>
>>> >>> You mean like putting from __future__ import division at the top of
>>> >>> your file? Or compiling with -3 (for Python 3 language semantics)?
>>> >>>
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