Scala - weird behaviour with Iterator.toList - scala

Question

I am new to Scala and I have a function as follows:
def selectSame(messages: BufferedIterator[Int]) = {
val head = messages.head
messages.takeWhile(_ == head)
}
Which is selecting from a buffered iterator only the elems matching the head. I am subsequently using this code:
val messageStream = List(1,1,1,2,2,3,3)
if (!messageStream.isEmpty) {
var lastTimeStamp = messageStream.head.timestamp
while (!messageStream.isEmpty) {
val messages = selectSame(messageStream).toList
println(messages)
}
Upon first execution I am getting (1,1,1) as expected, but then I only get the List(2), like if I lost one element down the line... Probably I am doing sth wrong with the iterators/lists, but I am a bit lost here.

Answer 1

Scaladoc of Iterator says about takeWhile:
Reuse: After calling this method, one should discard the iterator it
was called on, and use only the iterator that was returned. Using the
old iterator is undefined, subject to change, and may result in
changes to the new iterator as well.
So that's why. This basically means you cannot directly do what you want with Iterators and takeWhile. IMHO, easiest would be to quickly write your own recursive function to do that.
If you want to stick with Iterators, you could use the sameElements method on the Iterator to generate a duplicate where you'd call dropWhile.
Even better: Use span repeatedly:
def selectSame(messages: BufferedIterator[Int]) = {
val head = messages.head
messages.span(_ == head)
}
def iter(msgStream: BufferedIterator[Int]): Unit = if (!msgStream.isEmpty) {
val (msgs, rest) = selectSame(msgStream)
println(msgs.toList)
iter(rest)
}
val messageStream = List(1,1,1,2,2,3,3)
if (!messageStream.isEmpty) {
var lastTimeStamp = messageStream.head.timestamp
iter(messageStream0
}