Question

As far as I know, the isDefinedAt method should work like types contains method. But strangely enough, it behaves differently – it does not check elements for repeated occurence.

val randomizer = new Random

def next(acc: List[Int], n: Int): List[Int] = {
   if(n > 0) {
      val r = randomizer.nextInt(15)
      println("generating, r=" + r + " is defined=" + acc.isDefinedAt(r))
      if(!acc.isDefinedAt(r)) next(r :: acc, n - 1) // check for NO coincidence
      else next(acc, n)
   } else acc
}

println("indices = " + next(List[Int](), 6))

Ofcourse, I can use Sets for that instead of lists, but nevertheless, why does it act like this?

The output that I get is like

generating, r=8 is defined=false
generating, r=13 is defined=false
generating, r=2 is defined=false
generating, r=8 is defined=false
generating, r=9 is defined=false
generating, r=3 is defined=true
generating, r=2 is defined=true
generating, r=7 is defined=false

indices = List(7, 9, 8, 2, 13, 8)

Answer 1

All isDefinedAt does is tell you if a function does not accept an argument. That is, if isDefinedAt returns false, then you know the function doesn’t cover that value. For true, it may cover the value or throw an exception.

In the specific case of List (and all Seq), isDefinedAt(i) will return true if 0 <= i < list.size, and false otherwise, and list(i) will only throw an exception if isDefinedAt(i) returns false.

Anyway, let’s try it out:

scala> val list = List("a", "b", "c")
list: List[String] = List(a, b, c)

scala> list.isDefinedAt(0)
res0: Boolean = true

scala> list(0)
res1: String = a


scala> list.isDefinedAt(-1)
res3: Boolean = false

scala> list(-1)
java.lang.IndexOutOfBoundsException: -1


scala> list.isDefinedAt(2)
res5: Boolean = true

scala> list(2)
res6: String = c