C++ Vectors

C++ Vector

A vector in C++ is like a resizable array.

Both vectors and arrays are data structures used to store multiple elements of the same data type.

The difference between an array and a vector, is that the size of an array cannot be modified (you cannot add or remove elements from an array). A vector however, can grow or shrink in size as needed.

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

// Include the vector library
#include <vector>

Create a Vector

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

If you want to add elements at the time of declaration, place them in a comma-separated list, inside curly braces {}, just like with arrays:

// Create a vector called cars that will store strings
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Print vector elements
for (string car : cars) {
  cout << car << "\n";
}

Access a Vector

You can access a vector element by referring to the index number inside square brackets [].

Vectors, like arrays, are 0-indexed, meaning that [0] is the first element, [1] is the second element, and so on:

// Create a vector called cars that will store strings
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Get the first element
cout << cars[0];  // Outputs Volvo

// Get the second element
cout << cars[1];  // Outputs BMW

One advantage of using the vector library, is that it includes many useful functions. For example, you can access the first or the last element of a vector with the .front() and .back() functions:

// Create a vector called cars that will store strings
vector<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:

// Create a vector called cars that will store strings
vector<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 lets you know if an error occurs.

For example if the element is out of range:

// Create a vector called cars that will store strings
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Try to access an element that does not exist (throws an error message)
cout << cars.at(6);

Change a Vector Element

To change the value of a specific element, you can refer to the index number:

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

// Change the value of the first element
cars[0] = "Opel";

cout << cars[0];  // Now outputs Opel instead of Volvo

However, it is safer to use the .at() function:

vector<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 Volvo

Add Vector Elements

The biggest difference between a vector and an array is that vectors can grow dynamically. That means you can add or remove elements from the vector.

To add an element to the vector, you can use the .push_back() function, which will add an element at the end of the vector:

vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars.push_back("Tesla");

You can add as many elements as you want:

vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars.push_back("Tesla");
cars.push_back("VW");
cars.push_back("Mitsubishi");
cars.push_back("Mini");

Remove Vector Elements

To remove an element from the vector, you can use the .pop_back() function, which removes an element from the end of the vector:

vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars.pop_back();

Note: Elements are usually only added and removed from the end of the vector. If you need to add or remove elements from both ends, it is often better to use a deque instead of a vector.

Vector Size

To find out how many elements a vector has, use the .size() function:

vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.size();  // Outputs 4

Check if a Vector is Empty

There is also a function to find out whether a vector is empty or not.

The .empty() function returns
1 (true) if the vector is empty and 0 (false) if it contains one or more elements:

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

vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.empty();  // Outputs 0 (not empty)

Loop Through a Vector

You can loop through the vector elements by using a for loop combined with the .size() function:

vector<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:

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

for (string car : cars) {
  cout << car << "\n";
}

Tip: It is also possible to loop through vectors with an iterator, which you will learn more about in a later article.