foo.cc: In function ‘int main()’:
foo.cc:20:44: warning: comparison of integer expressions of different signedness: ‘long long int’ and ‘std::vector<long long int>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   20 |                 for(int j=0;prime[j]>n/i&&j<prime.size();j++)
      |                                           ~^~~~~~~~~~~~~