Modulo operator : differences between C and Python

[Erik de Castro Lopo]

Hi all,

Just recently I prototyped a relatively complex algorithm in Python
before converting it to C. During this conversion I noticed that
the modulo operator returns different results in C and Python if
the numerator is negative. For positive values they produce the same 
result.

----------------------------------------------------------------
erikd at coltrane > python
Python 2.1.2 (#1, Jan 18 2002, 18:05:45) 
[GCC 2.95.4  (Debian prerelease)] on linux2
Type "copyright", "credits" or "license" for more information.
>>> 123 % 12
3
>>> -123 % 12
9
>>> 
erikd at coltrane > cat mod_test.c 
#include <stdio.h>

int main (void)
{       int a = 123 ;

        printf (" % d %% 12 == %d\n",  a,  a % 12) ;
        printf (" % d %% 12 == %d\n", -a, -a % 12) ;
        return 0 ;
} 
erikd at coltrane > gcc -Wall mod_test.c -o mod_test
erikd at coltrane > ./mod_test 
  123 % 12 == 3
 -123 % 12 == -3
----------------------------------------------------------------

Anybody have any idea why this is?

Erik
-- 
+-----------------------------------------------------------+
  Erik de Castro Lopo  nospam at mega-nerd.com (Yes it's valid)
+-----------------------------------------------------------+
The government everybody loves to abuse sues the company everybody loves 
to hate. Throw in a bunch of faceless lawyers cross-examining techies 
[with] all the charisma of a video driver and you've got a spectacle of 
thoroughly miniscule proportions.