scala “not a member of type parameter”

Question

I'm trying to use Spark GraphX, and encountering what I think is a problem in how I'm using Scala. I'm a newbie to both Scala and Spark.

I create a graph by invoking my own function:

val initialGraph: Graph[VertexAttributes, Int] = sim.createGraph

VertexAttributes is a class I defined:

class VertexAttributes(var pages: List[Page], var ads: List[Ad], var step: Long, val inDegree: Int, val outDegree: Int)
extends java.io.Serializable
{
  // Define alternative methods to be used as the score
  def averageScore() =
  {
    this.ads.map(_.score).sum / this.ads.length
  }

  def maxScore() =
  {
    if(this.ads.length == 0) None else Some(this.ads.map(_.score).max)
  }

  // Select averageScore as the function to be used
  val score = averageScore _
}

After some computations, I use the GraphX vertices() function to get the scores for each vertex:

val nodeRdd = g.vertices.map(v => if(v._2.score() == 0)(v._1 + ",'0,0,255'") else (v._1 + ",'255,0,0'"))

But this won't compile, the sbt message is:

value score is not a member of type parameter VertexAttributes

I have googled this error message, but frankly can't follow the conversation. Can anyone please explain the cause of the error and how I can fix it?

Thank you.

PS Below is my code for the createGraph method:

// Define a class to run the simulation
class Butterflies() extends java.io.Serializable
{
  // A boolean flag to enable debug statements
  var debug = true

  // A boolean flag to read an edgelist file rather than compute the edges
  val readEdgelistFile = true;

  // Create a graph from a page file and an ad file
  def createGraph(): Graph[VertexAttributes, Int] =
  {
    // Just needed for textFile() method to load an RDD from a textfile
    // Cannot use the global Spark context because SparkContext cannot be serialized from master to worker
    val sc = new SparkContext

    // Parse a text file with the vertex information
    val pages = sc.textFile("hdfs://ip-172-31-4-59:9000/user/butterflies/data/1K_nodes.txt")
      .map { l =>
        val tokens = l.split("\\s+")     // split("\\s") will split on whitespace
        val id = tokens(0).trim.toLong
        val tokenList = tokens.last.split('|').toList
        (id, tokenList)
      }
    println("********** NUMBER OF PAGES: " + pages.count + " **********")

    // Parse a text file with the ad information
    val ads = sc.textFile("hdfs://ip-172-31-4-59:9000/user/butterflies/data/1K_ads.txt")
      .map { l =>
        val tokens = l.split("\\s+")     // split("\\s") will split on whitespace
        val id = tokens(0).trim.toLong
        val tokenList = tokens.last.split('|').toList
        val next: VertexId = 0
        val score = 0
        //val vertexId: VertexId = id % 1000
        val vertexId: VertexId = id
        (vertexId, Ad(id, tokenList, next, score))
      }
    println("********** NUMBER OF ADS: " + ads.count + " **********")

    // Check if we should simply read an edgelist file, or compute the edges from scratch
    val edgeGraph =
    if (readEdgelistFile)
    {
      // Create a graph from an edgelist file
      GraphLoader.edgeListFile(sc, "hdfs://ip-172-31-4-59:9000/user/butterflies/data/1K_edges.txt")
    }
    else
    {
      // Create the edges between similar pages
      //   Create of list of all possible pairs of pages
      //   Check if any pair shares at least one token
      //   We only need the pair id's for the edgelist
      val allPairs = pages.cartesian(pages).filter{ case (a, b) => a._1 < b._1 }
      val similarPairs = allPairs.filter{ case (page1, page2) => page1._2.intersect(page2._2).length >= 1 }
      val idOnly = similarPairs.map{ case (page1, page2) => Edge(page1._1, page2._1, 1)}
      println("********** NUMBER OF EDGES: " + idOnly.count + " **********")

      // Save the list of edges as a file, to be used instead of recomputing the edges every time
      //idOnly.saveAsTextFile("hdfs://ip-172-31-4-59:9000/user/butterflies/data/saved_edges")

      // Create a graph from an edge list RDD
      Graph.fromEdges[Int, Int](idOnly, 1);
    }

    // Copy into a graph with nodes that have vertexAttributes
    //val attributeGraph: Graph[VertexAttributes, Int] =
    val attributeGraph = 
      edgeGraph.mapVertices{ (id, v) => new VertexAttributes(Nil, Nil, 0, 0, 0) }

    // Add the node information into the graph
    val nodeGraph = attributeGraph.outerJoinVertices(pages) {
      (vertexId, attr, pageTokenList) =>
        new VertexAttributes(List(Page(vertexId, pageTokenList.getOrElse(List.empty), 0)),
                         attr.ads, attr.step, attr.inDegree, attr.outDegree)
    }

    // Add the node degree information into the graph
    val degreeGraph = nodeGraph
    .outerJoinVertices(nodeGraph.inDegrees)
    {
      case (id, attr, inDegree) => new VertexAttributes(attr.pages, attr.ads, attr.step, inDegree.getOrElse(0), attr.outDegree)
    }
    .outerJoinVertices(nodeGraph.outDegrees)
    {
      case (id, attr, outDegree) =>
        new VertexAttributes(attr.pages, attr.ads, attr.step, attr.inDegree, outDegree.getOrElse(0))
    }

    // Add the ads to the nodes
    val adGraph = degreeGraph.outerJoinVertices(ads)
    {
      (vertexId, attr, ad) =>
      {
        if (ad.isEmpty)
        {
          new VertexAttributes(attr.pages, List.empty, attr.step, attr.inDegree, attr.outDegree)
        }
        else
        {
          new VertexAttributes(attr.pages, List(Ad(ad.get.id, ad.get.tokens, ad.get.next, ad.get.score)),           
                               attr.step, attr.inDegree, attr.outDegree)
        }
      }
    }

    // Display the graph for debug only
    if (debug)
    {
      println("********** GRAPH **********")
      //printVertices(adGraph)
    }

    // return the generated graph
    return adGraph
  }
}

Answer 1

VertexAttributes in your code refers to a type parameter, not to the VertexAttributes class. The mistake is probably in your createGraph function. For example it may be like this:

class Sim {
  def createGraph[VertexAttributes]: Graph[VertexAttributes, Int]
}

or:

class Sim[VertexAttributes] {
  def createGraph: Graph[VertexAttributes, Int]
}

In both cases you have a type parameter called VertexAttributes . This is the same as if you wrote:

class Sim[T] {
  def createGraph: Graph[T, Int]
}

The compiler doesn't know that T has a score method (because it doesn't). You don't need that type parameter. Just write:

class Sim {
  def createGraph: Graph[VertexAttributes, Int]
}

Now VertexAttributes will refer to the class, not to the local type parameter.