comparing doubles with less than or greater than

[Chris Zaremba]

I understand that you shouldn't use equals for comparing two double values but does the same hold true for comparing a double to a literal using less than and greater than?

I have a double variable which holds an angle in degrees calculated using Math.asin and Math.toDegrees etc. If I then compare this to the literal 90 will the result always be true no matter how close a is to but less than 90 and false no matter how close a is to but greater than 90? I'm thinking if a was calculated to be 89.999999998 but the literal 90 is stored internally as 89.999999996;

double a = Math.toDegrees(Math.asin(0.3434));

if (a < 90) {
    // do stuff
}

[Campbell Ritchie]

Chris Zaremba wrote:I understand that you shouldn't use equals for comparing two double values but does the same hold true for comparing a double to a literal using less than and greater than?

Yes.

public class DoubleComparison
{
    public static void main(String[] args)
    {
        double d1 = Double.parseDouble(args[0]);
        double d2 = Double.parseDouble(args[1]);
        double d3 = 0d;
        for (int i = 0; i < Integer.parseInt(args[2]); i++)
        {
            d3 += d2;
        }
        System.out.printf("d3 < d1: %b%n", d3 < d1);
    }
}

campbell@computer_name:~/java$ java DoubleComparison 1.0 0.1 10
d3 < d1: ...

[Chris Zaremba]

Thanks.

[Campbell Ritchie]

You see what has happened? Adding 0.1 ten times is one of the better-known pitfalls in floating-point arithmetic. When you use asin, you never quite get back to 90°, so there are bound to be errors because of that imprecision.

[Chris Zaremba]

Yes thanks. I knew this was a problem when doing a == b but can now see it's the same issue here. I'm using it for drawing 3d pie segments and have adjusted the drawing method so it doesn't matter if it is +/- a little bit.

[Campbell Ritchie]

You're welcome