Introduction

A queue is a linear data structure in computer science that adheres to the First-In-First-Out (FIFO) principle. It is a group of elements where each element is added and removed sequentially. Operating systems, network protocols, and web applications are just a few of the applications that use queues. 

In circumstances where it is necessary to manage a series of tasks or events, queues are frequently used. For example, in an operating system, the operating system uses a queue to manage processes. A process is added to the end of the queue when it is launched. The first process in the queue is selected by the operating system and set to running when it is prepared to execute a process. Up until each and every process in the queue has been finished, this process keeps going.

There are many different ways to use queues. Arrays or linked lists are used most frequently as implementations. In an implementation using an array, the queue is shown as an array with two pointers, one pointing to the front and the other to the back of the queue. A linked list with a head pointer and a tail pointer is used to implement the queue in a linked list.

In this article, we will discuss the queue data structure in detail, including its implementation, operations, and real-world applications.

Implementation

There are several ways to implement a queue data structure, including using arrays, linked lists, or circular buffers.

Array implementation

In an array implementation of a queue, a fixed-size array is used to store the elements of the queue. Two pointers, front and rear, are used to keep track of the front and rear elements of the queue, respectively. When an element is added to the queue, it is added to the rear of the array, and the rear pointer is incremented. When an element is removed from the queue, it is removed from the front of the array, and the front pointer is incremented. If the front pointer becomes equal to the rear pointer, the queue is empty.

One disadvantage of the array implementation is that it has a fixed size, which limits the number of elements that can be stored in the queue. If the queue becomes full, it is not possible to add any more elements to it. To overcome this limitation, a circular buffer can be used.

Circular buffer implementation

A circular buffer is a data structure in which the end of the buffer is connected to the beginning of the buffer. In a circular buffer implementation of a queue, a circular buffer is used to store the elements of the queue. Two pointers, front and rear, are used to keep track of the front and rear elements of the queue, respectively. When an element is added to the queue, it is added to the rear of the buffer, and the rear pointer is incremented. When an element is removed from the queue, it is removed from the front of the buffer, and the front pointer is incremented. If the front pointer becomes equal to the rear pointer, the queue is empty.

The advantage of the circular buffer implementation is that it allows for a larger number of elements to be stored in the queue than the array implementation. However, it requires more complex indexing to manage the circular nature of the buffer.

Linked list implementation

In a linked list implementation of a queue, a linked list is used to store the elements of the queue. Each node in the linked list contains a data element and a pointer to the next node. Two pointers, front and rear, are used to keep track of the front and rear nodes of the queue, respectively. When an element is added to the queue, a new node is created and added to the rear of the linked list, and the rear pointer is updated to point to the new node. When an element is removed from the queue, the front node of the linked list is removed, and the front pointer is updated to point to the next node.

The advantage of the linked list implementation is that it allows for a dynamic number of elements to be stored in the queue, as new nodes can be added or removed as needed. 

However, it requires more memory than the array or circular buffer implementation, as each node in the linked list contains a data element and a pointer to the next node.

Operations

The queue data structure has several important operations that can be performed on it, including enqueue, dequeue, peek, and size.

Enqueue

The enqueue operation adds an element to the rear of the queue. In an array implementation, the element is added to the next available position in the array, and the rear pointer is incremented. In a linked list implementation, a new node is created and added to the rear of the linked list, and the rear pointer is updated to point to the new node.

Dequeue

The dequeue operation removes the front element from the queue. In an array implementation, the front element is removed by incrementing the front pointer. In a linked list implementation, the front node is removed, and the front pointer is updated to point to the next node.

Peek

The peek operation returns the front element of the queue without removing it. This operation is useful when you want to check the next element in the queue without actually removing it. In an array implementation, the front element can be accessed directly. In a linked list implementation, the data element of the front node can be accessed directly.

Size

The size operation returns the number of elements in the queue. This operation is useful when you want to check how many elements are in the queue at a given time. In an array implementation, the size can be calculated by subtracting the front pointer from the rear pointer. In a linked list implementation, the size can be calculated by iterating through the linked list and counting the number of nodes.

Real-world Applications

Queues are used in a wide variety of real-world applications, including operating systems, network protocols, and web applications.

Operating Systems

In an operating system, a queue is used to manage processes that are waiting to be executed. The operating system maintains a queue of processes that are waiting to be run on the CPU. When a process is ready to run, it is added to the end of the queue. The operating system then schedules the next process to run based on the FIFO principle, removing the process at the front of the queue and adding it to the CPU.

Network Protocols

In network protocols such as TCP/IP, queues are used to manage packets of data that are waiting to be transmitted or received. When a packet of data is sent over the network, it is added to a transmission queue. When a packet of data is received from the network, it is added to a receive queue. The network protocol then processes the packets in the receive queue based on the FIFO principle, removing the packet at the front of the queue and processing it.

Web Applications

In web applications, queues are used to manage requests that are waiting to be processed. When a user submits a request to a web application, the request is added to a request queue. The web application then processes the requests in the request queue based on the FIFO principle, removing the request at the front of the queue and processing it.

Conclusion

In a wide range of applications, the queue data structure is a straightforward but effective tool. It is a FIFO-compliant linear data structure that is useful for managing elements that must be handled in a particular order. Queues have several crucial operations, such as enqueue, dequeue, peek, and size, and they can be implemented using arrays, linked lists, or circular buffers. Operating systems, network protocols, and web applications are just a few areas where queues are used. Being a proficient programmer requires understanding the queue data structure.