How to fix this calculation error?

[MisterTams]

Why is my taxTotal rounding up like it should but why does finalTotal not round up? An example of this is purchasing the item that costs $21.00. With my subtotal at $21.00 and the calculated taxTotal equaling $1.58, then why does my finalTotal come out to $22.57?

Here is the function from my source code.

double checkout(double PurchaseSubTotal, double inBankBalance)
{
	srand(time(NULL)); //NULL is defined as 0 (zero)
	ofstream outFile; 
	
	int min = 1000;
	int max = 9999;
	int ReceiptID = rand()%1000 +1;
	int BankDebitID = (rand()%(max-min)+min);
	double taxTotal = 0.0750 * PurchaseSubTotal;
	double finalTotal = PurchaseSubTotal + taxTotal;
	double MoneyLeft = inBankBalance - finalTotal;
	double MoneyRemaining = (MoneyLeft * -1);

	cout << "\nWelcome to checkout!" << endl;
	
	if (inBankBalance < finalTotal)
	{
		cout << "US Debit: Declined" << endl;
		cout << "Unfortunately, you do not have enough funding in your piggy bank to complete this purchase!" << endl;
		cout << "You are short by exactly: $" << MoneyLeft << endl;
		cout << "Thank you! Please visit the Toy Store another time!" << endl;
	}

	else
	{
		outFile.open("ToyStoreReceipt.txt");   // Saving receipt from purchase to a text file on computer
		outFile << fixed << setprecision(2);
		outFile << "Subtotal: $" << PurchaseSubTotal<< endl; 
		outFile << "Sales tax: 7.5%" << endl;
		outFile << "Sales tax applied: $" << taxTotal << endl;
		outFile << "Total price: $" << finalTotal << endl;
		outFile << "EFT Debit: **** **** **** " << BankDebitID << endl;
		outFile << "US Debit: Approved" << endl;
		outFile << "Receipt ID: " << ReceiptID << endl;
		outFile << "\nThank you for your purchase! Please give our store a visit another time!" << endl;
		outFile.close();
		return MoneyLeft;
	}
}

[48T7M4Gy]

Sounds like rounding errors creeping in due to the limitations of double.

There are a couple of ways of eliminating this and one way is to perform all the calculations on cents as integers and convert to dollars as required.

[MisterTams]

kemort,

Could you show me a basic example of how to do this?
I would greatly appreciate it!

-MisterTams

[48T7M4Gy]

Better still read this for a more comprehensive understanding. It's just simple arithmetic so don't make it complicated for yourself - try it with pencil and paper :)

http://stackoverflow.com/questions/4008395/rounding-problem-with-double-type