Recursive Streams and collection in Scala
Recursive forms have their definition in terms of themselves like we have subfolders in folders which can further have subfolders. Recursive methods calling themselves is also a recursive form. Similar forms can be used to create a recursive stream. Example 1: Creating a lazy list
scala
// Scala program for recursive stream// creating object object Geeks { // Main method def main(args: Array[String]) { // second con can be recursive lazy val geek = Stream.cons(1, Stream.cons(5, Stream.empty)) println (geek) (geek take 4) foreach { x => println(x) } } } |
Output :
Stream(1, ?) 1 5
In the code above the second con can be recursive. Example 2: Create a lazy list by placing a recursive call to method
scala
// Scala program for resursive stream// creating object object Geeks { // Main method def main(args: Array[String]) { // con can be recursive def geek(n: Int): Stream[Int] = Stream.cons(n, geek(n+1)) // Creating lazy val lazy val q = geek(5) println(q) } } |
Output :
Stream(5, ?)
Example 3: Create a lazy list by placing a recursive call to method in a brief and clear manner
scala
// Scala program for resursive stream// creating object object Geeks { // Main method def main(args: Array[String]) { // recursive stream def geek(n: Int):Stream[Int] = n #:: geek(n+1) // Lazy value lazy val q = geek(5) println(q) } } |
Output :
Stream(5, ?)
Stream Collections
Stream collections in scala are very important as it allows need not to be explicitly lopped over. Declaratively things can be performed using functional combinators like map, filter, and flatMap. Streams are lazy lost collections. Example 4:
scala
// Scala program for stream collection// creating object object Geeks { // Main method def main(args: Array[String]) { // creating Stream def geek(n: Int):Stream[Int] = n #:: geek(n+1) lazy val g = geek(0) println (g) println (g.take(10).mkString(" ; ")) } } |
Output :
Stream(0, ?) 0 ; 1 ; 2 ; 3 ; 4 ; 5 ; 6 ; 7 ; 8 ; 9)
Example 5: Using filter
scala
// Scala program for filter list// creating object object Geeks { // Main method def main(args: Array[String]) { // Creating list val geek = List(1, 2, 3, 4) // Using filter val abc = geek filter { x => x > 2 } println(abc) } } |
Output :
List(3, 4)
filter is called over the list geek. It takes function as an argument and returns a Boolean, which is a function predicate. Every element is run through the function and elements are filtered out where the function returns false. It results in a new list and the original “geek” list is remain persistent. Example 6: Using filter in a much concise manner
scala
// Scala program for collection// creating object object Geeks { // Main method def main(args: Array[String]) { // Creating list val geek = List(1, 2, 3, 4) // Filter list val abc = geek filter { _ > 2 } println(abc) } } |
Output :
List(3, 4)
In the code above, there is no need to name the element. The element is referred by a shorter form via a_. Also there is no need to qualify the method call.
