#include <iostream>

// This program will go into an infinite loop if we
// enter a character that is not a valid integer.
int main( void )
{
        int num;
        std::cout << "Enter a number: ";
        std::cin >> num;

        while ( num != 0 ) {
                int square = num * num;
                std::cout << "Square of " << num
                     << " = " << square << '\n';
                std::cout << "Enter a number (0 to quit) \n";
                std::cin >> num;
        }
}
// The correct way to do it is like this:
#ifdef SENSIBLE
int main( void )
{
        int num;
        std::cout << "Enter a number: ";
        while ( std::cin >> num && num != 0 ) {
                int square = num * num;
                std::cout << "Square of " << num
                     << " = " << square << '\n';
                std::cout << "Enter a number (0 to quit) \n";
        }
}
#endif