#include <iostream>
#include <random>
#include <algorithm>

using namespace std;

int main()
{
    constexpr int sz = 20;
    int i[sz];

    {// fill with random data
      std::random_device rd;
      std::mt19937 mt(rd());
      std::uniform_int_distribution<int> dist(-9999, 9999);
      for(auto &x : i)
          x = dist(mt);
    }

    cout << "Random array is " << endl;
    //print random array
    for(auto x : i)
      cout << x << ' ';
    cout << endl;

    int idx[sz];    // to save indexes
    // set init values to -1
    fill(begin(idx), end(idx), -1);

    // scan array to find div by 3
    int *ptr = idx;
    for(auto &x : i)
      if(!(x%3)) // ?div -write index to idx
        *ptr++ = &x - begin(i);

    // print found indexes
    cout << "Indexes of the elements found " << endl;
    for(int j = 0 ; (j < sz ) && (idx[j] != -1); ++j)
          cout << idx[j] << " ";
    cout << endl;

    int ii[sz]; // here we put new - modified array
    copy(begin(i), end(i), begin(ii));
    //calculate new values
    for(int j = 0 ; (j < sz ) && (idx[j] != -1); ++j)
    {
        // exclude elements on the borders
        if((idx[j]>0)&&(idx[j]<(sz-1)))
            ii[idx[j]] = i[idx[j]-1]+i[idx[j]+1];
        else
            cout << "Most left or most right element skipped" << endl;
    }

    // print old values
    for_each(begin(i), end(i),   [](const int j){cout << j << " ";});
    cout << endl;

    // print new values
    for_each(begin(ii), end(ii), [](const int j){cout << j << " ";});
    cout << endl;

   return 0;
}