Negative Zero Help

[Xander314]

I am writing a program to find the coordinates of points on a unit circle. Everything works fine except for a minor aesthetic issue. Here is the code and then I'll discuss the problem:

public class PointsOnACircleV1
{
    public static void main(String[] args)
    {
        double radius = 1.0;
 
        double y1 = 0.0;
        double y2 = 0.0;
 
        for(double x1 = 1.0; x1 >= -1.0; x1 -= .1)
        {
            y1 = Math.sqrt(Math.pow(radius,2) - Math.pow(x1, 2));
            y2 = -y1;
            System.out.printf("%6.2f%8.2f%14.2f%8.2f%n", x1, y1, x1, y2);
        }
    }
}
 

When x1 = 1.0 or -1.0, y2 = -0.0. Obviously this is the same thing as 0.0 mathematically, but it just doesn't look good in the output. An suggestions? I've tried using a if-else loop like this to remedy the problem:

            if(y1 != 0.00)
            {
                y2 = -y1;
            }
            else
            {
                y2 = y1;
            }
 

This will fix the problem for the first zero but not the second one. It completely baffles me. Hope that's all clear. Any help would be greatly appreciated.

[vipersniper5]

I'm pretty sure the problem here is that the number your working with is not actually 0, but rather a decimal >-.1 for example -.001, the negative comes in because it is a negative number, but with the format specifiers your ignoring those extra digits. for a nice easy fix I would suggest something like:
if(y2>-.1&&y2<0)
y2=0;
before your print. There are other ways (rounding/truncation) but thats nice and simple and I think all thats needed for your program.
Hope it helps

[Xander314]

I'm pretty sure the problem here is that the number your working with is not actually 0, but rather a decimal >-.1 for example -.001, the negative comes in because it is a negative number, but with the format specifiers your ignoring those extra digits. for a nice easy fix I would suggest something like:
if(y2>-.1&&y2<0)
y2=0;
before your print. There are other ways (rounding/truncation) but thats nice and simple and I think all thats needed for your program.
Hope it helps

I removed the code that truncates the decimals and the number actually is zero. Or at least it's 0.000000. And when I do the math on paper I get zero for the problem values. This just baffles me.

[JosAH]

Here's something to read for both of you:

What Every Computer Scientist Should Know About Floating-Point Arithmetic.

kind regards,

Jos

[BigDaddyLH]

public class PointsOnACircleV1{
    public static void main(String[] args){
        double radius = 1.0;
        for(double x1 = 1.0; x1 >= -1.0; x1 -= .1) {
            double y1 = Math.sqrt(radius*radius - x1*x1);
            double y2 = -y1;
            if (y2 == -0.0)
                y2 = 0.0;
            System.out.printf("%6.2f%8.2f%14.2f%20.15f%n", x1, y1, x1, y2);
        }
    }
}

The first -0.00 is because there is such a thing as negative zero. This is not a "Java Thing" but part of how floating point numbers work. The second -0.00 is because of rounding error. As you can see when you run my version of the code, the last value isn't 0, it's -0.00000002107...