Related Articles
Recursive Streams and collection in Scala
• Last Updated : 29 Jul, 2019

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 program for resursive 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 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 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 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 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 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.

My Personal Notes arrow_drop_up