Factorial of a large number in python

Question

Here\'s my approach to factorials:

def factorial(n):
    \'\'\'Returns factorial of n\'\'\'
    r = 1
    for i in range(1, n + 1):
        r *= i
    return         


        

Answer 1

The slowdown in caused by a quadradic effect: as n gets larger you have to do more multiplications, but you also have to multiply larger numbers.

Finding a better algorithm won't be easy. You can try to exploit symmetries (as in FFT). It could also well pay to do multiplications in a different order, with intermediate results, such that you end up with multiplying only a few very big numbers at the end, but I haven't thought that to the end. In any case, you will have to find a law to exploit.

Look here for further inspiration.