scala: adjacency matrix graphs

Question

I am trying to understand how to build graphs in Scala, till now I have this:

import scala.collection.mutable.ArrayBuffer

object TestGraph {

  class Graph(vertices: Array[String], edges: ArrayBuffer[(Int, Int)]) {
    def size: Int = vertices.length
    def vertex(index: Int): String = vertices(index)
    def index(vertex: String): Int = vertices.indexOf(vertex)
    def printEdges(): Unit = { 
      for(v <- vertices) {
        print("vertex " + index(v) + ": ")
        for(e <- edges if(e._1 == index(v))) {
            print(e + " ")
        }
        println()
      }
    }
    def createAdjacencyMatrix() = {
      val adjacencyMatrix = Array.ofDim[Int](size, size)

    }
    def printAdjacencyMatrix(): Unit = {

    }
  }

   def main(args: Array[String]) {
    def vertices: Array[String] = Array("Seattle", "San Francisco", "Los Angeles",
        "Denver", "Kansas City", "Chicago", "Boston", "New York", 
        "Atlanta", "Miami", "Dallas", "Houston")

    def edges: ArrayBuffer[(Int, Int)] = ArrayBuffer(
        (0, 1), (0, 3), (0, 5), 
        (1, 0), (1, 2), (1, 3), 
        (2, 1), (2, 3), (2, 4), (2, 10), 
        (3, 0), (3, 1), (3, 2), (3, 4), (3, 5), 
        (4, 2), (4, 3), (4, 5), (4, 7), (4, 8), (4, 10), 
        (5, 0), (5, 3), (5, 4), (5, 6), (5, 7), 
        (6, 5), (6, 7), 
        (7, 4), (7, 5), (7, 6), (7, 8), 
        (8, 4), (8, 7), (8, 9), (8, 10), (8, 11), 
        (9, 8), (9, 11), 
        (10, 2), (10, 4), (10, 8), (10, 11), 
        (11, 8), (11, 9), (11, 10)
        )
        val graph = new Graph(vertices, edges)
        println("number of vertices in graph: " + graph.size)
        println("the vertex with index 1 is: " + graph.vertex(1))
        println("the index for Miami is: " + graph.index("Miami"))
        println("the edges for graph: "); graph.printEdges
        println("adjacency matric for graph: ");    
   }
}

I though to create an adjacency matrix and then to print it for the given graph. Is it a better way to fill the two dimensional array - something like I have done with printEdges()?

Answer 1

I allowed myself to refactor your code a bit:

class Graph(val vertex: IndexedSeq[String], edges: Seq[(Int, Int)]) {
    def size: Int = vertex.length
    val index: Map[String, Int] = vertex.zipWithIndex.toMap
    val adjacent = edges groupBy (_._1) mapValues (_ map (_._2))
    def adjacencyMatrix = adjacent mapValues (_.toSet) mapValues (0 to size map _)
    def printEdges: String = {
      for(idx <- 0 until size)
        yield f"vertex $idx: ${adjacent(idx) mkString " "}"
    } mkString "\n"
    def printAdjacencyList: String = adjacent mapValues (_ mkString ", ") mkString "\n"
    def printAdjacencyMatrix: String = adjacencyMatrix mapValues(_ mkString ", ") mkString "\n"
  }

and in def main:

println("number of vertices in graph: " + graph.size)
println("the vertex with index 1 is: " + graph.vertex(1))
println("the index for Miami is: " + graph.index("Miami"))
println("the edges for graph: ")
println(graph.printEdges)
println("adjacency list for graph: ")
println(graph.printAdjacencyList)
println("adjacency matrix for graph: ")
println(graph.printAdjacencyMatrix)