Question

I’m learning Scala and have a quick question: Can someone please explain to me why the following two sets of code yield different results?

def grey0(n: Int): List[List[String]]={
  if (n==0) List(Nil)
  else for(i<-List("0","1"); j<-grey0(n-1)) yield i :: j 
 }

versus

def grey1(n: Int): List[List[String]]={
  if (n==0) Nil
  else for(i<-List("0","1"); j<-grey0(n-1)) yield i :: j 
 }

The first option yields the result I am looking for. What I don’t understand is, why does the second option just return the empty list? I would have thought the other results would cons on to it and if anything, I would get some sort of flat list rather than a List[List[String]] (which is what I want).

Answer 1

In your first version grey0(0) will return a 1-element list that contains the empty list. grey0(1) will, for each element in grey0(0) create two lists, so you get a total of two lists because grey0(0) contains one list. Likewise grey0(2) will contain 4 lists because for each of the two elements in grey0(1) it creates 2 lists.

In your second version grey0(0) will return the empty list. grey0(1) will create two lists for each element in grey0(0). Since grey0(0) has 0 elements, that makes a total of 0*2=0 elements in the result.