Queue Data Structure: Types, Implementation, Applications

Queues are a fundamental data structure operating on the First-In-First-Out (FIFO) principle, meaning the first item added is the first to be removed. They are important for organizing and managing data in many applications, including operating systems, network protocols, and data processing pipelines. Queues are essentially used to manage threads in multithreading and implementing priority queuing systems. This blog will discuss different types of queue data structures, their basic operations, implementation, and queue applications.

What is a Queue?

A queue is an important data structure in programming. It follows the FIFO (First In, First Out) method and is open at both ends. Data insertion is done at one end, the rear end or the tail of the queue, while deletion is done at the other end, the front end or the head of the queue.

Real-Life Queue in Data Structure Example

Let’s consider a queue in data structure example. A line of people is waiting to buy a ticket at a cinema hall. A new person will join the line from the end, and the person standing at the front will be the first to get the ticket and leave the line. Similarly, in a queue data structure, data added will leave the queue first.

Some other applications of the queue in real life are:

• People on an escalator
• Cashier line in a store
• A car wash line
• One way exits

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Types of Queues in Data Structure

There are four different types of queues in data structures:

• Simple Queue
• Circular Queue
• Priority Queue
• Double-Ended Queue (Deque)

Simple Queue

Simple Queue is a linear data structure that follows the First-In-First-Out (FIFO) principle, where elements are added to the rear (back) and removed from the front (head).

• Ordered collection of comparable data kinds.
• Queue structure is FIFO (First in, First Out).
• When a new element is added, all elements added before the new element must be deleted to remove the new element.

Applications of Circular Queue

Resource Allocation: Simple Queues are is useful for resource allocation in operating systems that manage resource requests such as CPU, memory, and I/O devices.

Batch Processing: Queues accommodate batch jobs, for instance, tasks like data processing or rendering images, which are queued up for sequential execution.

Message Buffering: It helps to buffer the message in communication systems to ensure a smooth data flow among processes.

Circular Queue

A circular queue is a special case of a simple queue in which the last member is linked to the first, forming a circle-like structure.

• The last node is connected to the first node.
• Also known as a Ring Buffer, the nodes are connected end to end.
• Insertion takes place at the front of the queue, and deletion at the end of the queue.
• Example of circular queue application: Insertion of days in a week.

Applications of Circular Queue

• CPU Scheduling: Used in operating systems to manage processes.
• Data Buffering: Frequently used in data streaming and buffering applications.
• Simulation Systems: Helpful in simulating real-world systems and processes, e.g., traffic flow control.

Priority Queue

In a priority queue, the nodes will have some predefined priority in the priority queue. The node with the least priority will be the first to be removed from the queue. Insertion takes place in the order of arrival of the nodes.

Some of the applications of priority queue:

• Dijkstra’s shortest path algorithm
• Prim’s algorithm
• Data compression techniques like Huffman code

The diagram below shows how an application uses a priority queue for the items the user consumes.

Priority Queue Applications

• Data Compression: In methods like Huffman coding, priority queues organize characters based on how often they appear, which helps reduce file sizes.
• Event Simulation: Priority queues manage events in simulations, making sure that events happening sooner are processed before later ones.
• Top-k Elements: They can keep track of the top-k items coming from a data stream, allowing rapid access to the most important ones.

Deque (Double Ended Queue)

A double-ended queue, often abbreviated as deque (pronounced "deck"), is a type of data structure that allows elements to be added or removed from both the front and the rear, making it a versatile option for various applications. Unlike regular queues that follow the FIFO)principle, deques do not have this restriction.

Deque Applications:

• Undo/Redo Functions: Deques are useful in applications for keeping track of actions so users can go back or forward through their history.
• Store Web Browser History: In a web browser, deques can be used to easily store the visited pages to add new pages and remove old ones.
• Graph Traversal: In algorithms like Breadth-First Search (BFS), deques help efficiently manage which nodes to explore next.
• Palindrome Checking: Deques can check if a word or phrase reads the same backwards as forward by comparing letters from both ends.

Basic Queue Operations in Queue Data Structure

Below are the basic queue operations in data structure: 

Now, let's understand the two primary operations associated with the Queue data structure: enqueue and dequeue.

Enqueue Operation

Below are the steps to enqueue (insert) data into a queue

1. Check whether the queue is full or not.
2. If the queue is full – print the overflow error and exit the program.
3. If the queue is not full – increment the rear pointer to point to the next empty space.
4. Else add the element in the position pointed by Rear.
5. Return success.

Algorithm for Enqueue Operation

Dequeue Operation

Below are the steps to perform the dequeue operation

• Check whether the queue is full or not.
• If the queue is empty – print the underflow error and exit the program.
• If the queue is not empty – access the data where the front is pointing.
• Else increment the front pointer to point to the next available data element.
• Return success.

Algorithm for Dequeue Operation

Implementation of Queue

A queue can be implemented in two ways: 

• Sequential allocation: It can be implemented using an array. A queue implemented using an array can organize only a limited number of elements.
• Linked list allocation: It can be implemented using a linked list. A queue implemented using a linked list can organize unlimited elements.

Now, let’s move on to the application of queue.

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Queue applications in Data Structure

A queue data structure is generally used in scenarios where the FIFO approach (First In First Out) has to be implemented. The following are some of the most common queue applications in data structure: 

• Managing requests on a single shared resource, such as CPU scheduling and disk scheduling
• Handling hardware or real-time systems interrupts
• Handling website traffic
• Routers and switches in networking
• Maintaining the playlist in media players

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Conclusion

Understanding the queue data structure can help those willing to boost their programming and problem-solving skills. Once you know the different types of queues and how they are implemented, you can easily solve task scheduling, managing resources, and data processing problems, to name a few. Whether you are developing software for operating systems, managing network traffic, or building efficient algorithms, proper knowledge of queues will enable you to design more effective solutions. We encourage you to explore further and take up data structure courses that can help you understand how queue data structures work and are implemented so that you are better equipped to tackle complex programming challenges and improve your overall coding skills.

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