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)

If you call `isPrime(1)` your `for` loop will never execute and the function will never hit a `return` statement. — vacawama, Jun 16 '17 at 02:39

score 1 · Accepted Answer · answered Jun 16 '17 at 02:41

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.

answered Jun 16 '17 at 02:41

Sergey Kalinichenko

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It's important to note that your `false` and `true` conditions are correct; OP's conditions are the *opposite* of what they should have been. – NRitH Jun 16 '17 at 02:43
@NRitH That's definitely true - that's why I switched them around. Thank you! – Sergey Kalinichenko Jun 16 '17 at 02:45

score 0 · Answer 2 · answered Jun 16 '17 at 02:40

It's complaining because not all code paths end up returning something. What happens if number is less than 2? Then your for loop, and the return statements inside it, will never be called, and there's no return that happens after the for loop finishes.

Returning a Bool in a function

2 Answers2

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