Question

Ok so I thought this would be a snap, trying to practice Scala’s collection operators and my example is a list of points.

The class can calculate and return the distance to another point (as double).

However, fold left doesn’t seem to be the right solution – considering elements e1, e2, e3.. I need a moving window to calculate, I need the last element looked at to carry forward in the function – not just the sum

Sum {
  e1.dist(e2)
  e2.dist(e3)
  etc
} 

Reading the API I noticed a function called “sliding”, perhaps that’s the correct solution in conjunction with another operator. I know how to do this with loops of course, but trying to learn the scala way.

Thanks

import scala.math._

case class Point(x:Int, y:Int) {
  def dist(p:Point) = sqrt( (p.x-x)^2+(p.y-y)^2 )

}

object Point {

  //Unsure how to define this?
  def dist(l:Seq[Point]) =l.foldLeft(0.0)((sum:Double,p:Point)=>)

}

Answer 1

If you want to do it as a fold, you can, but you need the accumulator to keep both the total and the previous element:

l.foldLeft(l.head, 0.0){ 
  case ((prev, sum), p) => (p, sum + p.dist(prev)) 
}._2

You finish with a tuple consiting of the last element and sum, so use ._2 to get the sum part.

btw, ^ on Int is bitwise logical XOR, not power. Use math.pow.

The smartest way is probably using zipped, which is a kind of iterator so you don’t traverse the list more than once as you would using zip:

(l, l.tail).zipped.map( _ dist _ ).sum