The modulo operation on negative numbers in Python

Question

I\'ve found some strange behaviour in Python regarding negative numbers:

>>> -5 % 4
3

Could anyone explain what\'s going on?

Answer 1

It's also worth to mention that also the division in python is different from C: Consider

>>> x = -10
>>> y = 37

in C you expect the result

0

what is x/y in python?

>>> print x/y
-1

and % is modulo - not the remainder! While x%y in C yields

-10

python yields.

>>> print x%y
27

You can get both as in C

The division:

>>> from math import trunc
>>> d = trunc(float(x)/y)
>>> print d
0

And the remainder (using the division from above):

>>> r = x - d*y
>>> print r
-10

This calculation is maybe not the fastest but it's working for any sign combinations of x and y to achieve the same results as in C plus it avoids conditional statements.