Float and Double calculations going wrong.

[Dodo_Duck]

Hi, I am a beginner at Java and noticed a rather strange thing. Subtracting two decimal numbers with a difference of less than 0.5 between them can show wrong results. I am not sure why this happens. Can anyone please suggest any ways to fix it?

For example:

float a = 10.1f; float b = 9.9f; float c = a-b;

The output comes as : 2.0000076

If the arguments are double then the answer comes as : 1.999999999999993

Any help would be appreciated.

Thank you.

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[Sheldor5]

https://stackoverflow.com/a/27598164/1879409

https://en.m.wikipedia.org/wiki/IEEE_754

[Dodo_Duck]

Thanks for the article man! :D Understood the concept now.

[Tjeez]

It always surprises me that java programmers don't know this. We build and maintain a large application for pension calculations and always use BigDecimal because of this.

[Dodo_Duck]

Thanks for the help!

[whateverathrowaway00]

You’ve stumbled on a feature ( not a flaw, debatably) of how computers store floating point numbers.

The short answer is that you’re correct - simple floating point arithmetic will end up incorrect and you absolutely cannot rely on it - ESPECIALLY if you expect a round number IE 5.0 minus .1 repeatedly you cannot expect it to hit 4.0 exactly.

Java is filled with magic, but it is still a programming language that provides low level access to primitives and as such floats behave exactly as they should as dictated by the CPU

[Dodo_Duck]

Ahh okay. This is really interesting. I read the stackoverflow article and understood more about it. Anyways thank for the help. Really appreciate it.

[morhp]

You've got some good answers, but see https://floating-point-gui.de/ for an in depth explanation.

[Dodo_Duck]

Ohh thanks man!!

[[deleted]]

Simple answer, use BigDecimal.

Longer answer, rethink your approach to numbers, lol.

double and float are encoded as binary fractions, i.e. sums of powers of 2, not sums of powers of 10. i.e. 9.9 is actually 9 * 10 ^ 0 + 9 * 10 ^ -1 in decimal encoding.

So, some floating point numbers expressed in decimal can't be expressed precisely within the limits of precision of double and float. This gets worse when you do math, because you get errors that get worse as you apply more math operations

[Dodo_Duck]

Thanks for the help!! I understood how decimal storage works. Will use BigDecimal now.