List of Lists to HashMap[Index of List] -> List - Scala

Question

I have a List of Lists in Scala, that looks likes this: List(List(1, 5, 6, 10), List(1, 6), List(1, 3, 10), I want to convert it into a HashMap[Int, List[Int]] where the first Int is the index of each list and the List[Int] is the list itself. In the end the HashMap should look like

 HashMap[Int, List[Int]](
      0 -> List(1, 5, 6, 10),
      1 -> List(1, 6),
      2 -> List (1, 3, 10),)

Here's my approach, have a list 0 to the length of the list and zip it with this nested list, then somehow convert it into a HashMap. But I'm looking for something more idiomatic or neater. Any ideas?

evan.oman · Accepted Answer · 2016-06-07 21:00:48Z

3

A zipWithIndex with a map should do it:

scala> val a = List(List(1, 5, 6, 10), List(1, 6), List(1, 3, 10))
a: List[List[Int]] = List(List(1, 5, 6, 10), List(1, 6), List(1, 3, 10))

scala> a.zipWithIndex.map(_.swap).toMap
res0: scala.collection.immutable.Map[Int,List[Int]] = 
Map(
    0 -> List(1, 5, 6, 10),
    1 -> List(1, 6), 
    2 -> List(1, 3, 10)
)

Edit:

If you absolutely need a HashMap, things are a little messier:

scala> val a = List(List(1, 5, 6, 10), List(1, 6), List(1, 3, 10))
a: List[List[Int]] = List(List(1, 5, 6, 10), List(1, 6), List(1, 3, 10))

scala> val b = a.zipWithIndex.map(_.swap)
b: List[(Int, List[Int])] = List((0,List(1, 5, 6, 10)), (1,List(1, 6)), (2,List(1, 3, 10)))

scala> val hM = collection.immutable.HashMap[Int, List[Int]](b: _*)
hM: scala.collection.immutable.HashMap[Int,List[Int]] = 
Map(
    0 -> List(1, 5, 6, 10),
    1 -> List(1, 6),
    2 -> List(1, 3, 10)
)

edited Jun 7, 2016 at 21:00

answered Jun 7, 2016 at 20:17

evan.oman

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4 Comments

Kolmar Over a year ago

use _.swap instead of x => (x._2, x._1)

evan.oman Over a year ago

Added, I'll have to remember that!

Teodorico Levoff Over a year ago

Very nice. Is there any way to convert it into a HashMap instead of a Map. My function is complaining expected HashMap[Int, List[Int]], actual: Map[Int, List[Int]].

evan.oman Over a year ago

@TeodoricoLevoff see my edit for HashMap conversion

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