Recursive Streams and collection in Scala

Last Updated : 25 Apr, 2023

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 =&gt; 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(&quot; ; &quot;))  }  }

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 =&gt; x &gt; 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 { _ &gt; 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.