Modul (%) in python not like in C?

Arnaud Delobelle arnodel at googlemail.com
Sun Sep 9 21:39:48 CEST 2007


On Sep 9, 8:15 pm, "J. Cliff Dyer" <j... at sdf.lonestar.org> wrote:
> Dotan Cohen wrote:
> > FIrst of all, how is the % symbol (as in 70%6=4) called in English?
>
> > Second, in Turbo C -111%10=-1 however in python -111%10=9. Is one or
> > the other in error? Is this a known gotcha? I tried to google the
> > subject however one cannot google the symbol %. Thanks in advance.
>
> > Dotan Cohen
>
> The % operator is called "modulo" in English.  I don't think the
> difference in implementation is an error.  It's just a difference of
> calculation method.
>
> Python will always yield a number x = m%n such that 0 <= x < n, but
> Turbo C will always yield a number such that if x = m%n -x = -m%n.  That
> is, since 111 % 10 = 1, -111 % 10 = -1.  The two values will always
> differ by n (as used above).
>
> I'm sure there are mathematicians on the list who can give you a more
> technical, precise explanation of the reasons for the different results.

I don't know if there's a mathematical explanation of the difference,
but I prefer the behaviour of the Python modulo operator because for
any a, b, n, if  a = b (mod n) holds then the Python expression 'a % n
== b % n' evaluates to True.

--
Arnaud





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