Function to find the nth prime.

Primes.swift

extension Int {

    func isDivisible(by value: Int) -> Bool {
        return self % value == 0
    }
}

extension Array where Element == Int {

    func containsDivisor(of value: Int) -> Bool {
        return contains(where: value.isDivisible(by:))
    }
}

// 1-indexed
func prime(at index: Int) -> Int? {

    let numbers = 2...
    var primes: [Int] = []
    
    for number in numbers {

        guard primes.count < index else {
            break
        }

        if !primes.containsDivisor(of: number) {
            primes.append(number)
        }
    }

    return primes.last
}

print(prime(at: 100)!)