The std::vector lives in the <vector> header. So first, we need to add this line of code at the top of the program:

#include <vector>

For review, #include is a preprocessor directive that tells the compiler to include whatever library that follows. In our case that is the standard vector library.

And the syntax to create a vector looks like:

std::vector<type> name;

So to define a vector of ints called calories_today:

std::vector<int> calories_today;

Inside the angle brackets is the data type of the vector. After the angle brackets is the name of the vector.

Note: The type of the vector (i.e., what data type is stored inside) cannot be changed after the declaration.