Making Adjacency List in Scala with FoldLeft

Question

Given this function

  def makeAdjacencyList(edgeList: Seq[(String, String)]): Map[String, Seq[String]] = {
    val emptyMap = Map[String, Seq[String]]().withDefaultValue(Seq[String]())

    edgeList.foldLeft(emptyMap) {
      case (acc, curr) => {
        acc + (curr._1 -> (acc(curr._1) :+ curr._2))
        acc + (curr._2 -> (acc(curr._2) :+ curr._1))
      }
    }
  }

It takes in a edgeList of an undirected graph, for example:

Vector((S1,E1), (S1,D1), (S2,E2), (S2,D1), (S3,E1), (S3,D2))

The function however gives an output of

[E1 -> [S1, S3], D1 -> [S1, S2], E2 -> [S2], D2 -> [S3]]

but I was expecting an output of

[S1 -> [E1, D1], S2 -> [E2, D1], S3 -> [E1], E1 -> [S1, S3], D1 -> [S1, S2], E2 -> [S2], D2 -> [S3]]

It seems like the first line of the foldLeft (acc + (curr._1 -> (acc(curr._1) :+ curr._2))) is being ignored and I am not quite sure why.

jwvh · Accepted Answer · 2021-06-21 01:31:11Z

3

Both lines of code in the case block produces a result, but only the 2nd result is returned and becomes the next acc value.

This would fix it.

edgeList.foldLeft(emptyMap) {
  case (acc, curr) =>
    acc + (curr._1 -> (acc(curr._1) :+ curr._2)) +
          (curr._2 -> (acc(curr._2) :+ curr._1))
}

Here, on the other hand, is a different approach to achieve the desired output.

def makeAdjacencyList(edgeList: Seq[(String,String)]): Map[String,Seq[String]] =
  edgeList.flatMap(edge => Seq(edge, edge.swap))
          .groupMap(_._1)(_._2)

edited Jun 21, 2021 at 1:31

answered May 30, 2021 at 2:32

jwvh

51.3k7 gold badges42 silver badges70 bronze badges

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

Collectives™ on Stack Overflow

Making Adjacency List in Scala with FoldLeft

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related