How to calculate the 21! (21 factorial) in Swift?
math, swift
Solution
Unsigned 64 bit integer has a maximum value of 18,446,744,073,709,551,615. While 21! = 51,090,942,171,709,440,000. For this kind of case, you need a Big Integer type. I found a question about Big Integer in Swift. There's a library for Big Integer in that link.
BigInteger equivalent in Swift?
Problem
I am making fuction that calculates factorial in Swift. Like this: ``` func factorial(factorialNumber: UInt64) -> UInt64 { if factorialNumber == 0 { return 1 } else { return factorialNumber * factorial(factorialNumber - 1) } } let x = factorial(20) ``` This fuction can calculate until 20!. I think factorial(21) is bigger than UINT64_MAX. How to accurately calculate 21! (21 factorial) in Swift?