Introduction to Queue Data Structure

A queue is a data structure that stores items in an ordered sequence. It is often referred to as a “First-In-First-Out” (FIFO) structure, meaning the first item added to the queue is the first one to be removed. Queues are a fundamental data structure used in many computer algorithms and applications. 

A queue can be implemented using an array, linked list, or other data structure. When an item is added to the queue, it is placed at the end of the list. When an item is removed from the queue, it is taken from the front of the list. This ensures that the items are removed in the order in which they were added. 

Queues are used in many applications, such as in operating systems to manage processes, in communication protocols to store messages waiting to be sent or received, and in databases to store transactions that need to be processed. 

Queues are also used in real-world applications such as lines at stores or amusement parks. In these cases, the queue is used to ensure fairness by giving everyone an equal chance to be served. 

Overall, queues are an important data structure that is widely used in computer algorithms and applications. They are easy to implement, efficient to use, and serve a variety of purposes.

Implementation of Queue Data Structure 

Displaying basic push and pop operations in a Queue.
queue<int> q;

// queue is  (back)7 1 4 0 3(front)

cout<<"front element is "<<q.front(); //3
cout<<"back element is "<<q.back(); //7

q.pop(); // 3 is poped. Now front element is 0
cout<<"front "<<q.front();
cout<<"size of queue = "<<q.size(); // 4

The standard container classes deque and list fulfill these requirements. By default, if no container class is specified for a particular queue class instantiation, the standard container deque is used.

Types of Queue Data Structures 

  • Simple Queue: A simple queue, usually referred to as a linear queue, is the simplest fundamental type of queue. The Enqueue operation, which inserts an element, is performed at the back end while the Dequeue operation, which removes an element, is performed at the front end.
  • Circle Queue: In a circular queue, each queue member functions as a circle. With the exception of the end member being connected to the first element, a circular queue operates similarly to a linear queue. Its benefit is that memory is used more effectively. This is because an element can be added to a location in the queue where there is an empty space, i.e. when no element is present there.
  • Priority Queue:  The priority queue is a unique kind of queue. Its unique selling point is how it prioritizes the elements and queues them up. The element with the highest value may have the highest priority, resulting in a queue with values in decreasing order. In order to build a queue with increasing order of values, the priority can also be set so that the element with the lowest value receives the highest priority.
  • Dequeue: Another name for Dequeue is Double Ended Queue. The term “double-ended” refers to a queue that can have elements added or withdrawn from either end, as opposed to conventional queues where this may only be done from one end. This means that it might not adhere to the First In First Out characteristic.

Advantages of Queue Data Structures 

  • The management of a lot of data is easy and effective.
  • Operations like insertion and deletion may be done easily since it follows the first in first out rule.
  • When many people are utilizing a single service, queues are helpful.
  • While communicating data across processes, queues communicate quickly.
  • More data structures can be implemented using queues.

Disadvantages of Queue Data Structures 

  • Middle component insertion and deletion take a long time to complete.
  • Limited Space.
  • A new element can only be added to a traditional queue after the older ones have been removed.
  • An element search takes O(N) time.
  • A prior definition of the queue’s maximum size is required.

Applications of Queue Data Structures 

  • Multiprogramming: Multiprogramming is the term for running numerous programs simultaneously in the main memory. These numerous applications need to be organized, and queues are the best way to do so.
  • Network: Devices like a router or switch employ queues in networks. A mail queue is a directory that houses data and manages files for mail messages, hence it is another application of a queue.
  • Job Scheduling: The computer is tasked with carrying out a set number of jobs that are arranged to run sequentially. The processor, which is managed by a queue, is given these tasks one at a time.
  • Queues serve as waiting lists for a single shared resource when they are used in shared resources.

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