swift - Returning a Bool in a function

Question

I'm getting the error "missing return in function expected to return 'Bool'" but I can't figure out why.

func isPrime(_ number: Int) -> Bool {
   for primeDivisors in 2..<number {
       if number % primeDivisors == 0 {
           return true
       } else {
           return false
       }
   }
}

isPrime(13)

Answer 1

Your prime checker is incorrect, because the very first iteration of the loop returns a value. You have to go through the entire loop before deciding that the number is prime.

func isPrime(_ number: Int) -> Bool {
    for primeDivisors in 2..<number {
        if number % primeDivisors == 0 {
            return false
        }
    }
    return true
}

Note that this code is inefficient, because it continues checking divisibility more times than it is necessary: you can stop checking upon reaching square root of number.