Difference Between Queue And Deque In C++

In this guide, we will explore the variance between Queue and Deque in C++. Prior to delving into their distinctions, it is essential to understand the concepts of Queue and Deque.

Introduction of queue

A queue represents a fundamental data structure in C++ that follows the First-In-First-Out (FIFO) principle. Items are inserted at the back and removed from the front based on the "first come, first served" guideline. Due to its orderly organization, queues are beneficial in scenarios such as job scheduling, breadth-first search algorithms, and request handling in computer networks where tasks or data need to be processed in the sequence they arrive.

There exist several methodologies for constructing a queue in C++, with the primary ones being leveraging standard libraries or crafting a custom implementation. An adaptable container from the Standard Template Library (STL) called std::queue is commonly employed in C++ to streamline the queue's functionality. By incorporating this ready-made option and including the appropriate header, programmers can efficiently incorporate queues into their projects, thereby reducing development time and complexity.

Dequeue and enqueue are the fundamental actions that constitute a queue's functionality. Dequeue removes an item from the front of the queue, while enqueue appends an item to the rear. By maintaining the sequential order of elements, these processes guarantee that the earliest item in the queue will be the first one dequeued. Along with enqueue and dequeue, other common operations include peeking (viewing the front item without dequeuing) and checking if the queue is devoid of elements.

Developers often use data structures such as arrays or linked lists to store elements when implementing custom queues in C++. Arrays provide O(1) access to elements, but resizing may be required if the capacity is exceeded. On the contrary, linked lists have minimal overhead and support dynamic memory allocation, resulting in efficient insertion and deletion operations. The choice between these data structures depends on factors like the frequency of enqueue and dequeue operations and the expected size of the queue.

Program of Queue in C++:

Let's consider an example to demonstrate the Queue data structure in C++.

#include <iostream>
#include <queue>

int main() {
    // Creating a queue of integers
    std::queue<int> myQueue;

    // Enqueing elements into the queue
    myQueue.push(10);
    myQueue.push(20);
    myQueue.push(30);

    // Displaying the front element of the queue
    std::cout << "Front element of the queue: " << myQueue.front() << std::endl;

    // Dequeing elements from the queue and displaying them
    std::cout << "Dequeuing elements from the queue: ";
    while (!myQueue.empty()) {
        std::cout << myQueue.front() << " ";
        myQueue.pop();
    }
    std::cout << std::endl;

    // Checking if the queue is empty
    if (myQueue.empty()) {
        std::cout << "Queue is empty." << std::endl;
    } else {
        std::cout << "Queue is not empty." << std::endl;
    }

    return 0;
}

Output:

Front element of the queue: 10
Dequeuing elements from the queue: 10 20 30 
Queue is empty.

Explanation:

• ✅ The program starts by adding the required header files: #include<queue> to use the C++ Standard Template Library's (STL) data structure for the queue, and #include<iostream> for input and output operations.
• ✅ The std::queue syntax is used to declare a queue of integers called myQueue inside the main method. The entries that comprise this queue will be represented by integers.
• ✅ Next, the push function is used to enque three integers: 10, 20, and 30 into the myQueue. The push action complies with the First-In-First-Out (FIFO) concept by adding entries to the rear section of the queue.
• ✅ The application uses myQueue.front to display the front element of the queue to show how to access it with no dequeuing it. The reference to the queue's front element is returned through the aforementioned method.
• ✅ After that, items are dequed from the queue and shown using a while loop. The loop is guaranteed to run as long as the queue is not empty through the use of myQueue.empty. MyQueue.front fetches the queue's front element during the loop, and std::cout is used to show it. The front element is subsequently eliminated from the queue through the implementation of myQueue.pop.
• ✅ Using the empty function, the computer determines if the queue is empty after dequeuing every entry from it. It outcomes "Queue is empty" if there is no one waiting in the queue. If not, "Queue is not empty" is printed.

Introduction of Deque in C++

Double-ended queues, also known as deques in C++, serve as versatile data structures that offer efficient insertion and removal functions at both extremities. Unlike traditional queues, deques allow elements to be added and removed from either the front or the back, making them well-suited for a wide range of tasks that involve the real-time manipulation of data. These operations are executed with a constant time complexity in deques, ensuring swift retrieval of elements irrespective of their position within the data structure.

