scala - map & flatten shows different result than flatMap-CodePudding

val adjList = Map("Logging" -> List("Networking", "Game"))
// val adjList: Map[String, List[String]] = Map(Logging -> List(Networking, Game))

adjList.flatMap { case (v, vs) => vs.map(n => (v, n)) }.toList
// val res7: List[(String, String)] = List((Logging,Game))

adjList.map { case (v, vs) => vs.map(n => (v, n)) }.flatten.toList
// val res8: List[(String, String)] = List((Logging,Networking), (Logging,Game))

I am not sure what is happening here. I was expecting the same result from both of them.

CodePudding user response：

.flatMap is Map's .flatMap, but .map is Iterable's .map.

For a Map "Logging" -> "Networking" and "Logging" -> "Game" become just the latter "Logging" -> "Game" because the keys are the same.

val adjList: Map[String, List[String]] = Map("Logging" -> List("Networking", "Game"))
val x0: Map[String, String] = adjList.flatMap { case (v, vs) => vs.map(n => (v, n)) } 
//Map(Logging -> Game)
val x: List[(String, String)] = x0.toList 
//List((Logging,Game))

val adjList: Map[String, List[String]] = Map("Logging" -> List("Networking", "Game"))
val y0: immutable.Iterable[List[(String, String)]] = adjList.map { case (v, vs) => vs.map(n => (v, n)) } 
//List(List((Logging,Networking), (Logging,Game)))
val y1: immutable.Iterable[(String, String)] = y0.flatten
//List((Logging,Networking), (Logging,Game))
val y: List[(String, String)] = y1.toList
//List((Logging,Networking), (Logging,Game))

Also https://users.scala-lang.org/t/map-flatten-flatmap/4180