Map both keys and values of a Scala Map
ScalaScala Problem Overview
Scala's MapLike
trait has a method
mapValues [C] (f: (B) ⇒ C): Map[A, C]
But I sometimes want a different type:
mapKeysAndValues [C] (f: (A, B) ⇒ C): Map[A, C]
Is there a simple way to do this which I am missing? Of course, this can be done with a fold.
Scala Solutions
Solution 1 - Scala
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)}
Solution 2 - Scala
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
)
Solution 3 - Scala
m map (t => (t._1, t._2 + 1))
m map (t => t._1 -> t._2 + 1)
Solution 4 - Scala
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.
Solution 5 - Scala
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.
Solution 6 - Scala
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)