Easing into cats from Geekabyte For full story you can check it out on the blog.

Merging two maps

Given two maps of type Map[Sting, List[String]], combine them to create a resulting map where identical keys from the three maps have their list values concatenated in the combined map.

So for example, combining

Map("k1" -> List("One"), "k2" -> List("Zero")) and Map("k1" -> List("Two")) should give Map("k1" -> List("One", "Two"), "k2" -> List("Zero")).

Use the following test data for implementation:

val movies1 = Map(
"Will Smith" -> List("Wild Wild West", "Bad Boys", "Hitch"),
"Jada Pinkett" -> List("Woo", "Ali", "Gotham")
)

val movies2 = Map (
"Will Smith" -> List("Made in America"),
"Angelina Jolie" -> List("Foxfire", "The Bone Collector", "Original

Fiddle Around!


val movies1 = Map(
  "Will Smith" -> List("Wild Wild West", "Bad Boys", "Hitch"),
  "Jada Pinkett" -> List("Woo", "Ali", "Gotham")
)

val movies2 = Map (
  "Will Smith" -> List("Made in America"),
  "Angelina Jolie" -> List("Foxfire", "The Bone Collector", "Original Sin")
)

// Have your solution here that would produce
// Map(Will Smith -> List(Wild Wild West, Bad Boys, Hitch, Made in America), Angelina Jolie -> List(Foxfire, The Bone Collector, Original Sin), Jada Pinkett -> List(Woo, Ali, Gotham))

Scala Solution


val movies1 = Map(
  "Will Smith" -> List("Wild Wild West", "Bad Boys", "Hitch"),
  "Jada Pinkett" -> List("Woo", "Ali", "Gotham")
)

val movies2 = Map (
  "Will Smith" -> List("Made in America"),
  "Angelina Jolie" -> List("Foxfire", "The Bone Collector", "Original Sin")
)

val dZero = Map[String, List[String]]().withDefaultValue(List[String]())
val res = (movies1.toList ++ movies2.toList).foldLeft(dZero) {
  case (m, (k, v)) => m.updated(k, m(k) ++ v)
}

println(res)

Cats Solution

import cats._
import cats.data._
import cats.implicits._
  
val movies1 = Map(
  "Will Smith" -> List("Wild Wild West", "Bad Boys", "Hitch"),
  "Jada Pinkett" -> List("Woo", "Ali", "Gotham")
)

val movies2 = Map (
  "Will Smith" -> List("Made in America"),
  "Angelina Jolie" -> List("Foxfire", "The Bone Collector", "Original Sin")
)

val merged = movies1 |+| movies2