我已经尝试了这个方法,但它没有起作用:
val map:Map[String,String] = for {
tuple2 <- someList
} yield tuple2._1 -> tuple2._2
我该如何将一个 Tuple2 的列表转换成 Map 呢?
我已经尝试了这个方法,但它没有起作用:
val map:Map[String,String] = for {
tuple2 <- someList
} yield tuple2._1 -> tuple2._2
我该如何将一个 Tuple2 的列表转换成 Map 呢?
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)
需要使用:_*
,以提示编译器将列表用作varargs参数。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版本开始,您可以使用 toMap
方法:
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))
toMap
方法: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))
这适用于任何键值对集合。请注意,文档中对其重复键策略的说明如下:
重复的键将被后面的键覆盖:如果这是一个无序集合,则结果映射中的键未定义。
scala> import scala.collection.breakOut
import scala.collection.breakOut
scala> val ls = List("a","bb","ccc")
ls: List[java.lang.String] = List(a, bb, ccc)
scala> val map: Map[String,Int] = ls.map{ s => (s,s.length) }(breakOut)
map: Map[String,Int] = Map((a,1), (bb,2), (ccc,3))
scala> val map2: Map[String,Int] = ls.map{ s => (s,s.length) }.toMap
map2: Map[String,Int] = Map((a,1), (bb,2), (ccc,3))
scala>
listOf2Tuples
是明确的,而不是一个变量/常量时才有效。 - michael.kebetoMap
出现(scala 2.8) - krookedking