int ar[100] // ok
=================
int sz = 15;
int ar[sz] // error
=================
const int sz = 15;
int ar[sz] // ok
=================
int sz = 15; // or cin >> sz
vector<int> ar(sz) // ok
================
int sz = 15;
vector<int> ar;
for(int i = 0; i < sz; i++){
    ar.push_back(0);
}