Scala type mismatch when adding elements to hashmap

Question

I am representing a graph's adjacency list in Scala in the variable a.

        val a = new HashMap[Int, Vector[Tuple2[Int, Int]]] withDefaultValue Vector.empty
        for(i <- 1 to N) {                
            val Array(x, y, r) = readLine.split(" ").map(_.toInt)
            a(x) += new Tuple2(y, r)
            a(y) += new Tuple2(x, r)
        }

I am reading each edge in turn(x and y are nodes, while r is the cost of the edge). After reading it, I am adding it to the adjacency list.

However, when adding the Tuples containing a neighbouring node and a cost to the HashMap I get:

Solution.scala:17: error: type mismatch;
 found   : (Int, Int)
 required: String
                a(x) += new Tuple2(y, r)

I don't understand why it wants String. I haven't specified String anywhere.

Lodewijk Bogaards · Accepted Answer · 2016-01-02 17:51:00Z

3

+= is the operator for concatenating to a String.

You would probably want to do something like: a.update(x, a.getOrElse(x, Vector()) :+ (x, r)).

edited Jan 2, 2016 at 17:51

answered Jan 2, 2016 at 17:44

Lodewijk Bogaards

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Comments

Dima · Accepted Answer · 2016-01-02 22:06:37Z

2

Also, you are writing Java code in Scala. It compiles, but amounts to abuse of the language :/

Consider doing something like this next time:

val a = Range(1, N)
   .map { _ => readline.split(" ").map (_.toInt) }
   .flatMap { case Array(x, y, r) => 
       Seq(x -> (y, r), y -> (x, r)) 
   }
   .groupBy(_._1)
   .mapValues { _.map ( _._2) }

edited Jan 2, 2016 at 22:06

answered Jan 2, 2016 at 18:45

Dima

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