July 25, 2008 02:33pm pdt

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	[x] Attachment Details

Avoiding round-off error in calculating taxes

Zones: C++ Programming Language, C Programming Language

Tags: C++, C

This seems like an easy enough question, and hopefully it is for someone!

My application has to calculate sales tax. I had a user complain of it mis-calculating taxes at least on one testcase. On this particular sale the total items are $69.00. The tax rate is 0.065, but the computer stores it as 0.064999998 so when it multiplies the two it gets 4.4849997, instead of 4.485. Then I send it through the attached code to round it and it chooses 4.48, when it should technically be 4.49. Any ideas on how to correct this?

         
             double round(double value, int precision)
{
	double fraction,
		   temp;
 
	value = value*pow(10, precision);
 
	fraction = modf(value, &temp);
	if(fraction>=0.5) temp+=1.0;
	if(fraction<=-0.5) temp-=1.0;
 
	return temp*pow(0.1, precision);
}

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8.9

Question Stats

Zone: Programming

Question Asked By: david_johns

Solution Provided By: ozo

Participating Experts: 5

Solution Grade: A

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Comment by david_johns

I wanted to post back the final solution you all helped me to come up with. Feel free to offer additional suggestions, but I tested it for every dollar amount from $0.01 to $1000.00 and it worked without error. I am assuming it will be okay past there until I reach the limit my variable types allow. Note that I am not worried about amounts above $10,000.

         
             //Tax Rates are stored as longs, where 6000 means 6% for example
//Totals are stored as longs denoting cents
tax = taxableProducts * productsRate;
tax += taxableShipping * shippingRate;
 
remainder = tax%100000;
if(remainder>=50000) tax+=100000-remainder;
else if(remainder<=-50000) tax+=(-100000-remainder);
else tax-=remainder;
taxFloat = (float) round((double) tax/(double) 10000000, 2);
return taxFloat;

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