Convert List[(Int,String)] into List[Int] in scala

Question

My goal is to to map every word in a text (Index, line) to a list containing the indices of every line the word occurs in. I managed to write a function that returns a list of all words assigned to a index.

The following function should do the rest (map a list of indices to every word):

def mapIndicesToWords(l:List[(Int,String)]):Map[String,List[Int]] = ???

If I do this:

l.groupBy(x => x._2)

it returns a Map[String, List[(Int,String)]. Now I just want to change the value to type List[Int]. I thought of using .mapValues(...) and fold the list somehow, but I'm new to scala and don't know the correct approach for this.

So how do I convert the list?

converting List[(Int,String)] into List[Int] is just doing a .map(_._1) — Anahcolus
– Anahcolus, Commented Jun 2, 2018 at 14:04
@RameshMaharjan yeah l.groupBy(x => x._2).mapValues[List[Int]](f => f.map(_._1)) works great — Melkor
– Melkor, Commented Jun 4, 2018 at 9:21

Duelist · Accepted Answer · 2018-06-03 14:16:31Z

1

Also you can use foldLeft, you need just specify accumulator (in your case Map[String, List[Int]]), which will be returned as a result, and write some logic inside. Here is my implementation.

def mapIndicesToWords(l:List[(Int,String)]): Map[String,List[Int]] =
  l.foldLeft(Map[String, List[Int]]())((map, entry) =>
    map.get(entry._2) match {
      case Some(list) => map + (entry._2 -> (entry._1 :: list))
      case None => map + (entry._2 -> List(entry._1))
    }
  )

But with foldLeft, elements of list will be in reversed order, so you can use foldRight. Just change foldLeft to foldRight and swap input parameters, (map, entry) to (entry, map).

And be careful, foldRight works 2 times slower. It is implemented using method reverse list and foldLeft.

edited Jun 3, 2018 at 14:16

answered Jun 3, 2018 at 10:49

Duelist

1,5721 gold badge11 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

jwvh Over a year ago

According to the source code, foldRight is implemented via foldLeft and neither one of them are recursive (tail or otherwise).

Duelist Over a year ago

I know that all methods are implemented in imperative style, but I didn't know that foldRight is implemented as reverse.foldLeft. So, thank you for clarifying!

pedrorijo91 · Accepted Answer · 2018-06-03 09:49:52Z

0

scala> val myMap: Map[String,List[(Int, String)]]  = Map("a" -> List((1,"line1"), (2, "line")))
myMap: Map[String,List[(Int, String)]] = Map(a -> List((1,line1), (2,line)))

scala> myMap.mapValues(lst => lst.map(pair => pair._1))
res0: scala.collection.immutable.Map[String,List[Int]] = Map(a -> List(1, 2))

answered Jun 3, 2018 at 9:49

pedrorijo91

7,9359 gold badges48 silver badges89 bronze badges

Collectives™ on Stack Overflow

Convert List[(Int,String)] into List[Int] in scala

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related