Sliding Windows

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'

int k = 5;
vector<double> res(9 - k + 1);

void sol(vector<int> &v)
{
    int winst = 0, winen = 0; /* winst == window start, winen == windowend */
    double winsum = 0;      /* sum of window of size k */
    for (int winen = 0; winen < v.size(); winen++)
    {
        winsum += v.at(winen);
        if (winen >= k - 1)
        {
            res.at(winst) = winsum / k;
            winsum -= v.at(winst);
            ++winst;
        }
    }

    for (auto el : res)
    {
        cout << el << " ";
    }
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    vector<int> v = {1, 3, 2, 6, -1, 4, 1, 8, 2};
    sol(v);

    return 0;
}