C++ Primer 7.1

7.1.cpp

#include <iostream>

double harmonic_mean(double x, double y);

int main()
{
  using namespace std;

  double x, y;
  cout << "Please input x and y: ";
  while (cin >> x >> y) {
    cout << "harmonic mean is: " << harmonic_mean(x, y) << endl;

    if (0 == int(x) || 0 == int(y)) { break; }
    cout << "Please input x and y: ";
  }

  return 0;
}

double harmonic_mean(double x, double y)
{
  return 2.0 * x * y / (x + y);
}