一：功能

        求内积， 给定两个向量x，y，内积公式如下：
                  

二：用法

#include <iostream>
#include <vector>
#include <numeric>

int main() {
    std::vector<int> heights{1, 2, 3, 4, 5};
    std::vector<int> widths{2, 3, 4, 5, 6};

    auto total_area = std::inner_product(heights.begin(), heights.end(),
        widths.begin(), 0);
    std::cout << "total_area == " << total_area << "\n";

    total_area = 0;
    for(int i = 0; i < heights.size(); i++)
    {
       total_area += heights[i] * widths[i];
    }
    std::cout << "total_area == " << total_area << "\n";
}