我都試過,但它不工作:我將如何在Scala中產生一個不可變的.Map?
val map:Map[String,String] = for {
tuple2 <- someList
} yield tuple2._1 -> tuple2._2
我還能有別的會轉換Tuple2s的名單到地圖?
我都試過,但它不工作:我將如何在Scala中產生一個不可變的.Map?
val map:Map[String,String] = for {
tuple2 <- someList
} yield tuple2._1 -> tuple2._2
我還能有別的會轉換Tuple2s的名單到地圖?
我第一次嘗試是這樣的:
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)
但這裏是最好的解決辦法:
scala> val betterConversion = Map(country2capitalList:_*)
betterConversion: scala.collection.immutable.Map[java.lang.String,java.lang.String] = Map(England -> London, Germany -> Berlin)
的:_*
需要給編譯器的提示使用該列表作爲可變參數的參數。否則,它會給你:
scala> Map(country2capitalList)
<console>:6: error: type mismatch;
found : List[(java.lang.String, java.lang.String)]
required: (?, ?)
Map(country2capitalList)
^
從斯卡拉2.8上,你可以使用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))
斯卡拉2.8:
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>
在2.8,你可以使用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))
這將適用於任何成對的集合。請注意,文檔有此說關於它的重複的策略:
重複鍵將 後來鍵被覆蓋:如果這是一個無序 集合,關鍵是在 結果圖中是不確定的。
僅當'listOf2Tuples'顯式而不是變量/ val時纔有效 – 2010-05-04 08:18:32
修復了它的問題。 – 2010-05-04 08:56:44
+1,這個IMO是做這件事的最好方法。 :) – missingfaktor 2010-05-04 09:03:02