Question

I’m doing a scala course and one of the examples given is the sumInts function which is defined like :

  def sumInts(a: Int, b: Int) : Int = 
    if(a > b) 0  
    else a + sumInts(a + 1 , b)

I’ve tried to understand this function better by outputting some values as its being iterated upon :

class SumInts {
      def sumInts(a: Int, b: Int) : Int = 
        if(a > b) 0 else 
        {     
          println(a + " + sumInts("+(a + 1)+" , "+b+")")       
          val res1 = sumInts(a + 1 , b)
          val res2 = a
          val res3 = res1 + res2
          println("res1 is : "+res1+", res2 is "+res2+", res3 is "+res3)
          res3
        }
}

So the code :

object SumIntsMain {

    def main(args: Array[String]) {

      println(new SumInts().sumInts(3 , 6));

  }

}

Returns the output :

3 + sumInts(4 , 6)
4 + sumInts(5 , 6)
5 + sumInts(6 , 6)
6 + sumInts(7 , 6)
res1 is : 0, res2 is 6, res3 is 6
res1 is : 6, res2 is 5, res3 is 11
res1 is : 11, res2 is 4, res3 is 15
res1 is : 15, res2 is 3, res3 is 18
18

Can someone explain how these values are computed. I’ve tried by outputting all of the created variables but still im confused.

Answer 1

manual-human-tracer on:

return sumInts(3, 6) | a = 3, b = 6
3 > 6 ? NO
return 3 + sumInts(3 + 1, 6) | a = 4, b = 6
4 > 6 ? NO
return 3 + (4 + sumInts(4 + 1, 6)) | a = 5, b = 6
5 > 6 ? NO
return 3 + (4 + (5 + sumInts(5 + 1, 6))) | a = 6, b = 6
6 > 6 ? NO
return 3 + (4 + (5 + (6 + sumInts(6 + 1, 6)))) | a = 7, b = 6
7 > 6 ? YEEEEES (return 0)
return 3 + (4 + (5 + (6 + 0))) = return 18.

manual-human-tracer off.