James James - 8 months ago 28
Scala Question

What is an idiomatic Scala way to "remove" one element from an immutable List?

I have a List, which may contain elements that will compare as equal. I would like a similar List, but with one element removed. So from (A, B, C, B, D) I would like to be able to "remove" just one B to get e.g. (A, C, B, D). The order of the elements in the result does not matter.

I have working code, written in a Lisp-inspired way in Scala. Is there a more idiomatic way
to do this?

The context is a card game where two decks of standard cards are in play, so there may
be duplicate cards but still played one at a time.

def removeOne(c: Card, left: List[Card], right: List[Card]): List[Card] = {
if (Nil == right) {
return left
if (c == right.head) {
return left ::: right.tail
return removeOne(c, right.head :: left, right.tail)

def removeCard(c: Card, cards: List[Card]): List[Card] = {
return removeOne(c, Nil, cards)


I haven't seen this possibility in the answers above, so:

scala> def remove(num: Int, list: List[Int]) = list diff List(num)
remove: (num: Int,list: List[Int])List[Int]

scala> remove(2,List(1,2,3,4,5))
res2: List[Int] = List(1, 3, 4, 5)


scala> remove(2,List(2,2,2))
res0: List[Int] = List(2, 2)

Like a charm :-).