Map both keys and values of a Scala Map

Question

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.

Answer 1

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)}

Answer 2

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
)

Answer 3

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

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