Checking whether a number is prime in Scala

Question

I wanted to check if a number is prime or not. I wrote the following code, but it does not return any value:

 def isprime(x:Int) = {
     | if (x==1) false
     | else {
     | for (i <- 2 to x-1) {
     | if (x % i == 0) false
     | else true
     | }
     | }
     | }

Answer 1

What you did is a called defining a function so obviously it won't return anything and as a matter of fact, this function returns AnyVal which obviously won't help you much. I suspect that you actually need a Boolean type returned.

Since you are working with the REPL, you would want to define your function to check if a number is prime. I called it isPrime2 and then test it.

def isPrime2(i :Int) : Boolean = {
|     if (i <= 1)
|       false
|     else if (i == 2)
|       true
|     else
|       !(2 to (i-1)).exists(x => i % x == 0)
|   }
// isPrime2: (i: Int)Boolean

(1 to 10).foreach(i => if (isPrime2(i)) println("%d is prime.".format(i)))
// 2 is prime.
// 3 is prime.
// 5 is prime.
// 7 is prime.

I would even suggest a simpler version if you care not using if else conditions :

def isPrime1(n: Int): Boolean = ! ((2 until n-1) exists (n % _ == 0))

Which also returns a Boolean.

EDIT:

As @TheArchetypalPaul stated, which was also implied, the problem is that your for loop isn't resulting in any value - you calculate a true/false value, but then don't do anything with it. So your else clause doesn't result in any value (in fact, it produces Unit). You need to return false as soon as you find a divisor - and the way to do that is probably exists as in isPrime1.