Create Map in Scala using loop

Question

I am trying to create a map after getting result for each items in the list. Here is what I tried so far:

val sourceList: List[(Int, Int)] = ....
val resultMap: Map[Int, Int] = for(srcItem <- sourceList) {
  val result: Int = someFunction(srcItem._1)
  Map(srcItem._1 -> result)
}

But I am getting type mismatch error in IntelliJ and I am definitely not writing proper syntax here. I don't think I can use yield as I don't want List of Map. What is correct way to create Map using for loop. Any suggestion?

Sean Vieira · Accepted Answer · 2015-07-10 16:00:47Z

7

The simplest way is to create the map out of a list of tuples:

val resultMap = sourceList.map(item => (item._1, someFunction(item._1))).toMap

Or, in the monadic way:

val listOfTuples = for {
  (value, _) <- sourceList
} yield (value, someFunction(value))

val resultMap = listOfTuples.toMap

Alternatively, if you want to avoid the creation of listOfTuples you can make the transformation a lazy one by calling .view on sourceList and then call toMap:

val resultMap = sourceList.view
                          .map(item => (item._1, someFunction(item._1)))
                          .toMap

Finally, if you really want to avoid generating extra objects you can use a mutable Map instead and append the keys and values to it using += or .put

edited Jul 10, 2015 at 16:00

answered Jul 10, 2015 at 15:52

Sean Vieira

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