Question

I have tried this but it does not work:

val map:Map[String,String] = for {
    tuple2 <- someList
  } yield tuple2._1 -> tuple2._2

How else would I convert a List of Tuple2s into a Map?

Answer 1

My First try is this:

scala> val country2capitalList = List("England" -> "London", "Germany" -> "Berlin")
country2capitalList: List[(java.lang.String, java.lang.String)] = List((England,London), (Germany,Berlin))

scala> val country2capitalMap = country2capital.groupBy(e => e._1).map(e => (e._1, e._2(0)._2))
country2capitalMap: scala.collection.Map[java.lang.String,java.lang.String] = Map(England -> London, Germany -> Berlin)

But here is the best solution:

scala> val betterConversion = Map(country2capitalList:_*)
betterConversion: scala.collection.immutable.Map[java.lang.String,java.lang.String] = Map(England -> London, Germany -> Berlin)

The :_* is needed to give the compiler a hint to use the list as a varargs argument. Otherwise it will give you:

scala> Map(country2capitalList)
<console>:6: error: type mismatch;
 found   : List[(java.lang.String, java.lang.String)]
 required: (?, ?)
       Map(country2capitalList)
           ^

From Scala 2.8 on you can use toMap:

scala> val someList = List((1, "one"), (2, "two"))   
someList: List[(Int, java.lang.String)] = List((1,one), (2,two))

scala> someList.toMap
res0: scala.collection.immutable.Map[Int,java.lang.String] = Map((1,one), (2,two))