#pragma once
#include <iostream>
#include <stdio.h>
#include <vector>
#include <numeric> // for std::accumulate()
#include <cmath>   // for pow()

double coefficent_of_correlation(const std::vector<size_t>& values_x, const std::vector<size_t>& values_y)
{
    size_t mid_x = std::accumulate(values_x.begin(), values_x.end(), 0) / values_x.size();
    size_t mid_y = std::accumulate(values_y.begin(), values_y.end(), 0) / values_y.size();

    std::vector<long long int> xi_minus_mid_x;
    for (auto el : values_x) xi_minus_mid_x.push_back(el - mid_x);

    std::vector<long long int> xi_minus_mid_y;
    for (auto el : values_y) xi_minus_mid_y.push_back(el - mid_y);

    int long long upper_part = 0;
    for (size_t i = 0; i < xi_minus_mid_x.size(); i++) upper_part += xi_minus_mid_x.at(i) * xi_minus_mid_y.at(i);

    unsigned long long x_sum = 0;
    for (auto el : xi_minus_mid_x) x_sum += pow(el, 2);

    unsigned long long y_sum = 0;
    for (auto el : xi_minus_mid_y) y_sum += pow(el, 2);

    int long long lower_part = pow(x_sum * y_sum, 0.5);

    return (double)upper_part / lower_part;
}


int main()
{
    std::vector<size_t> x = {1, 2, 3, 4};
    std::vector<size_t> y = {3, 4, 1, 2};

    std::cout << coefficent_of_correlation(x, y);
    return 0;
}