Map over HashMap and avoid empty results

Question

I have an immutable HashMap:

val hashmap = Seq(1,2,3,3,2).groupBy(identity).map({x => x._1 -> x._2.length})

println(hashmap)
// HashMap(1 -> 1, 2 -> 2, 3 -> 2)

I try to filter it by value (2) and get the keys:

val res = hashmap.map({ case (key, value) => if (value == 2) key})

println(res)
// List((), 2, 3)

However, it returns an empty tuple if there is a key/value pair in hashmap which does not satisfy value==2.

Where do these empty tuples come from? Is there an easy approach how to avoid them?

I want to have a List(2, 3) as a result.

Online code

Answer 1

Omitting else clause

if (value == 2) key

is equivalent to

if (value == 2) key else ()

which is why

hashmap.map({ case (key, value) => if (value == 2) key})

evaluates to

val res0: scala.collection.immutable.Iterable[AnyVal] = List((), 2, 3)

Instead try collect which is semantically equivalent to map+filter like so

hashmap.collect { case (key, value) if value == 2 => key }
// val res2: scala.collection.immutable.Iterable[Int] = List(2, 3)