Queue in Scala

Queue

Queue is a first-in, first-out (FIFO) data structure. Scala offers both an immutable queue and mutable queue. This recipe demonstrates the mutable queue.

You can create an empty, mutable queue of any data type:

import scala.collection.mutable.Queue

var ints = Queue[Int]()

var fruits = Queue[String]()

var q = Queue[Person]()

You can also create a queue with initial elements :

val q = Queue(1, 2, 3)

 Add elements with +=, ++=, enqueue :

Once you have a mutable queue, add elements to it using +=, ++=, and enqueue, as shown in the following examples:

import scala.collection.mutable.Queue

var q = Queue[String]()

q += "apple"

println(q)

//Queue(apple)

q += ("kiwi", "banana")

println(q)

//Queue(apple, kiwi, banana)

q ++= List("cherry", "coconut")

println(q)

//Queue(apple, kiwi, banana, cherry, coconut)

q.enqueue("pineapple")

println(q)

//Queue(apple, kiwi, banana, cherry, coconut, pineapple)

Dequeue

Because a queue is a FIFO, you typically remove elements from the head of the queue, one element at a time, using dequeue:

var q = Queue(“apple”, “kiwi”, “banana”, “cherry”, “coconut”, “pineapple”)

q.dequeueprintln(q)
//Queue(kiwi, banana, cherry, coconut, pineapple)

DequeueFirst, DequeueAll

You can also use the dequeueFirst and dequeueAll methods to remove elements from the queue by specifying a predicate

var q = Queue(“apple”, “kiwi”, “banana”, “cherry”, “coconut”, “pineapple”)

q.dequeueFirst(_.startsWith("b"))

println(q)

// Queue(apple, kiwi, cherry, coconut, pineapple)

//Queue(apple, kiwi, banana, cherry, coconut, pineapple)

q.dequeueAll(_.length > 6)

println(q)

//Queue(apple, kiwi, banana, cherry)