// Vectors are sequence containers representing arrays that can change in size.

// Library to include 
#include <vector> 

vector<int> a;                                  // Declare an empty vector, a.
a.push_back(1);                                 // Appends/Adds an element whose value is 1 to vector a. 
std::cout << a.at(0) << std::endl;              // Accessing index 0 of vector a. 
a.size();                                       // Returns the size of the vector
a.at(0) = 420;                                  // Changing the value at index 0 to 420.