Difference Between Array-Based Queue and List-Based Queue

In this article, we will discuss the difference between the Array-Based Queue and List-Based Queue in C++. But before discussing their differences, we must know about the Queue in C++ with its advantages, and disadvantages.

What is the Queue?

In computer science and programming, a queue is a basic abstract data type (ADT). It represents an elemental collection where the primary operations are to add elements to the end of the collection (enqueue) and remove elements from the front of the collection (dequeue). It follows the First-In-First-Out (FIFO) principle, meaning that the element inserted first is the one to be removed first.

Queue Operations:

There are several queue operations in C++. Some main operations are as follows:

1. Enqueue (Add): Enqueuing is the process of adding an element to the rear (end) of the queue. The element is inserted at the rear, and if the queue is not full, the rear pointer is increased to accommodate the new element.
2. Dequeue (Remove): Using the dequeue (remove) operation, an element is removed from the front of the queue. The element at the front of the queue is removed, and if the queue is not empty, the front pointer is raised to point to the next element.
3. Peek/Front: Using the peek/front method, the element at the front of the queue is returned without being removed. In a case that the queue is empty, the "peek" operation might return an unusual result (like null or throw an exception), signifying that there are no elements to peek.
4. IsEmpty: Use the IsEmpty operation to determine whether the queue is empty. It returns true if there are no elements in the queue and false otherwise.

Advantages of Queue:

There are several advantages of the queue in C++. Some main advantages are as follows:

1. FIFO (First-In-First-Out) Order: According to the strict FIFO order of queues, the first item added to a queue will be the first one removed. This feature is useful in task scheduling and networking request processing.
2. Simple and Efficient: When implemented using linked lists or dynamic arrays, queue operations like enqueue and dequeue have a common time complexity of O(1). Due to their simplicity, queues are effective for a wide range of applications.
3. Concurrency Control: Queues are typically used in distributed or multithreaded systems to enable communication between threads or processes. They provide coordinated information transmission between different system components.
4. Resource Management: Queues are useful for regulating bandwidth in networking and for managing CPU scheduling in operating systems. They can also be helpful in other circumstances when control over resource access is required.
5. Buffering: Queues can serve as temporary data storage buffers in situations when the rate of production and consumption are different. It happens frequently in circumstances such as data processing, when data must be processed at a certain rate.

Disadvantages of Queue:

There are several disadvantages of the queue in C++. Some main disadvantages are as follows:

1. Limited Access: Unlike arrays or lists, queues provide elements with limited access. It is inappropriate in scenarios where elements must be available at random since we can only access those at the front and rear of the queue.
2. Fixed Capacity Overhead: Overhead may occur in systems with fixed-size capacities when a queue full and more enqueue operations are declined. While it may be mitigated, dynamic queue resizing introduces complexity and can reduce performance.
3. Potential for Blocking: A thread may get blocked until another thread completes the appropriate operation if it tries to enqueue to a full queue or dequeue from an empty queue. This blocking behaviour may introduce a delay in some systems.
4. Memory Overhead: Implementations of linked lists can demand more memory for each element than arrays because they require additional memory to store the pointer to the next element.
5. Not Suitable for All Data Structures: Queues are excellent for FIFO data management. However, they might not be the best choice in situations when prioritized data processing or other access patterns are needed. In such cases, other data structures like stacks or priority queues may be more appropriate.

What is the Array-Based Queue?

A basic data structure in computer science, array-based queues are used to store elements in sequential order according to the First-In-First-Out (FIFO) principle.

An array-based queue is a type of linear data structure where elements are inserted at the rear (end) and removed from the front (beginning) of the queue using a fixed-size array.

The data elements of an array-based queue are usually stored in a fixed-size array. Indexes track both the front and the rear of the queue.

When they are queued, the rear pointer is incremented, and elements are inserted at the index indicated by the rear pointer.

The element at the index specified by the front pointer is removed, and the front pointer is increased when elements are dequeued.

The front and rear pointers may wrap around the array if necessary to utilize the entire array efficiently.

Advantages of Array-Based Queue:

There are several advantages of the Array-Based Queue in C++. Some main advantages are as follows:

1. Simple Implementation: When compared to linked list-based implementations, array-based queues are comparatively easy to implement. They require only an array and two indices to keep track of the front and rear of the queue.
2. Random Access: Unlike linked list-based systems, elements in array-based queues can be retrieved randomly. It can be helpful when iterating over the queue or in other scenarios where direct access to elements is required.
3. Efficient Memory Usage: When compared to linked list-based systems, array-based queues usually use less memory. Their memory overhead per element decreases because they do not have to deal with the overhead of maintaining pointers for linked nodes.

Disadvantages of Array-Based Queue:

There are several disadvantages of the Array-Based Queue in C++. Some main disadvantages are as follows:

1. Fixed Size: Array-based queues have a limited size, which limits the number of elements they can store. If the queue fills up, subsequent enqueue operations will fail until the queue is dynamically extended, which increases overhead and complexity.
2. Memory Wastage: Memory waste can occur from using an extremely large array size, especially if the queue contains few elements. However, if the array size is set too small, the queue will fill up too quickly, and new elements could be rejected.
3. Costly Resize Operations: In terms of time complexity, dynamically expanding an array to accommodate new elements can be an expensive operation. In most cases, resizing involves allocating the old array, copying the existing elements, and allocating the new array.

What is the List-Based Queue?

A queue data structure that uses an array or list as its implementation is called a "List-Based Queue". Queues are a fundamental concept in computer science because they operate on the First-In-First-Out (FIFO) principle. In a list-based queue, elements are inserted at the rear (enqueue) and removed from the front (dequeue).

Implementation

• An array or linked list can be used to implement a list-based queue.
• When using an array, dynamic resizing may be necessary to effectively handle different numbers of elements.

Complexity Analysis:

Time Complexity

• In a list-based queue, the enqueue and dequeue operations usually have an O(1) time complexity.
• In the worst case, dequeuing in a linked list implementation could need traversing the list, which would result in O(n) time complexity.

Space Complexity

• A list-based queue has an O(n) space complexity, whereby n is the queue's total number of elements.
• In a linked list implementation, more space can be needed to keep pointers or metadata.

Advantages of List-Based Queue

There are several advantages of the List-Based Queue in C++. Some main advantages are as follows:

1. Dynamic Size: In contrast to array-based queues, List-based queues can dynamically change in size to support an infinite number of entries. As a result, they become more adaptive, making them appropriate for applications in which the queue's size changes over time.
2. Efficient Enqueue and Dequeue: Implementing list-based queues using doubly linked lists usually results in efficient enqueue and dequeue operations. With the time complexity of O(1), operations like adding or removing elements from the front and back of a linked list are fast.

Disadvantages of List-Based Queue

There are several disadvantages of the List-Based Queue in C++. Some main disadvantages are as follows:

1. Less Predictable Performance: Compared to array-based queues, the efficiency of enqueue and dequeue operations for list-based queues might be compromised. Accessing non-contiguous memory locations during a linked list traverse may result in cache misses and occasionally worse performance.
2. No Random Access: List-based queues do not allow for random access to elements, unlike arrays. It can be inefficient for large queues to traverse the list from the front or rear in order to access elements at any position in the queue.

Difference between Array-Based Queue and List-Based Queue:

There are several differences between Array-based Queue and List-based Queue. Some main differences between these queues are as follows: