Re: Comparing floating point values in Java

From:

Philipp <sicsicsic@freesurf.ch>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 30 Dec 2006 21:00:44 +0100

Message-ID:

<1167508844_429@sicinfo3.epfl.ch>

Arne Vajh?j wrote:

Philipp wrote:

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?

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

Your example is not equivalent to the example in your link.

Yes I'm aware of that. The point in the link is that compiler
optimization can lead to unequal floats even if exactly the same
operations were performed on it. Is this also true for Java?

But in general you should not use == to compare floating
points in Java either.

Agree

Your example are just more simple than real life code.

No! (If I ask the question it's because I met this problem)

I was calculating an array of float values from some user input. But
sometimes (rarely) the calculation cannot be done (this is correct
behavior).
I then want to mark these values by setting them to -1 and treat this
special case later on. So I later need to test for -1 in my array.

Philipp