#include <iostream>
#include <stdexcept>

// Function declarations
double current( double voltage, double resistance );

// Function definitions
int main() {
  double const voltage{ 12.0 };

  double resistances[]{
    4.0, 2.0, 0.0, -3.0, 6.0
  };

  for ( double resistance : resistances ) {
    std::cout << std::endl
              << "Resistance: " << resistance << " ohms"
              << std::endl;

    try {
      double amps{ current( voltage, resistance ) };

      std::cout << "Current: " << amps << " A"
                << std::endl;
    } catch ( std::invalid_argument const &e ) {
      std::cout << ">> Cannot calculate the current: "
                << e.what() << std::endl;
    }
  }

  return 0;
}

// Calculate the current through a resistance.
double current( double voltage, double resistance ) {
  if ( resistance <= 0.0 ) {
    throw std::invalid_argument{
      "resistance must be positive"
    };
  }

  return voltage/resistance;
}