CODEWITHSUNDEEP
scala
SnarkShark
SnarkShark

Reputation: 380

Error when I use my val in function, but not when I access val on its own in REPL.

I am writing Scala code to search a QuadTree. I'm fairly new to Scala, so this may be a simple question: Why do I get a error: value rec not a member of QuadTree when I try to use tree.rec in a function, BUT if I ask the REPL to evaluate tree.rec, it successfully returns the data?

Anyway, I've got these classes to start with:

case class Rectangle (minx: Int, maxx: Int, miny: Int, maxy: Int)

abstract class QuadTree
case class Node (value : Int, nw : QuadTree, ne: QuadTree, sw: QuadTree, se: QuadTree, rec: Rectangle) extends QuadTree
case class Leaf (value : Int, rec: Rectangle) extends QuadTree
case class Empty (value: Int, rec: Rectangle) extends QuadTree

So I'm defining a Rectangle, and then a QuadTree, which can has a value, and either contains a Node, a Leaf, or Empty.

Next I have a function that checks to see if two rectangles overlap called rectangles_Overlap, which returns True if two input Rectangles overlap, and returns False if there is no overlap.

If I define

val query = Rectangle(1, 8, 8, 16)
val tree1 = Node(13,Node(12,Leaf(7,Rectangle(1,4,12,16)),Leaf(3,Rectangle(4,8,12,16)),Leaf(2,Rectangle(1,4,8,12)),Empty(0,Rectangle(4,8,8,12)),Rectangle(1,8,8,16)),Empty(0,Rectangle(8,16,8,16)),Empty(0,Rectangle(1,8,1,8)),Leaf(1,Rectangle(8,16,1,8)),Rectangle(1,16,1,16))

If I want to get the size of the tree, I can type into the REPL

scala> tree1.rec
res0: Rectangle = Rectangle(1,16,1,16)

I can also use my rectangles_Overlap method to see if the query Rectangle overlaps with the tree Rectangle. 

scala> rectangles_Overlap(query, tree1.rec)
res1: Boolean = true

But if I try to use rectangles_Overlap(query, tree1.rec) in another function, its unhappy!

def queryBoolean(query: Rectangle, tree: QuadTree): Boolean = {
    if(rectangles_Overlap(query, tree.rec)) {
        println("Yay they overlap so I can do other stuff...")
        // I want to add other code here after I get this working
        true
    } else { 
        println("Nah, they don't overlap, don't need to do anything")
        false
    }
}

And then call the function with queryBoolean(query, tree1), I get this error:

<console>:17: error: value rec is not a member of QuadTree
       if (rectangles_Overlap(query, tree.rec)) {

So my question again is: Why can I evaluate tree1.rec in the REPL, but cannot use tree.rec in a function without an error?

My original thought was that perhaps I was overloading something... but rec is always a Rectangle in all of the case clase ... extends QuadTree. So I don't believe I am overloading anything?

Upvotes: 0

Views: 71

Answers (2)

Apurva Singh
Apurva Singh

Reputation: 5000

That is true. rec i.e. type of Rectangle, is not a member of QuadTree. If you change from QuadTree to Node, it will work. Otherwise it must be there in QuadTree too.

def queryBoolean(query: Rectangle, tree: Node): Boolean

Add to QuadTree as val (val is needed to access it), and then you need to override and call super constructor as below:

  abstract class QuadTree(val rec: Rectangle)
  case class Node (value : Int, nw : QuadTree, ne: QuadTree, sw: QuadTree, se: QuadTree, override val rec: Rectangle) extends QuadTree(rec)
  case class Leaf (value : Int, override val rec: Rectangle) extends QuadTree(rec)
  case class Empty (value: Int, override val rec: Rectangle) extends QuadTree(rec)

Upvotes: 2

Davis Broda
Davis Broda

Reputation: 4125

This is because rec is not defined for class QuadTree, but only for its subclasses, therefore there is no value to call. The reason that this works properly on REPL is because of the way it is defined. You define it as 

val tree1 = Node(/*some params*/)

Which means it has type Node, instead of QuadTree. As Node has a defined rec parameter, everything works out fine. However in the method, you pass in a raw QuadTree, so no rec is defined.

To get it to work, either add rec to the QuadTree class, or pass in a specific subtype, which has the correct parameters defined.

Upvotes: 1

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