How to join two lists in Scala?

Question

We have two lists:

val a:List[(String, Int, Int)] = List(("apple", 3, 25),("orange", 4, 47))

val b:List[(String, String)] = List(("mango", "25"),("orange", "50"))

Which is the best method to Join a and b to get:

val c : List[(String, Int, Int, String)] = List(("orange", 4, 47, "50"))

Answer 1

Iterate over the first list and lookup the values of the second list in a map mb. The .flatMap makes the entry dissappear, if the .get returns None.

 val mb = b.toMap

 a.flatMap{case (ka,va,vva) => mb.get(ka).map(vb => (ka,va,vva,vb))}

Answer 2

You can concatenate the lists, and then group them by the first element of your tuple:

val groupedTuples: Map[String, List[(String, String)]] = (a ++ b).groupBy(_._1)
val c: Map[String, List[String]] =  groupedTuples.mapValues(_.map(_._2))

This will result in

Map(mango -> List(25), orange -> List(4, 50), apple -> List(3))