Question

I’m trying to understand the add method in below class taken from book ‘Programming in Scala – Second Edition’.
Is this correct :
The method ‘add’ takes defines an operator which of type Rational. I don’t know what is occurring within the new Rational :

numer * that.denom + that.numer * denom,
denom * that.denom

How are numer & denom be assigned here ? , Why is each expression separated by a comma ?
Entire class :

class Rational(n: Int , d: Int) {

  require(d != 0)

  private val g = gcd(n.abs, d.abs)
  val numer = n / g
  val denom = d/ g

  def this(n: Int) = this(n, 1)

  def add(that: Rational): Rational = 
    new Rational(
        numer * that.denom + that.numer * denom,
        denom * that.denom
        )

  override def toString = numer +"/" + denom

  private def gcd(a: Int, b: Int): Int = 
    if(b == 0) a else gcd(b, a % b)
}

Answer 1

That two expressions are the arguments to the Rational constructor, thus they will be the private vals n and d respectively. For example

class C(a: Int, b: Int)
val c1 = new C(1, 2) // here 1, 2 is like the two expressions you mention

The add method implements rational number addition:

 a     c     a*d     c*b     a*d + c*b
--- + --- = ----- + ----- = -----------
 b     d     b*d     b*d        b*d

where

 a = numer
 b = denom
 c = that.numer
 d = that.denom