Scala：类型不匹配错误

Question

I am new to scala and I am practicing it with k-means algorithm following the tutorial from k-means

I am confused by this part of this tutorial:

var newCentroids = pointsGroup.mapValues(ps => average(ps)).collectAsMap()  

This causes a type mismatch error because function average needs a Seq , while we give it an Iterable . How can I fix this? What caused this error?

Answer 1

Well Seq is a sub-type of Iterable but not vice-versa, so it is not possible to convert these types in the type systems.

There is an explicit conversion available by writing average(ps.toSeq) . This conversion will iterate the Iterable and collect the items into a Seq .

Answer 2

We could easily replace Seq with Iterable in provided solution for average function:

def average(ps: Iterable[Vector]) : Vector = {
  val numVectors = ps.size
  var out = new Vector(ps.head.elements)
  ps foreach ( out += _)
  out / numVectors
}

Or even in constant space:

def average(ps: Iterable[Vector]): Vector = {
  val numVectors = ps.size

  val vSize = ps.head.elements.length

  def element(index: Int): Double = ps.map(_(index)).sum / numVectors

  new Vector(0 until vSize map element toArray)
}