tiny offset in calculation

[pie_man]

Sometimes when i try to add, subtract, multiply, divide, etc. two doubles, i get this tiny offset of ±0.000000000000001. For example, when i try to multiply 2.6 by 3.0, instead of getting 7.8, i get 7.800000000000001. Could someone please tell me how to fix this?

[sunde887]

You can use formatting to specify how many decimal places. That's the only fix I can think of. Using floating point numbers in computers is simply not exact. There are a lot of resources on this topic around the internet if you are willing to read to understand it. Otherwise, you can just understand that using floating point will not produce exact results, and you should be careful if calculations require exact results.

To format with String.format or System.out.printf(), use the %f flag, look up how to use them, and specify how many decimal places to show.

[JosAH]

What Every Computer Scientist Should Know About Floating-Point Arithmetic

kind regards,

Jos

[pie_man]

Thanks for the help :)

[neilcoffey]

There are a couple of approaches to this:

(1) not care about the inaccuracy in the last decimal place and treat it as a display problem. As mentioned, you can do something like System.out.println("%.2f", number); to display to (in this case) 2 decimal places; (See the NumberFormat class if you need more control)
(2) if you actually care about the inaccuracy for some reason, then see the BigDecimal class.