Scala：更改地图内列表中的值

Question

I'm facing a problem: I want to change one element inside but the code is changing all elements. I must be doing something wrong. Here's what I have:

var critics: Map[String, List[(String, Double)]] = Map(
  "First" -> List(("a", 1.0), ("b", 2.0)),
  "Second" -> List(("c", 3.0), ("d", 4.0))
  )

  def setNewRating(keyMap: String, param1: String, newRating: Double) = {
    val newList = critics(keyMap).map{case (param1, _) => (param1, newRating); case x => x}
    critics = critics + (keyMap -> newList)
    }

    println(critics)

    setNewRating("First", "b", 5.0)

    println(critics)

so, I'm looking for the key "First" and getting the List I want to change the "b" from 2.0 to 5.0 but the result I'm getting is:

critics: Map[String,List[(String, Double)]] = Map(First -> List((a,1.0), (b,2.0)), Second -> List((c,3.0), (d,4.0)))
setNewRating: (keyMap: String, param1: String, newRating: Double)Unit
Map(First -> List((a,1.0), (b,2.0)), Second -> List((c,3.0), (d,4.0)))
Map(First -> List((a,5.0), (b,5.0)), Second -> List((c,3.0), (d,4.0)))

The code changed all the values!

thanks in advance,

Answer 1

This should work. the param1 you use in the case clause .map{case (param1, _) is not the same variable param1 defined in the argument. It is a new variable param1 which is assigned the value of the first element of the Tuple2 that shadows the method argument.

def setNewRating(keyMap: String, param1: String, newRating: Double) = {
  val newList = critics(keyMap) map {
    case (x, _) if x == param1 => (param1, newRating)
    case x => x
  }
  critics = critics + (keyMap -> newList)
}

Answer 2

You need to put param1 in backticks:

  def setNewRating(keyMap: String, param1: String, newRating: Double) = {
    val newList = critics(keyMap)
     .map { 
       case (`param1`, _) => param1 -> newRating 
       case x => x
     }
     critics = critics + (keyMap -> newList)
  }

The problem is that case (param1, _) means "match anything, and store the value into new local variable param1 ". That's not what you want.

 case (`param1`, _) => 

On the other hand, means "match the value that is the same as the outer variable param1 ".

Alternatively, if you renamed param1 to Param1 , it would work too (apparently, scala matching assumes that everything starting with a capital letter is constant). I am not suggesting you do that, only mentioning it for completeness. Just use backticks.

(Also, using var like this makes me cringe, you really should find a way to avoid it, but that wasn't your question ...)

Answer 3

If you are willing to change from List to Set , you might just be missing MultiMap trait.

A trait for mutable maps with multiple values assigned to a key.

This class is typically used as a mixin. It turns maps which map A to Set[B] objects into multimaps that map A to B objects.

You can find the reference here