How to sum two neighbours in a list in scala - scala

Question

If you have one Integer list in Scala, and you want to iterate through it and sum every two neighbours with the same value and return this as a list, how would one do that ?
So for example:
List(4, 4, 2, 6) => List(8, 2, 6)
I'm completely new to Scala, but I can imagine that pattern match or map could be useful.
def sumSameNeighbours: List[Int] => List[Int] = {
ls match {
case l1::l2:ls => l1 == l2
}
}
This is what I can think of.
EDIT: How would I have to change the code in order to iterate from right to left instead from left to right?
So that f.e. it would be:
List(2, 2, 2, 6, 4) => List(2, 4, 6, 4)
instead of
List(2, 2, 2, 6, 4) => List(4, 2, 6, 4)

Answer 1

This is pretty close to your suggestion and seems basically to work:
import scala.annotation.tailrec
def sumSameNeighbors( ls : List[Int] ) : List[Int] = {
#tailrec
def walk( unsummed : List[Int], reverseAccum : List[Int] ) : List[Int] = {
unsummed match {
case a :: b :: rest if a == b => walk( rest, a + b :: reverseAccum )
case a :: rest => walk( rest, a :: reverseAccum )
case Nil => reverseAccum.reverse
}
}
walk( ls, Nil )
}

Answer 2

Note: Based on final OP's specifications clarification, this answer doesn't exactly fit the question requirements.
Here is a solution using List.grouped(2):
list.grouped(2).toList
.flatMap {
case List(a, b) if a == b => List(a + b)
case l => l
}
The idea is to group consecutive elements by pair. If the pair has the same elements, we return their sum to be flatMaped and otherwise both elements untouched.
List(4, 4, 2, 6) => List(8, 2, 6)
List(2, 4, 4, 2, 6) => List(2, 4, 4, 2, 6)
List(2) => List(2)
List(9, 4, 4, 4, 2, 6) => List(9, 4, 8, 2, 6)

Answer 3

Another way using foldRight, which I find a good default for this sort of traversing a collection while creating a new one:
list.foldRight(List.empty[Int]) {
case (x, y :: tail) if x == y => (x + y) :: tail
case (x, list) => x :: list
}
Output of List(2, 2, 2, 6, 4) is List(2, 4, 6, 4) as requested.
The main thing I wasn't clear on from your examples is what the output should be if summing creates new neighbours: should List(8, 4, 2, 2) turn into List(8, 4, 4) or List(16)? This produces the second.