C and Python - different behaviour of the modulo (%) operation

I have found that the same mod operation produces different results depending on what language is being used.

In Python:

-1 % 10

produces 9

In C it produces -1 !

1. Which one is the right modulo?
   
2. How to make mod operation in C to be the same like in Python?

Solution 1:

1. Both variants are correct, however in mathematics (number theory in particular), Python's modulo is most commonly used.
2. In C, you do ((n % M) + M) % M to get the same result as in Python. E. g. ((-1 % 10) + 10) % 10. Note, how it still works for positive integers: ((17 % 10) + 10) % 10 == 17 % 10, as well as for both variants of C implementations (positive or negative remainder).

Solution 2:

Python has a "true" modulo operation, while C has a remainder operation.

It has a direct relation with how the negative integer division is handled, i.e. rounded towards 0 or minus infinite. Python rounds towards minus infinite and C(99) towards 0, but in both languages (n/m)*m + n%m == n, so the % operator must compensate in the correct direction.

Ada is more explicit and has both, as mod and rem.