Different results for the floating point operations

Question

I'm having a small issue, I have some code that calculates a mean value from a given set of data, I do this from several different computers (some with linux, some with windows, some 32bits and others 64bits), but when I watch the database results the some values that should be equal are slightly different.

This makes no difference for my program but I was wondering why it was this way, I was under the impression that floating point operations with the same word lenght should yield the same result (I was obviously wrong).

This is a small sample code that I wrote to check:

public class Test{
    public static void main(String... args){
        double counter = 0, value = 1./10;

        for (int i = 0; i < 1000000; i++){
            counter += value;
        }

        System.out.println(counter);
    }
}

It basically calculates 1000000 / 10, I know that the result won't be exact but in some pcs it prints a number that is slightly less than 100000 and in other cases it prints a number slightly greater.

score 5 · Accepted Answer · edited May 23 '17 at 12:27

5

If you really need the same error on all the platforms you should use the strictfp keyword in your method declaration, check out the selected answer on this post: When should I use the "strictfp" keyword in java?

Let it be noted that strictfp only works on your current block, and does not propagates though the code, so if you are calling some methods in the middle they should have strictfp too (also you should use StrictMath instead of Math).

At any rate if this is not affecting your code, just let it be... strict floating point operations have a higher computational cost that regular floating point operations.

edited May 23 '17 at 12:27

Community

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answered Apr 15 '12 at 22:45

Federico Vera

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Wow, I am doing Java for such a long time and I have read a lot about Java, but I have never heard of this keyword. Thanks for enlightening me. ^^ – Michael Schmeißer Apr 15 '12 at 22:53
@user1335148 Yes, strictfp is a Java keyword! – Federico Vera Apr 15 '12 at 22:54
@Fido Thanks for the tip, I've used java for a while now, and as MichaelSchmeißer have never heard of the strictfp keyword. – user1335148 Apr 15 '12 at 23:36

score 2 · Answer 2 · answered Apr 15 '12 at 22:37

2

I would think it depends on compiler optimization techniques used. E.g. with no optimizations, you get a lot of addition, resulting in probably loss of precision, with a compiler that can understand the operations involved, the whole loop might be replaced with counter = value*1000000

The precision of double and its operations are set precisely in the language

answered Apr 15 '12 at 22:37

Attila

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Is it possible to manually set compiler optimizations on Java? or is it just a decision of the JVM? in which case nothing could be done... – user1335148 Apr 15 '12 at 22:47
Yes, you [can set](http://developers.sun.com/solaris/articles/options.html) optimization options – Attila Apr 15 '12 at 23:03

score 1 · Answer 3 · answered Apr 15 '12 at 22:41

1

Different plate forms have different ways to represent floating point.

To get same results you should limit the precision to a specific number of decimal places.

answered Apr 15 '12 at 22:41

Tasawer Khan

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Different results for the floating point operations

3 Answers3