Scala List indexOf has bug?

Question

I am reading scala API of List, I found out: indexOf(elem: A, from: Int)

so I run following code:

scala> List(1,3,5,8).indexOf(8,-2)
res393: Int = 1

scala> List(1,3,5,8).indexOf(8,-3)
res391: Int = 0

why is this result? My Scala version is 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0).

Answer 1

It is a bug (although indeed there's no clear documentation of the expected results when from is negative) - and thanks to your question, it will be fixed in 2.12.2: https://github.com/scala/scala/pull/5621

So - good catch!

Answer 2

This behavior isn't a bug, simply undefined (although quite surprising, and inconsistent depending on the collection you're using).

---

On List, .indexOf will end up calling .indexWhere (on LinearSeqOptimized), where a negative number will end up shifting the result:

override /*SeqLike*/
def indexWhere(p: A => Boolean, from: Int): Int = {
  var i = from                 // -3
  var these = this drop from   // no effect
  while (these.nonEmpty) {
    if (p(these.head))
      return i                 // may return a negative number

    i += 1
    these = these.tail
  }
  -1
}

---

As noted by @adamwy in the comments, calling .indexOf with a negative number on a Vector will not have the same result; it will call .indexWhere (on IndexedSeqOptimized):

override /*SeqLike*/
def indexWhere(p: A => Boolean, from: Int): Int = {
  val start = from max 0        // completely ignores negatives 'from' input
  negLength(start + segmentLength(!p(_), start))
}