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8.9

Avoiding round-off error in calculating taxes

Asked by david_johns in 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?

Get your answer NOW

         
             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);
}

20091111-EE-VQP-89 / EE_QW_2_20070628

Avoiding round-off error in calculating taxes

Asked by david_johns in C++ Programming Language, C Programming Language

Tags: C++, C

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