C++ Queues

C++ Queue

A queue stores multiple elements in a specific order, called FIFO.

FIFO stands for First in, First Out. To visualize FIFO, think of a queue as people standing in line in a supermarket. The first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO in computer science and programming.

Unlike vectors, elements in the queue are not accessed by index numbers. Since queue elements are added at the end and removed from the front, you can only access an element at the front or the back.

To use a queue, you have to include the <queue> header file:

  // Include the queue library
#include <queue>

Create a Queue

To create a queue, use the queue keyword, and specify the type of values it should store within angle brackets <> and then the name of the queue, like: queue<type> queueName.

// Create a
queue of strings called cars
queue<string> cars;

Note: The type of the queue (string in our example) cannot be changed after its been declared.

Note: You cannot add elements to the queue at the time of declaration, like you can with vectors:

  queue<string> cars = {"Volvo", "BMW",
"Ford", "Mazda"};

Add Elements

To add elements to the queue, you can use the .push() function after declaring the queue.

The .push() function adds an element at the end of the queue:

Example

  // Create a queue of strings
queue<string> cars;

// Add elements to the queue
cars.push("Volvo");
cars.push("BMW");
  cars.push("Ford");
cars.push("Mazda");

Access Queue Elements

You cannot access queue elements by referring to index numbers, like you would with arrays and vectors.

In a queue, you can only access the element at the front or the back, using .front() and .back() respectively:

Example

// Access the
front element (first and oldest)
cout << cars.front();  //
Outputs "Volvo"

// Access the
back element (last and newest)
cout << cars.back();  //
Outputs "Mazda"

Change Front and Back Elements

You can also use .front and .back to change the value of the front and back elements:

Example

// Change the value of the front element
cars.front() = "Tesla";

// Change the value of the back element
cars.back() = "VW";

//
Access the front element
cout << cars.front();  // Now outputs
"Tesla" instead of "Volvo"

// Access the back element
cout << cars.back();  // Now outputs "VW" instead
of "Mazda"

Remove Elements

You can use the .pop() function to remove an element from the queue.

This will remove the front element (the first and oldest element that was added to the queue):

Example

  // Create a queue of strings
queue<string> cars;

// Add elements to the queue
cars.push("Volvo");
cars.push("BMW");
  cars.push("Ford");
cars.push("Mazda");

// Remove the
  front
  element (Volvo)
cars.pop();

// Access the front
  element (Now BMW)
  cout << cars.front();

Get the Size of a Queue

To find out how many elements there are in a queue, use the .size() function:

Example

cout
<< cars.size();

Check if the Queue is Empty

Use the .empty() function to find out if the queue is empty or not.

The .empty() function returns 1 (true) if the queue is empty and 0 (false) otherwise:

Example

  queue<string> cars;
cout << cars.empty();
  // Outputs 1 (The queue is empty)

Example

queue<string> cars;

cars.push("Volvo");
cars.push("BMW");
cars.push("Ford");
cars.push("Mazda");

cout
<< cars.empty();
// Outputs 0 (not empty)

Stacks and Queues

Queues are often mentioned together with Stacks, which is a similar data structure described in the previous page.