# option 1 – iterate from 1 to n
def factorial(n):
    # we start with the "n" itself
    result = n
    # for all numbers from 1 to n-1:
    for i in range(1,n):
        # multiply "result" by "i"
        result *= i
    return result
print factorial(8)
    
# option 2 – iterate from n to 1
def factorial(n):
    # we start with 1
    result = 1
    # for all numbers from 1 to n:
    for i in range(1,n+1):
        # multiply "result" by "i"
        result *= i
    return result
print factorial(8)
    
# option 3 – don't use loops at all!
def factorial(n):
    # if n is equal to zero, return 1 (0! = 1)
    if n == 0:
        return 1
    # if n in higher than zero, run the function again with n-1 → we will arrive to 0 in the end, which will return 1 (n * n-1 * n-2 * … * 2 * 1 * 1)
    return n * factorial(n - 1)
print factorial(8)