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

python c math modulo
psihodelia picture psihodelia · Dec 15, 2009 · Viewed 18.3k times · Source

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?

Answer

Alex B picture Alex B · Dec 15, 2009
  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).

Related questions

Calling C/C++ from Python?

What would be the quickest way to construct a Python binding to a C or C++ library? (I am using Windows if this matters.)

Read More
Convert Python program to C/C++ code?

is it possible to convert a Python program to C/C++? I need to implement a couple of algorithms, and I'm not sure if the performance gap is big enough to justify all the pain I'd go through when doing …

Read More
It is more efficient to use if-return-return or if-else-return?

Suppose I have an if statement with a return. From the efficiency perspective, should I use if(A > B): return A+1 return A-1 or if(A > B): return A+1 else: return A-1 Should I prefer one or another …

Read More