Modular exponentiation fails for large mod in C++

Question

This is the code I\'m using for calculating (n^p)%mod. Unfortunately, it fails for large values of mod (in my case mod = 10000000000ULL) w

Answer 1

One of the possible issues here seems to be that when you do (a*b)%c, the a*b part itself could overflow, resulting in a wrong answer. One way to work around that is to use the identity that

(a*b)%c 

is equivalent to

(a%c * b%c)%c

This will prevent overflows in the intermediate multiplications as well.