Flash cards
Review the key moves
What is the main idea behind C++ Vectors?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
// ___ the vector library #include <vector>Put the learning moves in the order that makes the concept easiest to apply.
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 .
Example
// Create a vector called cars that will store strings vector<string> cars;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:
Example
// 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";
}Note
The type of the vector ( string in our example) cannot be changed after its been declared.
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:
Example
// 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 BMWOne 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:
Example
// 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:
Example
// 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:
Example
// 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:
Example
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 VolvoHowever, it is safer to use the .at() function:
Example
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 VolvoAdd 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:
Example
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.size();
// Outputs 4Check 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:
Example
vector<string> cars;
cout << cars.empty();
// Outputs 1 (The vector is empty)Example
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:
Example
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:
Example
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 chapter.