idris75

The difference between map, flatMap is a little confusing for beginers - this example might help:
This can be tested on a spark shell or scala CLI:

scala> val l = List(1,2,3,4,5)

scala> l.map(x => List(x-1, x, x+1))
res1: List[List[Int]] = List(List(0, 1, 2), List(1, 2, 3), List(2, 3, 4), List(3, 4, 5), List(4, 5, 6))

scala> l.flatMap(x => List(x-1, x, x+1))
res2: List[Int] = List(0, 1, 2, 1, 2, 3, 2, 3, 4, 3, 4, 5, 4, 5, 6)