scala - 我将如何在 Scala 中生成一个 immutable.Map？

Question

我试过这个，但它不起作用:

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

我还能如何将 Tuple2 列表转换为 Map？

Answer 1

我的第一次尝试是这样的:

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)

但这是最好的解决方案:

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

:_*需要给编译器一个使用列表作为可变参数参数的提示。否则它会给你:

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

从 Scala 2.8 开始，您可以使用 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))