Question

Suppose I have two lists:

val a = List('a', 'b', 'c')
val b = List('a', 'b', 'c', 'd')

I want to get the element which is not in the first list (in this case it’s ‘d’). I know I can do this with a loop, but is there any fancy functional way to do this quickly in one line?

I’ve been looking at the Scala List API, but could only found union and intersection (which will give me List(‘a’, ‘b’, ‘c’, ‘d’) and List(‘a’, ‘b’, ‘c’) respectively)

Answer 1

I think you can use b -- a. Here is the documentation from scala:

def -- [B >: A] (that: List[B]) : List[B]
Computes the difference between this list and the given list that.
that
the list of elements to remove from this list.
returns this list without the elements of the given list that.
deprecated: use list1 filterNot (list2 contains) instead

Sorry for the deprecated method, here is the current good one: list1 filterNot (list2 contains)

def filterNot (p: (A) ⇒ Boolean) :

List[A] Selects all elements of this
list which do not satisfy a predicate.
p the predicate used to test elements.
returns a new list consisting of all
elements of this list that do not
satisfy the given predicate p. The
order of the elements is preserved.
definition classes: TraversableLike