Division of Integers as Double in Scala

Question

I have a file with information about the age of people and the number of friends they have. I'm using Spark RDD's and Scala to calculate the average number of friends by age like this:

    val friendsByAge: Array[(Int, Double)] = (
      actualData
      .mapValues((value: Int) => {(value, 1)})
      .reduceByKey((acc: (Int, Int), value: (Int, Int)) => (acc._1 + value._1, acc._2 + value._2))
      .mapValues((tuple: (Int, Int)) => tuple._1.toDouble / tuple._2)
      .collect()
    )

This works. However, if I remove the tuple._1.toDouble conversion from the last step (where I'm dividing two integers) I get the following error:

type mismatch;
 found   : Array[(Int, Int)]
 required: Array[(Int, Double)]

I thought that, when dividing two integers, if I type that the result is Double it should return a double. If this is not the case, why is this simple code returning a double when I divide two integers?

scala> val a: Double = 10/5
val a: Double = 2.0

scala> println(a)
2.0

scala> println(10/5)
2

In summary, I know how to solve it using tuple._1.toDouble but I want to know what's going on. I'm new to Scala and I have a Python background so I feel like understanding the basic concepts is key. Thanks in advance.

Answer 1

let's dig a bit more on the provided code

scala> val a: Double = 10/5
val a: Double = 2.0

scala> println(a)
2.0

scala> println(10/5)
2

The first one, looks like it solves the problem, but it is not exactly correct. If you do 10 / 3 the result will not be the expected one

scala> val a: Double = 10 / 3
val a: Double = 3.0

scala> val a: Double = 10.toDouble / 3
val a: Double = 3.3333333333333335

scala> val a: Double = 10 / 3.toDouble
val a: Double = 3.3333333333333335

So, when you write a number in scala, the compiler will infer that is an Int (Unless you add a type).

val a = 1          // infers Int
val b = 1: Double  // 1 is converted to Double and then `b` infers Double
val c: Double = 1  // 1 is infered as Int and then cast as Double

If you look at the signatures of the method / from Int class you will see

  def /(x: Byte): Int
  def /(x: Short): Int
  def /(x: Char): Int
  def /(x: Int): Int
  def /(x: Long): Long
  def /(x: Float): Float
  def /(x: Double): Double

So, the method says what is the return type. Remember also that in scala everything is an object and every value has a type. You can also take a look at how operators work in scala