Java double addition gives incorrect answers

Hi, part of an application I'm writing needs to increment a double by 0.0001, however the application seems to give the wrong result

for example I'm expecting

double currentVal = -0.0003;
		double increment = 0.0001;
		for( int i = 0; i < 6; i++)
		{
			System.out.println(currentVal);
			currentVal = currentVal + increment;
		}

to return...

-3.0E-04
-2.0E-04
-1.0E-04
0.0
1.0E-04
2.0E-04

however it prints out..

-3.0E-4
-1.9999999999999998E-4
-9.999999999999998E-5
2.710505431213761E-20
1.0000000000000003E-4
2.0000000000000004E-4

Please can anyone help?

TIA

Without going into the standard of how floats and doubles are stored; don't expect a floating point to be correct. If you need to check a value of a floating point give it a rounding range which it can be in.

If you look at the results from the expected you will see theres not much difference:
expected = -2.0E-04
result = -1.9999999999999998E-4
diff = 2.0E-20

Dmail is absolutely correct - and it's one of those "fundamentals" that anybody who writes a computer program should really be aware of.

"Floating point" vs "Integer" is the fundamental distinction, but the theory behind floating point is quite large.

Interestingly, "newer" is not necessarily "better". FORTRAN did a lot better job than C, and C does a better job than Java (as far as I know, Java and C# are pretty much "the same thing" as far as floating point).

Here are a few links that might be of interest:

http://www.cs.princeton.edu/introcs/91float/

http://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf

http://www.ibiblio.org/pub/languages/fortran/ch1-2.html

http://developers.sun.com/prodtech/c...fp_errors.html

... and, my favorite ...

http://www.cs.berkeley.edu/~wkahan/ARITH_17.pdf

Originally Posted by dmail Nobody is correct 100% of the time.

but doubles are correct 99.999999999999% of the time

Thanks for you help, I've managed to get the results I want by using something along the lines of

double currentVal = -0.0003;
double increment = 0.0001;
		
DecimalFormat Currency = new DecimalFormat("#0.0000");   
		
currentVal += increment;
currentVal = Double.valueOf(Currency.format(currentVal));

Originally Posted by graemef but doubles are correct 99.999999999999% of the time

better than humans then

The key thing to be aware of is that floating point operations are generally neither "correct" nor "wrong" - they're *approximations*.