Generating a Map[String, List[String]] from List[(String,String)]

Question

I got the following list of pairs:

 List(("US","New York"),("England","London"),("US","Los Angeles"),("England","Manchester"),("US","Washington"))

I need to generate a Map[Country, List[Cities]]:

Map("US" -> List("New York", "Los Angeles", "Washington"), "England" -> List("London", "Manchester"))

The problem that if I use toMap() directly the values with same keys are removed.

The story so far:

list.groupBy(el => el).map(el => el._1 -> ?)

Arjan · Accepted Answer · 2012-11-03 17:30:59Z

4

using groupBy:

list.groupBy(_._1).mapValues(_.map(_._2))

using fold:

list.foldLeft(Map.empty[String, List[String]]) { case (m, (k, v)) =>
  m.updated(k, v :: m.getOrElse(k, List()))
}

answered Nov 3, 2012 at 17:18

Arjan

21.8k2 gold badges57 silver badges49 bronze badges

Sign up to request clarification or add additional context in comments.

1 Answer 1