A vector is a list of objects.

They are ordered: 1, 2, 3 is different to 1, 3, 2
The objects themselves are the elements of the vector
- We say that a vector is a “sequence of elements”
The positions of the elements in the vector are their indices/indexes. The first element is at index 0 (not index 1!), the second element is at index 1 (not index 2!) and so on. We say that vectors are ‘zero-indexed’ for this reason: the positions of the elements start at 0, not 1.
The vector header needs to be included in your source code in order to use vectors

1 Declaring and Initialising

The syntax to declare a vector called name where the elements of name are all of the data type type is std::vector<type> name;:

// The vector header needs to be included in your source code in order to use vectors
#include <vector>

int main() {
    // Declare a vector of floating-point numbers
    std::vector<double> numbers;
}

The data type of a vector cannot be changed once you have declared it.

To declare and intialise at the same time, assign values to the variable using curly brackets:

#include <vector>
#include <string>

int main() {
    // Declare and initialise four vectors of different types
    std::vector<int> squares = {0, 1, 4, 9, 16};
    std::vector<double> coordinates = {51.5002149, -0.1249473};
    std::vector<char> letters = {'a', 'b', 'c', 'd'};  // Single quotation marks
    std::vector<std::string> beatles = {"John", "Paul", "George", "Ringo"};  // Double quotation marks
}

If you don’t know what elements will be going into your vector but you do know how many elements there will be, you can provide a size in round brackets when you declare it. This is known as presizing:

#include <vector>

int main() {
    // Declaring and presizing
    std::vector<int> variable(2);
}

2 Info about a Vector

2.1 Length

Get the number of elements in a vector using size():

#include <vector>
#include <string>
#include <iostream>

int main() {
    // Declare and initialise
    std::vector<std::string> beatles = {"John", "Paul", "George", "Ringo"};
    // Get the length
    std::cout << beatles.size() << "\n";
}

2.2 Indexing

Get the element that exists at a particular index by using square brackets: variable[2] will return the element in the third position (2nd index):

#include <vector>
#include <iostream>

int main() {
    // Declaring and initialising
    std::vector<int> squares = {0, 1, 4, 9, 16};
    std::vector<double> coordinates = {51.5002149, -0.1249473};
    std::vector<char> letters = {'a', 'b', 'c', 'd'};
    std::vector<std::string> beatles = {"John", "Paul", "George", "Ringo"};
    // Indexing
    std::cout << squares[0] << "\n";
    std::cout << coordinates[1] << "\n";
    std::cout << letters[2] << "\n";
    std::cout << beatles[3] << "\n";
}

0
-0.124947
c
Ringo

2.3 Iterating (and Displaying)

Loop through a vector (‘iterate over it’) by using a for loop with the size of the vector as your terminal condition. Then, you can use the counter to index it. By iterating over a vector this way you can, as an example, display each element in turn:

#include <vector>
#include <iostream>
#include <string>

int main() {
    // Declare and initialise
    std::vector<std::string> beatles = {"John", "Paul", "George", "Ringo"};
    // Iterate
    for (int i = 0; i < beatles.size(); i++) {
        // Display
        std::cout << beatles[i] << " ";
    }
    std::cout << "\n";
}

John Paul George Ringo

3 Editing a Vector

3.1 Updating Values

You can edit each element in turn by iterating with a for loop and using the counter to both get and update the element at that index:

#include <vector>
#include <iostream>

int main() {
    // Declare and initialise
    std::vector<int> squares = {0, 1, 4, 9, 16};
    // Iterate over vector
    for (int i = 0; i < squares.size(); i++) {
        // Update the element by adding 1 to it
        squares[i] = squares[i] + 1;
    }
    // Display
    for (int i=0; i<squares.size(); i++) {
        std::cout << squares[i] << " ";
    }
    std::cout << "\n";
}

1 2 5 10 17

3.2 Adding Elements

Use .push_back() to push a new element to the back of the vector (aka append to the vector):

#include <vector>
#include <iostream>
#include <string>

int main() {
    // Declare and initialise
    std::vector<std::string> beatles = {"John", "Paul", "George", "Ringo"};
    // Append
    beatles.push_back("Pete");
    // Display
    for(int i=0; i<beatles.size(); i++) {
        std::cout << beatles[i] << " ";
    }
    std::cout << "\n";
}

John Paul George Ringo Pete

3.3 Removing Elements

Use .pop_back() to pop an element out from the back of the vector:

#include <vector>
#include <iostream>

int main() {
    // Declare and initialise
    std::vector<char> letters = {'a', 'b', 'c', 'd'};
    // Remove
    letters.pop_back();  // No argument inside the round brackets
    // Display
    for (int i=0; i<letters.size(); i++) {
        std::cout << letters[i] << " ";
    }
    std::cout << "\n";
}

a b c

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Introduction to C++:Vectors