How to access and update a value in a mutable map of map of maps

Question

I've a three-level data structure (indentation and line breaks for readability):

scala> import scala.collection.mutable.Map
import scala.collection.mutable.Map

scala> val m = Map("normal" -> Map("home" -> Map("wins" -> 0, "scores" -> 0),
                                   "away" -> Map("wins" -> 0, "scores" -> 0)))
m: scala.collection.mutable.Map[java.lang.String,
   scala.collection.mutable.Map[java.lang.String,
   scala.collection.mutable.Map[java.lang.String,Int]]] = 
Map((normal,Map(away -> Map(wins -> 0, scores -> 0),
     home -> Map(wins -> 0, scores -> 0))))

Accessing the innermost data (scores) requires a lot of typing:

import org.scalatest.{Assertions, FunSuite}

class MapExamplesSO extends FunSuite with Assertions {
  test("Update values in a mutable map of map of maps") {
    import scala.collection.mutable.Map
    // The m map is essentially an accumulator
    val m = Map("normal" -> 
                Map("home" -> Map("wins" -> 0, "scores" -> 0),
                    "away" -> Map("wins" -> 0, "scores" -> 0)
                  )
          )
    //
    // Is there a less verbose way to increment the scores ?
    //
    assert(m("normal").apply("home").apply("scores") === 0)

    val s1 = m("normal").apply("home").apply("scores") + 1
    m("normal").apply("home").update("scores", s1)

    assert(m("normal").apply("home").apply("scores") === 1)

    val s2 = m("normal").apply("home").apply("scores") + 2
    m("normal").apply("home").update("scores", s2)

    assert(m("normal").apply("home").apply("scores") === 3)
  }
}

Is there a less verbose way to modify the value of scores ?

I'm a Scala newbie, so all other observations of the code above are also welcome.

score 21 · Accepted Answer · answered Dec 01 '10 at 12:59

21

You don't have to use "apply" just do it normally with "()"

m("normal")("home")("scores") = 1

answered Dec 01 '10 at 12:59

wheaties

35,646
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3

That works in this example, but won't work if m("normal")("home") hasn't already been defined. – Mark Oct 12 '16 at 00:46

score 10 · Answer 2 · answered Dec 01 '10 at 13:00

10

You can write

m("normal").apply("home").apply("scores")

as

m("normal")("home")("scores")

However I'm not sure if such a structure is a good idea. Maybe you should consider encapsulating this functionality in a specialized class.

answered Dec 01 '10 at 13:00

Landei

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BenjaminJackman · Answer 3 · 2010-12-06T01:39:55.943

Adding a local helper function is always good way to reduce code duplication:

class MapExamplesSO {
  def test {
    import scala.collection.mutable.Map
    // The m map is essentially an accumulator
    var m = Map("normal" -> 
                Map("home" -> Map("wins" -> 0, "scores" -> 0),
                    "away" -> Map("wins" -> 0, "scores" -> 0)))


    //Local Helper returns (Old, New)
    def updateScore(k1 : String,k2 : String,k3 : String)
                   (f : Int => Int) : (Int, Int) = {
      val old = m(k1)(k2)(k3)
      m(k1)(k2)(k3) = f(old)
      (old, m(k1)(k2)(k3))
    }

    assert(m("normal")(home")("scores") === 0)
    assert(updateScore("normal","home","scores")(_+1)._2 === 1)
    assert(updateScore("normal","home","scores")(_+2)._2 === 3)
  }
}

[Edit made code tighter]

Clever ! (I just hope not too clever:) Took me a few readings before I realised what is going on here. Great answer, thanks ! — user272735, Dec 01 '10 at 20:01

Daniel C. Sobral · Answer 4 · 2010-12-02T11:24:15.817

Less verbose:

assert(m("normal")("home")("scores") === 0)

val s1 = m("normal")("home")("scores") + 1
m("normal")("home")("scores") = s1

assert(m("normal")("home")("scores") === 1)

val s2 = m("normal")("home")("scores") + 2
m("normal")("home")("scores") = s2

assert(m("normal")("home")("scores") === 3)

This takes advantage of the fact that both apply and update have syntactic sugars for them as seen above. Shorter still:

// On REPL, put both these definitions inside an object instead
// of entering them on different lines
def scores = m("normal")("home")("scores")
def scores_=(n: Int) = m("normal")("home")("scores") = n

assert(scores === 0)

val s1 = scores + 1
scores = s1

assert(scores === 1)

val s2 = scores + 2
scores = s2

// Just so you see these updates are being made to the map:
assert(m("normal")("home")("scores") === 3)

Which takes advantage of the syntactic sugar for getters and setters (the getter definition must exist for the setter definition to work).

This is also a very helpful answer - thanks ! I wish many upvotes to you too. Getter/setter explained for beginners: http://www.dustinmartin.net/2009/10/getters-and-setters-in-scala/ — user272735, Dec 02 '10 at 07:15

How to access and update a value in a mutable map of map of maps

4 Answers4