// Program calculates the miles per gallon 
// per tank of gas and overall given the 
// input data of miles driven and number 
// of gallons used for each tank 
// Exercise 2.16 from D&D
#include<iostream.h>

main ()
{
  float gallons, total_gallons, miles, 
        total_miles, mpg;

  // initialize
  total_gallons = 0;
  total_miles = 0;

  // initialize input (for sentinal value)
  cout << "Enter the gallons used (-1 to end): ";
  cin >> gallons;

  while (gallons != -1) 
    {
      if (gallons == 0)
	{
	  cout << "ERROR: gallons = 0, please try again";
	  cout << endl;
	}
      else
	{
	  total_gallons += gallons;
	  cout << "Enter the miles driven: ";
	  cin >> miles;
      	  total_miles += miles;
	  cout << "The miles / gallon for this tank was " 
	       << ( miles / gallons) << endl;
	  cout << endl;
	}
      cout << "Enter the gallons used (-1 to end): ";
      cin >> gallons;
    }
  
  // Summation Phase
  if (total_gallons != 0)
    {
      cout << endl;
      cout << "The ovrall average miles / gallon was " 
	   << (total_miles / total_gallons)
	   << endl;
    }

  return 0;
}