Question

basically I’m not really a Java/Scala fan, but unfortunately I’m forced to use it for my studies. Anyways, I was given an assignment:

What the program gets is a list of objects like: Mark(val name String, val style_mark Int, val other_mark Int).

How can I use groupBy, to group the marks by name, and get an average for style_mark and other_mark?

Mark("John", 2, 5)
Mark("Peter", 3, 7)
Mark("John", 4, 3)

Should return:

Mark("John", 3, 4)
Mark("Peter", 3, 7)

Thats the code:

class Mark(val name: String, val style_mark: Int, val other_mark: Int) {}

object Test extends Application
  {
  val m1 = new Mark("Smith", 18, 16);
  val m2 = new Mark("Cole", 14, 7);
  val m3 = new Mark("James", 13, 15);
  val m4 = new Mark("Jones", 14, 16);
  val m5 = new Mark("Richardson", 20, 19);
  val m6 = new Mark("James", 4, 18);

  val marks = List(m1, m2, m3, m4, m5, m6);

  def avg(xs: List[Int]) = xs.sum / xs.length

  marks.groupBy(_.name).map { kv => Mark(kv._1, avg(kv._2.map(_.style_mark)), avg(kv._2.map(_.other_mark))) }

  println(marks);
  }

Any help would be greatly appreciated,

Paul

Answer 1

As you already said, we can use groupBy to group the Marks by name. Now we have a Map where each key is the name and the value is a list of Marks with that name.

We can now iterate over that Map and replace each key-value pair with a Mark-object that has the key as its name, and the average of the style_marks in the list as its style_mark and the average of other_marks in the list as its other_mark. Like this:

def avg(xs: List[Int]) = xs.sum / xs.length
marks.groupBy(_.name).map { kv =>
  Mark(kv._1, avg(kv._2.map(_.style_mark)), avg(kv._2.map(_.other_mark)))
}