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

I also thought it was a strange behavior of Python. It turns out that I was not solving the division well (on paper); I was giving a value of 0 to the quotient and a value of -5 to the remainder. Terrible... I forgot the geometric representation of integers numbers. By recalling the geometry of integers given by the number line, one can get the correct values for the quotient and the remainder, and check that Python's behavior is fine. (Although I assume that you have already resolved your concern a long time ago).

Answer 2

In python, modulo operator works like this.

>>> mod = n - math.floor(n/base) * base

so the result is (for your case):

mod = -5 - floor(-1.25) * 4
mod = -5 - (-2*4)
mod = 3

whereas other languages such as C, JAVA, JavaScript use truncation instead of floor.

>>> mod = n - int(n/base) * base

which results in:

mod = -5 - int(-1.25) * 4
mod = -5 - (-1*4)
mod = -1

If you need more information about rounding in python, read this.