The Standard Template Library (STL) simplifies the process of creating a deque in C++ by providing a predefined container named std::deque. Programmers can make use of this container along with its array of benefits in their software by including the appropriate header. The std::deque container offers a user-friendly interface for handling deques in C++, effectively managing memory intricacies and resizing tasks.

A deque consists of a resizable array composed of fixed-size blocks that can each hold multiple elements. This structure allows for flexible scaling when items are inserted or deleted, optimizing memory usage. Unlike vectors that may reallocate their entire storage, deques resize by managing data in smaller chunks, minimizing the overhead of resizing operations.

The primary benefits of a deque over other sequential containers, like vectors, include its efficient insertion and deletion operations.

Deco retains a consistent time complexity for these actions on both extremes, while vectors provide constant-time retrieval of elements but linear-time insertion or removal at the start of the process. Deques prove particularly valuable in scenarios such as managing sliding windows in algorithms or constructing a double-ended queue where items must be added or removed efficiently from both ends.

When utilizing deques in C++, developers have the flexibility to perform various operations like inserting and deleting elements, accessing elements at specific positions, and traversing through the deque thanks to the numerous member functions and iterators at their disposal. Furthermore, deques support random access, enabling efficient retrieval of elements using index-based notation. By leveraging these capabilities, programmers can design efficient and customizable data structures tailored to their specific requirements.

Program of Deque in C++

Let's consider a scenario to demonstrate the Deque data structure in C++.

#include <iostream>
#include <deque>

int main() {
    // Creating a deque of integers
    std::deque<int> myDeque;

    // Adding elements to the back of the deque
    myDeque.push_back(10);
    myDeque.push_back(20);
    myDeque.push_back(30);

    // Adding elements to the front of the deque
    myDeque.push_front(5);
    myDeque.push_front(0);

    // Displaying the elements of the deque
    std::cout << "Elements of the deque: ";
    for (int num : myDeque) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    // Removing elements from the front and back of the deque
    myDeque.pop_front();
    myDeque.pop_back();

    // Displaying the modified deque
    std::cout << "Modified deque after pop operations: ";
    for (int num : myDeque) {
        std::cout << num << " ";
    }
    std::cout << std::endl;

    // Accessing elements at specific positions in the deque
    std::cout << "Element at position 2: " << myDeque.at(2) << std::endl;

    return 0;
}

Output:

Elements of the deque: 0 5 10 20 30 
Modified deque after pop operations: 5 10 20 
Element at position 2: 20

Explanation:

• ✅ In this example, the std::deque syntax is used to declare a deque containing integers called myDeque inside the main method. The elements of this deque will be integers.
• ✅ After that, using the pushfront and pushback functions, elements are added to the deque's front and rear, respectively. In this instance, in deque is added to in the following order: 0, 5, 10, 20, and 30.
• ✅ Next, a for loop is used to iterate over each element of the deque and output it using std::cout to demonstrate the contents comprising the deque. Next, items are eliminated from the deque's front and rear by using the popfront and popback functions. As a result, the elements 0 and 30 are eliminated within the deque.
• ✅ Using a for loop akin to the last one, the changed deque following the pop operations is shown once more. Elements 5, 10, and 20 are currently present in the deque. Lastly, the element at index 2 may be retrieved by using the at function, which returns elements in the deque structure at the given index. Following that, using std::cout, the value of the element at index 2, which is 20, is printed.

Main difference between Queue and Deque in C++

There are several key distinctions between the queue and deque in C++. Some primary variances are outlined below:

Conclusion:

In essence, although both Queue and Deque data structures are utilized in C++ for handling collections of elements, they vary in ways that cater to diverse programming needs. The concept of First-In-First-Out (FIFO) is strictly adhered to by queues, simplifying the process of adding elements from the back and removing them from the front. Conversely, Deque offers a dual-ended approach to data manipulation, providing greater flexibility with the ability to insert and delete elements at both ends. Deques are preferred for scenarios that demand efficient manipulation at either end, like implementing sliding windows or versatile dynamic arrays, whereas Queues are typically used in situations where maintaining a strict order is crucial, such as task scheduling or breadth-first search algorithms.

Moreover, these two data structures contrast in terms of how they are represented in the standard library, the operations they support, efficiency factors, the presence of iterators, and the scenarios in which they are typically utilized. This allows developers the freedom to choose the most suitable option based on the specific requirements of their applications.