How to find consecutive elements in a collection in Scala

Question

I am trying to find a way to find consecutive elements in a collection in Scala based on a desired element being in the first position. I'd like to achieve this with functional programming, so I'm struggling on coming up with an approach.

Basically, I have a list foo which can contain either A, B or C. 'A' being the desired value, so it is necessary that 'A' is first, or else, the resulting bar should be empty.

From the first element, I need to extract the consecutive elements up until there is a change in the element value, then create a new list bar which contains the sequence of elements prior to the one that was different.

So for example, what I wanna do is

List('A', 'A', 'B', 'C', 'A', 'B') => List('A', 'A')

At the same time, if the first element is not A, then the resulting List should be empty

List('B', 'A', 'A', 'C') => List()

I have tried to apply foreach on a List foo and check if the current element is the desired element to add it to a list, but due to Scala immutability (I know you can use mutable but I'd rather not) I don't know how to keep track of the previous elements and output the collection

Answer 1

If the first element always has to be A, then takeWhile does what you want.

scala> List('A', 'A', 'B', 'C', 'A', 'B').takeWhile(_ == 'A')
val res3: List[Char] = List(A, A)