map, flatMap, filter and for comprehension in Scala

.map
When map is applied on some collection of type A, it returns a new collection of the same type with elements of type B.

    val list = List(1,2,3) // this is calling apply method on companion object
    println(list)
    // map
    println(list.map(_ + 1)) // returns List (same type) of type Int
    println(list.map(_ + " is a number")) // returns List (same type) of type String

.filter
Takes a lambda that takes one param and returns boolean. Values are filtered when boolean is true.

  println(list.filter(_ % 2 == 0)) // prints 2

.flatMap
flatten (reduces) the hierarchy by one level each time it is applied

    val toPair = (x: Int) => List(x, x+1)
    println(list.flatMap(toPair))

Example with both map and flatMap

val numbers = List(1, 2, 3, 4)
    val chars = List('a', 'b', 'c', 'd')
    val colors = List("White", "Black")
    val combinations = numbers.flatMap(n => chars.flatMap(c => colors.map(co => c + "" + n + " " + co)))
    println(combinations)

for comprehensions
since the above maps and flatMaps are difficult to read, we use for comprehensions and the compiler will transform the for-comprehension to maps and flatMaps for us

    val forCombinations = for {
        n <- numbers
        c <- chars
        co <- colors
    } yield "" + c + "" + n + " " + co
    println(forCombinations)