Why does this Scala line return a Unit?

Question

Here's a bit of Scala code to sum the values from 1 to 9 which are divisible by 3 or 5. Why does line 5 return a Unit and not a Boolean type?

object Sample {

    def main(args : Array[String]) {
        val answer = (1 until 10).foldLeft(0) ((result, current) => {
            if ((current % 3 == 0) || (current % 5 == 0)) {
                result + current
            }
        })

        println(answer)
    }

}

Answer 1

The if expression has a type of Unit because there is no else clause. Thus sometimes it returns nothing (Unit) so the entire expression has type Unit.

(I assume you meant to ask why it didn't return Int, not Boolean)

Answer 2

Here is my solution:

scala> val answer = (1 until 10) filter( current => (current % 3 == 0) || (current % 5 == 0)) sum
answer: Int = 23

Notice the filter instead of if's.

Another one in a even more idiomatic Scala:

( for( x <- 1 until 10 if x % 3 == 0 || x % 5 == 0 ) yield x ) sum

Answer 3

Can we ever be too idiomatic? YES WE CAN!

Set(3,5).map(k => Set(0 until n by k:_*)).flatten.sum

[Edit]

Daniel's suggestion looks better:

Set(3,5).flatMap(k => 0 until n by k).sum

Answer 4

The closest working code to what you did is this:

object Euler {
    def main(args : Array[String]) {
        val answer = (1 until 10).foldLeft(0) ((result, current) =>
            if ((current % 3 == 0) || (current % 5 == 0))
                result + current
            else
                result
        )

        println(answer)
    }
}