按键合并地图 [英] Merge maps by key

问题描述

说我有两张地图:

val a = Map(1 -> "one", 2 -> "two", 3 -> "three")
val b = Map(1 -> "un", 2 -> "deux", 3 -> "trois")

我想通过键合并这些映射，应用一些函数来收集值(在这种情况下，我想将它们收集到seq中，给出:

I want to merge these maps by key, applying some function to collect the values (in this particular case I want to collect them into a seq, giving:

val c = Map(1 -> Seq("one", "un"), 2 -> Seq("two", "deux"), 3 -> Seq("three", "trois"))

感觉应该有一种很好的习惯用法.

It feels like there should be a nice, idiomatic way of doing this.

推荐答案

scala.collection.immutable.IntMap 有一个intersectionWith方法可以精确地执行您想要的操作(我相信):

scala.collection.immutable.IntMap has an intersectionWith method that does precisely what you want (I believe):

import scala.collection.immutable.IntMap

val a = IntMap(1 -> "one", 2 -> "two", 3 -> "three", 4 -> "four")
val b = IntMap(1 -> "un", 2 -> "deux", 3 -> "trois")

val merged = a.intersectionWith(b, (_, av, bv: String) => Seq(av, bv))

这给您IntMap(1 -> List(one, un), 2 -> List(two, deux), 3 -> List(three, trois)).请注意，它会正确忽略仅在a中出现的密钥.

This gives you IntMap(1 -> List(one, un), 2 -> List(two, deux), 3 -> List(three, trois)). Note that it correctly ignores the key that only occurs in a.

作为旁注:我经常发现自己想要来自unionWith，intersectionWith等功能. .2.0/doc/html/Data-Map.html> Scala中Haskell的Data.Map .我不认为有任何原则上的理由只能在IntMap上使用它们，而不是在基本的collection.Map特性上使用.

As a side note: I've often found myself wanting the unionWith, intersectionWith, etc. functions from Haskell's Data.Map in Scala. I don't think there's any principled reason that they should only be available on IntMap, instead of in the base collection.Map trait.

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