Iterator values in Scala vs cached values with vals

Question

I don't understand this behavior. I want to check if a value is in a circle created on a grid (as an array). Defining the two sides of the circle..

val circleLeft  = Vector(5,14,23..)
val circleRight = Vector(5,16,27,..)

I'm checking with the below function this condition. With the first one:

def insideCircle(idx: Int): Boolean = {
  val l = circleLeft.toIterator
  val r = circleRight.toIterator
  while (l.hasNext && r.hasNext) {
    if(idx < r.next && idx > l.next) return true
  }
  return false
}

It returns always true. With this second one

def insideCircle(idx: Int): Boolean = {
  val l = circleLeft.toIterator
  val r = circleRight.toIterator
  while (l.hasNext && r.hasNext) {
    val x1 = r.next
    val x2 = l.next
    println(x2,x1)
    if(idx < x1 && idx > x2) return true
  }
  return false
}

It works properly, i.e. insideCircle(15) = true, insideCircle(17) = false

Is there something here different..?

Victor Moroz · Accepted Answer · 2016-09-10 12:02:33Z

4

&& is called short-circuit method (in Scala it's implemented with by-name argument). In your first example l.next is only executed if idx < r.next. In the second example l.next is executed always.

UPDATE

I would suggest more functional way:

def insideCircle(idx: Int): Boolean = {
  val l = circleLeft.toIterator
  val r = circleRight.toIterator
  r.zip(l).find {
    case (x1, x2) =>
      idx < x1 && idx > x2
  }
  .isDefined
}

edited Sep 10, 2016 at 12:02

answered Sep 10, 2016 at 11:46

Victor Moroz

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3 Comments

LowFieldTheory Over a year ago

so, to make it an AND condition I should close the two conditions between parentheses? EDIT: Beautiful, thank you!

Victor Moroz Over a year ago

Not sure I understand your question, you need to evaluate both .nexts before calling &&. Compare { println("left"); false } && { println("right"); true } and { println("left"); false } & { println("right"); true } (& is not a short-circuit method)

LowFieldTheory Over a year ago

"you need to evaluate both .nexts before calling &&" now I understand better, ok

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