Addition of doubles (newbie question)

Hi. In subtracting one double from another, namely 45.32 - 45.31, I get a result of 0.00999999999999801. According to all the math I've ever learned, the answer should be .01.

I asked my professor about this, and his response was that "45.31 may be a repeating decimal." To which I responded, "I have no idea what that means." I tried another forum, where someone treated me like I was a complete moron.

I just don't understand what's happening, and if someone could explain it to me very simply, I'd greatly appreciate it.

Thank you.

OP has been given a link and an explanation already and became abusive. Do not waste your time with this idiot.

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Hello archyb,

It would be worth doing some reading about repeating decimals. There is lot's of information online.

Heres the gist of it though:

Quote:

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A decimal representation of a real number is called a repeating decimal (or recurring decimal) if at some point it becomes periodic: there is some finite sequence of digits that is repeated indefinitely. For example, the decimal representation of 1/3 = 0.3333333... (spoken as "0.3 repeating", or "0.3 recurring") becomes periodic just after the decimal point, repeating the single-digit sequence "3" infinitely. A somewhat more complicated example is 3227/555 = 5.8144144144..., where the decimal representation becomes periodic at the second digit after the decimal point, repeating the sequence of digits "144" infinitely.

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The link provided in your other forum post is also useful:

What Every Computer Scientist Should Know About Floating-Point Arithmetic

I know it's an awful lot of reading and to a beginner it can be very daunting but even just a quick read will help you pick up some basic information that helps lay the foundation for the knowledge you need to acquire.

I also suggest reading - Java theory and practice: Where's your point?

You can round your answer to get the expected output:

Code Java:

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public class DoubleTest {
 
	/**
	 * JavaProgrammingForums.com
	 */
	public static void main(String[] args) {
 
		double x = 45.32;
		double y = 45.31;
		double answer = (x - y);
 
		answer = Math.round(answer*100)/100.0d;		
		System.out.println(answer);
 
	}
 
}

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