Cannot calculate factorials bigger than 20! ! How to do so?

Question

I am using unsigned long long integer format in order to calculate big factorials. However my code fails at some point can you have a look at it? Actually it is part of a la

Answer 1

A long long is only so big, and thus can only represent numbers so big. If you need an exact representation of bigger integers, you'll need to use something else (some 3-rd party library or some datatype you make yourself); if you don't need it to be exact, then you could use double's instead.