Re: Comparing floating point values in Java

From:

"Daniel Pitts" <googlegroupie@coloraura.com>

Newsgroups:

comp.lang.java.programmer

Date:

30 Dec 2006 10:33:25 -0800

Message-ID:

<1167503605.184515.248200@48g2000cwx.googlegroups.com>

Philipp wrote:

Hello,
I'm aware of problems (rounding, NaN etc) when comparing floating point
values in computers.
In C++ this goes a bit further as you cannot compare with certitude
floating point numbers even if you have made exactly the same operations
on each of them (see eg:
http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.18 )

My question: In Java, will unchanged values compare strictly true if equal?
Is it _guaranteed_ that the following code does output "true" on all JVMs?

Thanks for answers. Phil

example code:
public class Test {
    public static void main(String[] args) {
        float a = 1;
        float b = 1;
        System.out.println("a == b?: " + (a == b));
    }
}

Look into StrictMath.

It might also be worthwhile to learn how to calculate error, and
compare signifigant figures only. If you only need a "probably equal",
then you can use a delta, eg:

public class FloatEquality {
    public static boolean areEqual(float a, float b, float delta) {
        return Math.abs(a - b) < delta;
    }
    public static float DELTA_F = 0.00000001f;
    public static void main(String[] args) {
        float a = 1f;
        float b = 1f;
        System.out.println("a == b?: " + areEqual(a, b, DELTA_F);
    }
}

Hope this helps.

Daniel.