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.
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";
}
Try it Yourself »
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 BMW
Try it Yourself »
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:
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();
Try it Yourself »
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);
Try it Yourself »
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);
Try it Yourself »
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 Volvo
Try it Yourself »
However, 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 Volvo
Try it Yourself »
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:
Example
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars.push_back("Tesla");
Try it Yourself »
You can add as many elements as you want:
Example
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars.push_back("Tesla");
cars.push_back("VW");
cars.push_back("Mitsubishi");
cars.push_back("Mini");
Try it Yourself »
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:
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 4
Try it Yourself »
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:
Example
vector<string> cars;
cout << cars.empty();
// Outputs 1 (The vector is empty)
Try it Yourself »
Example
vector<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.empty();
// Outputs 0 (not empty)
Try it Yourself »
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";
}
Try it Yourself »
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";
}
Try it Yourself »
Tip: It is also possible to loop through vectors with an iterator, which you will learn more about in a later chapter.
