Vectors

A vector is a dynamic list of items that can shrink and grow in size. It can only store values of the same data type.

#include <vector>

std::vector<type> name;

To use vectors, it is necessary to #include the vector library.

The data type of its elements must be specified when the vector is created. Afterwards, the type cannot be changed.

Create a Vector with Size

std::vector<int> grades(10);

Create and Initialize a Vector

std::vector<double> order = {3.99, 12.99, 2.49};

An index refers to an element’s position within an ordered list, like a vector or an array. The first element has an index of 0.

A specific element in a vector or an array can be accessed via index, using a name[index] syntax:

std::vector<double> order = {3.99, 12.99, 2.49};

// What's the first element?
std::cout << order[0];

// What's the last element?
std::cout << order[2];

.at()

The .at() function provides a safer way of accessing elements in a vector. It performs bounds checking on the vector and will throw an error if there is an attempt to access an element that is out of bounds:

// First element
std::cout << order.at(0);

// Last element
std::cout << order.at(2);

// Out of bounds
std::cout << order.at(100);

The code above will print the following error:

terminate called after throwing an instance of 'std::out_of_range'
what():  vector::_M_range_check: __n (which is 100) >= this->size() (which is 3)
Aborted (core dumped)

There are two different ways to insert elements into the vector.

.push_back()

This method pushes elements to the back of a vector:

std::vector<int> v = {1,2,3,4};

v.push_back(5);

int n = v.size();

std::cout << "The last element is: " << v[n - 1];
// Output: The last element is: 5

.insert()

This method inserts new elements before the element at the specified position.

std::vector<int> v = {1,2,3,4};

// Insert at beginning
v.insert(v.begin(), 0);

// Insert at end
v.insert(v.end(),6);

std::cout << "The first element is: " << v[0] << "\n";
// Output: The first element is: 0

std::cout << "The last element is: " << v[5] << "\n";
// Output: The last element is: 6

To create a vector named grade with three items and then insert elements to the last of the vector using .push_back() and .insert() functions:

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