映射 Scala Map 的键和值

Scala 的 MapLike trait 有一个方法

mapValues [C] (f: (B) ⇒ C): Map[A, C]

但有时我想要一种不同的类型:

mapKeysAndValues [C] (f: (A, B) ⇒ C): Map[A, C]

有没有一个简单的方法可以做到这一点，我错过了吗？当然，这可以通过折叠来完成。

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最佳答案

map method iterates though all (key, value) pairs. You can use it like this:

val m = Map("a" -> 1, "b" -> 2)


val incM = m map {case (key, value) => (key, value + 1)}
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What about this code:

val m = Map(1 -> "one", 2 -> "two")
def f(k: Int, v: String) = k + "-" + v
m map {case (k, v) => (k, f(k, v))}

Which produces:

 Map(1 -> 1-one, 2 -> 2-two)

This can be packaged into utility method:

def mapKeysAndValues[A,B,C](input: Map[A,B], fun: (A, B) => C) =
input map {case(k,v) => (k, fun(k, v))}

Usage:

mapKeysAndValues(
Map(1 -> "one", 2 -> "two"),
(k: Int, v: String) => k + "-" + v
)
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You could create a utility class:

class MyMapLike[K,V](m:MapLike[K,V,_]){
def mapKeysAndValues[R](f: (K, V) => R)={
m.map{case (k,v)=> f(k,v)}
}
}
object MyMapLike{
implicit def maplike2mymaplike[K,V](ml:MapLike[K,V,_]):MyMapLike[K,V]=new MyMapLike(m)


}


import MyMapLike._
Map(1 -> "one", 2 -> "two").mapKeysAndValues(k,v=>v*k)

Code not tested but it should work somehow simmilar like that.

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With some Scalaz:

scala> def fst[A, B] = (x: (A, B)) => x._1
fst: [A, B]=> (A, B) => A


scala> Map(1 -> "Lorem", 2 -> "Ipsum").map(fst &&& Function.tupled(_.toString + _))
res1: scala.collection.immutable.Map[Int,java.lang.String] = Map(1 -> 1Lorem, 2 -> 2Ipsum)

I like @tenshi's solution better.

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m map (t => (t._1, t._2 + 1))


m map (t => t._1 -> t._2 + 1)
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scala> val m = Map("a" -> 1, "b" -> 2)
val m: scala.collection.immutable.Map[String,Int] = Map(a -> 1, b -> 2)


// Convert to anther map
scala> m.map(kvp => (kvp._1, kvp._2 + 1))
val res1: scala.collection.immutable.Map[String,Int] = Map(a -> 2, b -> 3)


// Convert to a different Iterable type
scala> m.map(kvp => s"${kvp._1} -> ${kvp._2 + 1}")
val res2: scala.collection.immutable.Iterable[String] = List(a -> 2, b -> 3)

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