Scala map : How to add new entries

Question

I have created my scala map as :

val A:Map[String, String] = Map()

Then I am trying to add entries as :

val B = AttributeCodes.map { s =>

    val attributeVal:String = <someString>
    if (!attributeVal.isEmpty)
    {
      A + (s -> attributeVal)
    }
    else
      ()
  }

And after this part of the code, I am seeing A is still empty. And, B is of type :

Pattern: B: IndexedSeq[Any]

I need a map to add entries and the same or different map in return to be used later in the code. However, I can not use "var" for that. Any insight on this problem and how to resolve this?

A += (s -> attributeVal) or A updated (s, attributeVal). [source] — TeWu
– TeWu, Commented Feb 13, 2019 at 16:16
I tried and see this compilation error : value update is not a member of Map[String,String] [ERROR] A(s)=attributeVal — VictorGram
– VictorGram, Commented Feb 13, 2019 at 16:19

VictorGram · Accepted Answer · 2019-02-13 20:35:04Z

4

Scala uses immutability in many cases and encourages you to do the same.

Do not create an empty map, create a Map[String, String] with .map and .filter

val A = AttributeCodes.map { s =>
      val attributeVal:String = <someString>
      s -> attributeVal
}.toMap.filter(e => !e._1.isEmpty && !e._2.isEmpty)

edited Feb 13, 2019 at 20:35

VictorGram

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answered Feb 13, 2019 at 16:22

Ivan Stanislavciuc

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Comments

user31601 · Accepted Answer · 2019-02-13 16:24:51Z

1

In Scala, the default Map type is immutable. <Map> + <Tuple> creates a new map instance with the additional entry added.

There are 2 ways round this:

Use scala.collection.mutable.Map instead:

val A:immutable.Map[String, String] = immutable.Map()

AttributeCodes.forEach { s =>
  val attributeVal:String = <someString>
  if (!attributeVal.isEmpty){
    A.put(s, attributeVal)
  }
}

Create in immutable map using a fold:

val A: Map[String,String] = AttributeCodes.foldLeft(Map(), { m, s =>
  val attributeVal:String = <someString>
  if (!attributeVal.isEmpty){
    m + (s -> attributeVal)
  } else {
    m
  }
}

answered Feb 13, 2019 at 16:24

user31601

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