Queue in Scala

A queue is a first-in, first-out (FIFO) data structure. Scala offers both an immutable queue and a mutable queue. A mutable queue can be updated or extended in place. It means one can change, add, or remove elements of a queue as a side effect. Immutable queue, by contrast, never change.

In Scala, Queue is implemented as a pair of lists. One is used to insert the elements and second to contain deleted elements. Elements are added to the first list and removed from the second list. The two most basic operations of Queue are Enqueue and Dequeue.

• Enqueue – Adding an element at the end of the queue.
• Dequeue – Deleting an element from the beginning of the queue.

Methods in Queue:

1. +=: This method is used to add a single element in the end of the queue.
2. ++=: This method is used to Insert more than one the element in the end of the queue.
3. clear: Remove all elements from the queue.
4. dequeue: Returns the first element in the queue
5. enqueue: Adds all the elements to the queue.
6. equals: Checks if two queues are structurally identical.
7. front: Returns the first element in the queue.
8. isEmpty: Check if the queue is empty or not.

Below are simple Scala programs to demonstrate these operations:

Example 1:

// Scala program for illustrating Queue

// Import Queue
import scala.collection.mutable._

// Creating object
object GfG
{
    // Main method
    def main(args:Array[String])
    {
        // Initialize a queue
        var q1 = Queue(1, 2, 3, 4, 5)

        // Print the elements of queue
        print("Queue Elements: ")
        q1.foreach((element:Int) => print(element+" "))

        // Print the first element of the queue
        var firstElement = q1.front
        println("\nFirst element in the queue: "+ firstElement)

        // Enqueue 10 in the queue
        q1.enqueue(10)

        // Print the elements of queue
        print("Queue Elements after enqueue: ")
        q1.foreach((element:Int) => print(element+" "))

        // Dequeue first element from the queue
        var deq = q1.dequeue

        // Print the elements of queue
        print("\nQueue Elements after dequeue: ")
        q1.foreach((element:Int) => print(element+" "))

        // Print the Dequeued element
        print("\nDequeued element: " + deq)

        // using isEmpty method
        println("\nQueue is empty: "+ q1.isEmpty)
    }
}

Output:

Queue Elements: 1 2 3 4 5 
First element in the queue: 1
Queue Elements after enqueue: 1 2 3 4 5 10 
Queue Elements after dequeue: 2 3 4 5 10 
Dequeued element: 1
Queue is empty: false

Example 2:

// Scala program for illustrating Queue

// Import Queue
import scala.collection.mutable._

// Creating object
object GfG
{
    // Main method
    def main(args:Array[String])
    {
        // Initialize a queue
        var fruits = Queue[String]()

        // Adding elements to the queue
        fruits.enqueue("apple")
        fruits.enqueue("banana")
        fruits.enqueue("mango")
        fruits.enqueue("guava")

        // Print the elements of queue
        print("Queue Elements: ")
        fruits.foreach((element:String) => print(element+" "))

        // Print the first element of the queue
        var firstElement = fruits.front
        println("\nFirst element in the queue: "+ firstElement)

        // Enqueue pineapple in the queue
        fruits.enqueue("pineapple")

        // Print the elements of queue
        print("Queue Elements after enqueue: ")
        fruits.foreach((element:String) => print(element+" "))

        // Dequeue first element from the queue
        var deq = fruits.dequeue

        // Print the elements of queue
        print("\nQueue Elements after dequeue: ")
        fruits.foreach((element:String) => print(element+" "))

        // Print the Dequeued element
        print("\nDequeued element: " + deq)

        // Using clear method
        println("\nclear the queue: "+ fruits.clear)

        // Using isEmpty method
        println("\nqueue is empty: "+ fruits.isEmpty)

    }
}

Output:

Queue Elements: apple banana mango guava 
First element in the queue: apple
Queue Elements after enqueue: apple banana mango guava pineapple 
Queue Elements after dequeue: banana mango guava pineapple 
Dequeued element: apple
clear the queue: ()

queue is empty:true