[Python-ideas] integer dividion in R -- PS

[Xavier Ho]

On Fri, May 7, 2010 at 7:43 PM, Mark Dickinson <dickinsm at gmail.com> wrote:

>  No: PEP 238 is (partly) about how integer division is expressed in
> Python;  not about its semantics.  Python's choice for integer
> division with negative arguments goes back much further.  From
> Misc/HISTORY:
>
> ==================================
> ==> RELEASE 0.9.6 (6 Apr 1992) <==
> ==================================
>
> [...]
>
> New features in 0.9.6:
>
> [...]
>
> - Division and modulo for long and plain integers with negative operands
>  have changed; a/b is now floor(float(a)/float(b)) and a%b is defined
>  as a-(a/b)*b.  So now the outcome of divmod(a,b) is the same as
>  (a/b, a%b) for integers.  For floats, % is also changed, but of course
>  / is unchanged, and divmod(x,y) does not yield (x/y, x%y)...
> [...]
>

Nice! Thanks for sharing.


>
>
> Personally, I've always liked Python's behaviour in this regard:  for
> the few times that I've needed an 'x % y' operation that works with
> both positive and negative x, more often than not x-y*floor(x/y) turns
> out to be what I need.  I've lost count of the number times I've had
> to write something awkward like:
>
>        /* adjust for the exponent;  first reduce it modulo _PyHASH_BITS */
>        e = e >= 0 ? e % _PyHASH_BITS : _PyHASH_BITS-1-((-1-e) %
> _PyHASH_BITS);
>
> in C.
>

Wow. I would never write code like that, but you got a fair point.

Cheers,
Xav

(Mark: Sorry, I forgot to reply to the correct list.)
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