Partial Sum

 

#include <iostream>
#include <vector>
using namespace std;


int partial_sum(vector<int>& sum, int x, int y) {
    return sum[y] - sum[x - 1];
}

void sum_array(vector<int>& v, vector<int>& sum) {
    sum.push_back(v[0]);
    for (auto i = 1; i < v.size(); i++) {
        sum.push_back(sum[i - 1] + v[i]);
    }

}

int main()
{
    vector<int> v = { 7, 3, 5, 6, 7, 9, 3, 5, 11 };
    vector <int> sum;
    sum_array(v, sum);
//    for (auto i = sum.begin(); i != sum.end(); i++)
//        cout << *i << " ";

    cout << partial_sum(sum, 3, 7);
    return 0;
}