It should be in the arguments of the function.

when you click on TRAIN you get the kata trainer, where the declaration for the function odd_or_even is provided:

#include <string>
#include <vector>

std::string odd_or_even(const std::vector<int> &arr)
{
    return ""; // your code here
}

...and you write your code between the curly braces in place of the line that says, "your code here"