Scala Stream

The Stream is a lazy lists where elements are evaluated only when they are needed. This is a scala feature. Scala supports lazy computation. It increases performance of our program. Streams have the same performance characteristics as lists.

Syntax :

val str = 1 #:: 2 #:: 3 #:: Stream.empty

In scala a List can be constructed with :: operator, whereas a Stream can be constructed with the #:: operator method, using Stream.empty at the end of the expression. In above syntax the head of this stream is 1, and the tail of it has 2 and 3.

Operations on Stream

Create a Stream: Below is the examples to create a streams in Scala.
Example :

// Program to creating an empty stream

// Creating object
object GFG
{
    // Main method
    def main(args:Array[String])
    {
        // Creating stream
        val stream = 1 #:: 2#:: 8 #:: Stream.empty
        println(stream)
    }
}

Output:

Stream(1, ?)

In the above output, we can see that second element is not evaluated. Here, a question mark is displayed in place of element. Scala does not evaluate list until it is required. The tail is not printed, because it hasn’t been computed yet. Streams are specified to lazy computation.

Create a Stream using Stream.cons : We can also create a Stream by using Stream.cons . A package import scala.collection.immutable.Stream.cons is used for creating stream.

Example :

// Program to creating an stream
// using cons
import scala.collection.immutable.Stream.cons

// Creating object
object GFG
{
    // Main method
    def main(args:Array[String])
    {
        // Creating stream
        val stream2: Stream[Int] = cons(1, cons(2, cons(3, Stream.empty) ) )
        println(s"Elements of stream2 = ${stream2}")
    }
}

Output:

Elements of stream2 = Stream(1, ?)

Using take function on stream: take function is used to take elements from stream. Below is the example of using take function.

Example :

// Program to Using take function on stream

// Creating object
object GFG
{
    // Main method
    def main(args:Array[String])
    {
        // Creating stream
        val stream = 1 #:: 2#:: 8 #:: Stream.empty
        println(stream)

        // Taking elements from stream
        print("Take first 2 numbers from stream = ")
        stream.take(2).print
        print("\nTake first 10 numbers from stream2 = ")
        stream.take(10).print
    }
}

Output :

Stream(1, ?)
Take first 2 numbers from stream = 1, 2, empty
Take first 10 numbers from stream2 = 1, 2, 8, empty

When we wanted to take 10 numbers from a Stream, although it only contained 3 elements, it did not throw any IndexOutOfBoundsException.

Using map function on stream: map function is used to perform operation on stream.

Example :

// Scala program to using map function on stream

// Creating object
object GFG
{
    // Main method
    def main(args:Array[String])
    {
        // Creating stream
        val stream = 1 #:: 2#:: 8 #:: Stream.empty
        println(stream)

        // map elements from stream
        println(stream.map{_+5})
    }
}

Output:

Stream(1, ?)
Stream(6, ?)

In above example by using map function we are transforming the input collection to a new output collection.

Initialize an empty Stream: Below code shows how to initialize an empty Stream.
Example :

// Program to create empty stream

// Creating object
object GFG
{
    // Main method
    def main(args:Array[String])
    {
        // Creating empty stream
        val emptyStream: Stream[Int] = Stream.empty[Int]
        println(s"Empty Stream = $emptyStream")
    }
}

Output:

 Empty Stream = Stream()