I'll like to check if I have understood flatten and flatMap functions correctly.
1) Am I correct that flatten works only when a collection constitutes of other collections. Eg flatten would work on following lists
//list of lists
val l1 = List(List(1,1,2,-1,3,1,-4,5), List("a","b"))
//list of a set, list and map
val l2 = List(Set(1,2,3), List(4,5,6), Map('a'->"x",'b'->"y"))
But flatten will not work on following
val l3 = List(1,2,3)
val l4 = List(1,2,3,List('a','b'))
val s1 = "hello world"
val m1 = Map('h'->"h", 'e'->"e", 'l'->"l",'o'->"0")
'flatten' method would create a new list consisting of all elements by removing the hierarchy. Thus it sort of 'flattens' the collection and brings all elements at the same level.
l1.flatten
res0: List[Any] = List(1, 1, 2, -1, 3, 1, -4, 5, a, b)
l2.flatten
res1: List[Any] = List(1, 2, 3, 1, 5, 6, (a,x), (b,y))
2) 'flatMap' first applies a method to elements of a list and then flattens the list. As we noticed above, the flatten method works if lists have a hierarchy (contain other collections). Thus it is important that the method we apply to the elements returns a collection otherwise flatMap will not work
//we have list of lists
val l1 = List(List(1,1,2,-1,3,1,-4,5), List("a","b"))
l1 flatMap(x=>x.toSet)
res2: List[Any] = List(5, 1, -4, 2, 3, -1, a, b)
val l2 = List(Set(1,2,3), List(1,5,6), Map('a'->"x",'b'->"y"))
l2.flatMap(x=>x.toSet)
res3: List[Any] = List(1, 2, 3, 1, 5, 6, (a,x), (b,y))
val s1 = "hello world"
s1.flatMap(x=>Map(x->x.toString))
We notice above that s1.flatten didn't work but s1.flatMap did. This is because, in s1.flatMap, we convert elements of a String (characters) into a Map which is a collection. Thus the string got converted into a collection of Maps like (Map('h'->"h"), Map('e'->"e"), Map('l'->"l"),Map ('l'->"l"),Map('o'->"o")....) Thus flatten will work now. Note that the Map created is not Map('h'->"h", 'e'->"e", 'l'->"l",....).
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