Modul (%) in python not like in C?

Bryan Olson fakeaddress at nowhere.org
Mon Sep 10 12:09:34 CEST 2007


Dennis Lee Bieber wrote:
> 	The best answer is probably to be found from the definition of
> divmod()

The divmod() function is one of those little delights that reminds
me why I love Python, but I do not think it answers the question
here. The definition of divmod() references the '%' operation, and
not the other way around.

   http://docs.python.org/lib/built-in-funcs.html
   http://docs.python.org/ref/binary.html


>>>> divmod(70, 6)
> (11, 4)
>>>> 6*11 + 4
> 70
>>>> divmod(-70, 6)
> (-12, 2)
>>>> 6 * -12 + 2
> -70

Also:

 >>> divmod(70, -6)
(-12, -2)
 >>> -6*-12 + -2
70

 >>> divmod(-70, -6)
(11, -4)
 >>> -6*11 + -4
-70

> Or in general terms
> 
> (a, b) = divmod(x, y)
> x = y * a + b

> IOWs, the definition of modulo, and the definition of integer division,
> are linked...

Right. Thus given integers x and y, y!=0, Python and C agree:

    x == (y * (x / y)) + (x % y)

The disagreement is how integer division rounds. In C, integer
division rounds toward zero. In Python, integer division rounds
downward.


-- 
--Bryan



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