Scala: Best way to filter & map in one iteration

Question

I'm new to Scala and trying to figure out the best way to filter & map a collection. Here's a toy example to explain my problem.

Approach 1: This is pretty bad since I'm iterating through the list twice and calculating the same value in each iteration.

val N = 5
val nums = 0 until 10
val sqNumsLargerThanN = nums filter { x: Int => (x * x) > N } map { x: Int => (x * x).toString }

Approach 2: This is slightly better but I still need to calculate (x * x) twice.

val N = 5
val nums = 0 until 10
val sqNumsLargerThanN = nums collect { case x: Int if (x * x) > N => (x * x).toString }

So, is it possible to calculate this without iterating through the collection twice and avoid repeating the same calculations?

Answer 1

Could use a foldRight

nums.foldRight(List.empty[Int]) {
  case (i, is) =>
    val s = i * i
    if (s > N) s :: is else is
  }

A foldLeft would also achieve a similar goal, but the resulting list would be in reverse order (due to the associativity of foldLeft.

Alternatively if you'd like to play with Scalaz

import scalaz.std.list._
import scalaz.syntax.foldable._

nums.foldMap { i =>
  val s = i * i
  if (s > N) List(s) else List()
}