QUEUE IN C++ STL

In this article, you will learn about the C++ queue and its methods with some useful example programs which would help you understand the concept better.

QUEUE IN C++ STL

Queues are a type of container adaptors that operate in a first in first out (FIFO) type of arrangement. In a queue, the elements are inserted from the rear and removed from the front.

Creating a Queue

Before using the queue, we must include the <queue> header which has all the operations of the queue.

So now that we have included the <queue> header file we can create a queue using the following syntax.

Syntax:

queue<type> q;

Here the type indicated the type of data that we want to store in the queue which is similar to the stack syntax.

// create a queue of integer data type
queue<int> integer_queue;

// create a queue of string data type
queue<string> string_queue;

Queue Methods

Now let’s look at some queue Methods. The STL queue has several methods to perform different operations on a queue.

• push(i) – this method is used to insert the element I at the end of the queue.
• pop() – the pop() method is used to remove an element from the queue and the element is removed from the front of the queue.
• front()  – Returns the first element in the queue
• back() – Returns the last element or the latest element in the queue.
• size() – Returns the size i.e., the number of elements in the queue.
• empty() – This empty() method is similar to the stack empty() method it checks whether the queue is empty or not and returns true if the queue is empty.

Adding Elements to a Queue

We will use the push() method to insert an element to the rare end of the queue.

For example,

#include <iostream>
#include <queue>

using namespace std;

int main() {

  // create a queue of string
  queue<string> animals;


  // push elements into the queue
  animals.push("Cat");
  animals.push("Dog");

  
  cout << "Queue: ";

  // print elements of queue
  // loop until queue is empty
  while(!animals.empty()) {

    // print the element
    cout << animals.front() << ", ";

    // pop element from the queue
    animals.pop();
  }

  cout << endl;
 
  return 0;
}

Output

Queue: Cat, Dog,

In the above example, we have created a queue of strings called animals and added elements using the push() method.

animals.push("Cat");
animals.push("Dog");

To print the queue we use the while loop along with the front() and pop() methods.

Removing Elements from a Queue

We use the pop() method which removes the element from the front of the queue.

For example,

#include <iostream>
#include <queue>
using namespace std;

// function prototype for display_queue utility
void display_queue(queue<string> q);

int main() {

  // create a queue of string
  queue<string> animals;

  // push element into the queue
  animals.push("Cat");
  animals.push("Dog");
  animals.push("Fox");
  
  cout << "Initial Queue: ";
  display_queue(animals);
  
  // remove element from queue
  animals.pop();

  
  cout << "Final Queue: ";
  display_queue(animals);
  
  return 0;
}

// utility function to display queue
void display_queue(queue<string> q) {
  while(!q.empty()) {
    cout << q.front() << ", ";
    q.pop();
  }

  cout << endl;
}

Output

Initial Queue: Cat, Dog, Fox, 
Final Queue: Dog, Fox, 

Accessing the Elements from a Queue

We can access the elements of a queue by using either of the functions, front() or back()

front() – returns the element from the front of the queue

back() – returns the element from the back of the queue

For example,

#include <iostream>
#include <queue>
using namespace std;

int main() {

  // create a queue of int
  queue<int> nums;

  // push element into the queue
  nums.push(1);
  nums.push(2);
  nums.push(3);
  
  // get the element at the front
  int front = nums.front();

  cout << "First element: " << front << endl;
  
  // get the element at the back
  int back = nums.back();

  cout << "Last element: " << back << endl;
  
  return 0;
}

Output

First element: 1
Last element: 3

In the above example, 1 was inserted first so the front() method returns 1 and the last element that was inserted into the queue was 3 so the back() method returns 3.

That is all for this article. Let’s learn about the priority queue in the next one.

Happy Coding!