Challenge B: It’s prime time

When I’m acquainting myself with a programming language, one of the first things I do is write a function to determine whether or not a number is prime. That’s your first challenge.

First, write the following function:

func isNumberDivisible(_ number: Int, by divisor: Int) -> Bool

You’ll use this to determine if one number is divisible by another. It should return

true when number is divisible by divisor.

Hint: You can use the modulo (%) operator to help you out here. Next, write the main function:

func isPrime(_ number: Int) -> Bool

This should return true if number is prime, and false otherwise. A number is prime if it’s only divisible by 1 and itself. You should loop through the numbers from 1 to

the number and find the number’s divisors. If it has any divisors other than 1 and itself, then the number isn’t prime. You’ll need to use the isNumberDivisible(_:by:) function you wrote earlier.

Use this function to check the following cases:

isPrime(6) // false isPrime(13) // true isPrime(8893) // true

Hint 1: Numbers less than 0 should not be considered prime. Check for this case at the start of the function and return early if the number is less than 0.

Hint 2: Use a for loop to find divisors. If you start at 2 and end before the number itself, then as soon as you find a divisor, you can return false.

Hint 3: If you want to get really clever, you can simply loop from 2 until you reach the square root of number rather than going all the way up to number itself. I’ll leave it as an exercise for you to figure out why. It may help to think of the number 16, whose square root is 4. The divisors of 16 are 1, 2, 4, 8 and 16.