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Difference between Queue and Deque in C++

  • Difficulty Level : Easy
  • Last Updated : 14 Dec, 2020

Queue: A Queue is a linear data structure that follows a First In First Out (FIFO) order in which the operations are performed. It is a type of container adaptor where elements are inserted into one end of the container and deleted from the other.

Functions:

  • empty(): Tests whether the queue is empty.
  • size(): Returns the unsigned int, size of the queue.
  • queue::front() and queue::back(): front() function returns a reference to the first element or the oldest of the queue. back() function returns a reference to the last or the newest element of the queue.
  • push(k) and pop(): push() function adds the element ‘k’ at the end of the queue. pop() function deletes the element from the beginning of the queue and reduces its size by 1.
  • swap(): exchanges the elements of two different queues of the same type but may or may not of the same size.
  • emplace(): it is used to insert a new element at end of the queue.

Syntax:

queue <data_type> q

Below is the program to illustrate the same:



C++




// C++ program to demonstrate the
// working of queue
#include <bits/stdc++.h>
using namespace std;
  
// Driver Code
int main()
{
    // Declare a queue
    queue<int> q;
  
    // Insert elements in the queue
    q.push(10);
    q.push(5);
    q.push(15);
    q.push(1);
  
    // Delete elements from the queue
    q.pop();
    q.pop();
  
    cout << "Elements in Queue are: ";
  
    // Print the element stored
    // in queue
    while (!q.empty()) {
        cout << q.front() << ' ';
  
        // Pop the front element
        q.pop();
    }
  
    return 0;
}
Output:
Elements in Queue are: 15 1

Deque: Deque is a sequence container with the ability of expansion and contraction on both ends. It is a template of Standard Template Library or STL in C++is. It is similar to vectors but are more efficient for the insertion and deletion of elements. Contiguous storage allocation in deque may not be guaranteed as in vectors.

Functions:

  • max_size(): Returns the maximum number of elements deque can contain.
  • push_back() and push_front(): push_front( ) push the elements into a deque from the front and push_back( ) push elements into a deque from the back.
  • pop_front() and pop_back(): pop_front() function is used to pop elements from a deque from the front and pop_back( ) function is used to pop elements from a deque from the back.
  • clear() and erase():  clear is used to remove all the elements from the deque and erase is used to remove some specified elements.
  • insert(): increases the container side by inserting element in the specified position.
  • resize(): changes the size of the element’s container as per requirement.
  • rbegin() and rend(): rbegin() points to the last element of the deque whereas rend points to the position before the beginning of the deque.
  • at() and swap(): at() points to the position of the element given in the parameter and swap( ) is used two swap elements of two deques.
  • emplace_front() and emplace_back(): these two functions are used to insert new elements in the container at the beginning and at the end of deque respectively.

Syntax:

deque<data_type> dq

Below is the program to illustrate the same:

C++




// C++ program to demonstrate the
// working of deque
#include <bits/stdc++.h>
using namespace std;
  
// Driver Code
int main()
{
    // Declare a deque
    deque<int> dq;
  
    // Insert element in the front
    dq.push_front(10);
    dq.push_front(5);
    dq.push_front(3);
  
    // Delete elements from the the front
    dq.pop_front();
    dq.pop_front();
  
    // Insert elements in the back
    dq.push_back(1);
    dq.push_back(50);
    dq.push_back(2);
  
    // Delete elements from the the back
    dq.pop_back();
    dq.pop_back();
  
    cout << "Elements in deque are: ";
  
    // Print the element stored
    // in deque
    while (!dq.empty()) {
        cout << " " << dq.front();
        dq.pop_front();
    }
  
    return 0;
}
Output:
Elements in deque are:  10 1

Below is the tabular difference between the queue and deque:

S.No.

Queue

Deque

1Insertion can be done through the front end only.Insertion is possible through both ends.
2Deletion of elements is possible through the front end only.Deletion of elements possible through both ends.
3Elements can not be accessed through iterators.Elements can be accessed through iterators.
4Implemented as container adaptors.Implemented generally as some form of a dynamic array.
5The stack can not be implemented using a queue.A stack can be implemented using deque.

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