Thread: Why does this fail?

I have tried in vain to debug this. The logic seems to fail around dimes. For most of the values I put in, the logic works fine. However sometimes, there will be a "penny" left in difference and other times there will be a negative number. Any ideas how this happens?

Code:

#include <iostream>
#include <string>
using namespace std;

int main()
{
// self documenting code begin
	cout << "This program calculates the amount of change\ngiven back after a purchase";
// self documening code end
	cout << "\nEnter the purchase amount:  ";
	double purchase;
	cin >> purchase;
	cout << "\nEnter the amount of money given:  ";
	double money;
	cin >> money;
	double difference = money - purchase;
	double change = difference;
	int dollar = 1;
	double halfDollar = .5;
	double quarter = .25;
	double dime = .1;
	double nickel = .05;
	double penny = .01;
	for (int dollars = 0; difference >= 1; dollars++)
	{
		cout << difference <<"  DEBUGGING LOGIC - subtracting a dollar\n"; // for debugging
		difference = difference - dollar;
		cout << difference <<"  DEBUGGING LOGIC - I subtracted a dollar\n"; // for debugging
	}
	for (int halfDollars = 0; difference >= 0.50; halfDollars++)
	{
		cout << difference <<"  DEBUGGING LOGIC - subtracting a half dollar\n"; // for debugging
		difference = difference - halfDollar;
		cout << difference <<"  DEBUGGING LOGIC - I subtracted a half dollar\n"; // for debugging
	}
	for (int quarters = 0; difference >= 0.25; quarters++)
	{
		cout << difference <<"  DEBUGGING LOGIC - subtracting a quarter\n"; // for debugging
		difference = difference - quarter;
		cout << difference <<"  DEBUGGING LOGIC - I subtracted a quarter\n"; // for debugging
	}
	for (int dimes = 0; difference >= 0.10; dimes++)
	{
		cout << difference <<"  DEBUGGING LOGIC! - subtracting a dime\n"; // for debugging
		difference = difference - dime;
		cout << difference <<"  DEBUGGING LOGIC - I subtracted a dime\n"; // for debugging
	}
	for (int nickels = 0; difference >= 0.05; nickels++)
	{
		cout << difference <<"  DEBUGGING LOGIC - subtracting a nickel\n"; // for debugging
		difference = difference - nickel;
		cout << difference <<"  DEBUGGING LOGIC - I subtracted a nickel\n"; // for debugging
	}
	for (int pennies = 0; difference > 0.01; pennies++)
	{
		cout << difference <<"  DEBUGGING LOGIC! - subtracting a penny\n"; // for debugging
		difference = difference - penny;
		cout << difference <<"  DEBUGGING LOGIC - I subtracted a penny\n"; // for debugging
	}
	if (difference > 0.01)
	{
		cout << difference <<"  DEBUGGING LOGIC! - Added a penny to make it right?\n"; // for debugging
		++pennies;
	}
	if (difference < 0)
	{
		cout << difference <<"  DEBUGGING LOGIC! - Subtracted a penny to make it right?\n"; // for debugging
		--pennies;
	}

	 // Self documenting output to screen
	cout << difference <<"  THE END RESULT OF DIFFERENCE DEBUGGING LOGIC!\n"; // for debugging
	cout << "\nPlease give the customer "  << change << " back in change\n";
	cout <<  dollars << " dollars\n";
	cout <<  halfDollars << " half dollars\n";
	cout <<  quarters << " quarters\n";
	cout <<  dimes << " dimes\n";
	cout <<  nickels << " nickels\n";
	cout <<  pennies << " pennies\n";
	cin.ignore();
	return 0;
	}