// namespace3.cc
#include <iostream>

namespace physics {
  double mean(const double& a, const double& b) {
    return (a+b)/2.;
  }
}

void printMean(const double& a, const double& b) {
  double z1 = physics::mean(a,b);

  using namespace std; // using std namespace within this method!
  cout << "physics::mean(" << a << "," << b << ") = " << z1 << endl;
}

int main() {

   double x = 3;
   double y = 4;

   printMean(x,y);
   cout << "no namespace available in the main!" << endl;

   return 0;
}