Native lesson simulator
Queue operations
Enqueue at the back, dequeue from the front.
A queue is FIFO: the front item leaves first.
A deque (stands for d ouble- e nded queue ) however, is more flexible, as elements can be added and removed from both ends (at the front and the back). You can also access elements by index numbers.
// Include the deque library #include <deque>Create a Deque
To create a deque, use the deque keyword, and specify the type of values it should store within angle brackets <> and then the name of the deque, like: deque< type > dequeName .
Example
// Create a deque called cars that will store strings
deque<string> cars;If you want to add elements at the time of declaration, place them in a comma-separated list, inside curly braces {} :
Example
// Create a deque called cars that will store strings deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"}; // Print deque elements
for (string car : cars) {
cout << car << "\n";
}Note
The type of the deque ( string in our example) cannot be changed after its been declared.
Access a Deque
You can access a deque element by referring to the index number inside square brackets [] .
Deques are 0-indexed, meaning that [0] is the first element, [1] is the second element, and so on:
Example
// Create a deque called cars that will store strings deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"}; // Get the first element cout << cars[0]; // Outputs Volvo // Get the second element cout << cars[1]; // Outputs BMWYou can also access the first or the last element of a deque with the .front() and .back() functions:
Example
// Create a deque called cars that will store strings deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"}; // Get the first element cout << cars.front(); // Get the last element cout << cars.back();To access an element at a specified index, you can use the .at() function and specify the index number:
Example
// Create a deque called cars that will store strings deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"}; // Get the second element cout << cars.at(1); // Get the third element cout << cars.at(2);Note
The .at() function is often preferred over square brackets [] because it throws an error message if the element is out of range:
Example
// Create a deque called cars that will store strings deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"}; // Try to access an element that does not exist (will throw an exception) cout << cars.at(6);Change a Deque Element
To change the value of a specific element, you can refer to the index number:
Example
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
// Change the value of the first element cars[0] = "Opel"; cout << cars[0]; // Now outputs Opel instead of VolvoHowever, it is safer to use the .at() function:
Example
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
// Change the value of the first element cars.at(0) = "Opel"; cout << cars.at(0); // Now outputs Opel instead of VolvoAdd Deque Elements
To add elements to a deque, you can use .push_front() to insert an element at the beginning of the deque and .push_back() to add an element at the end:
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
// Add an element at the beginning cars.push_front("Tesla"); // Add an element at the end cars.push_back("VW");Remove Deque Elements
To remove elements from a deque, use .pop_front() to remove an element from the beginning of the deque and .pop_back() to remove an element at the end:
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
// Remove the first element cars.pop_front(); // Remove the last element cars.pop_back();Deque Size
To find out how many elements a deque has, use the .size() function:
Example
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.size();
// Outputs 4Check if a Deque is Empty
Use the .empty() function to find out if a deque is empty or not.
The .empty() function returns 1 ( true ) if the deque is empty and 0 ( false ) otherwise:
Example
deque<string> cars;
cout << cars.empty();
// Outputs 1 (The deque is empty)Example
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.empty();
// Outputs 0 (not empty)Loop Through a Deque
You can loop through the deque elements by using a for loop combined with the .size() function:
Example
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (int i = 0; i < cars.size(); i++) {
cout << cars[i] << "\n";
}You can also use a for-each loop (introduced in C++ version 11 (2011), which is cleaner and more readable:
Example
deque<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (string car : cars) {
cout << car << "\n";
}Tip
It is also possible to loop through deques with an iterator , which you will learn more about in a later chapter.