Going nuts over modulo of two doubles

Question

consider:
double x=2.0;
double y=0.1;
double z=x%y;
boolean perfectFit = (Math.round(x/y)==x/y);
System.out.println("x="+x+",y="+y+",z="+z+",perfectFit="+perfectFit);
// surprisingly (at least for me),
// while perfectFit is true (as expected), z is not zero at all:
x=2.0,y=0.1,z=0.0999999999999999,perfectFit=true
am I missing something?

Answer 1

What happens is quite normal and you will understand this if you try:
public class Modulo {
    public static void main (String [] args) {
        double x = 2.0;
        double y = 0.00001;
        double z=x%y;
        boolean perfectFit = (Math.round(x/y)==x/y);
        System.out.println("rounded: "+Math.round(x/y));
        System.out.println("not rounded: "+x/y);
        System.out.println("modulo: "+z);
        System.out.println(perfectFit);       
}You see Math.round(double) returns a long and you play with double. That means that you lose a bit of presision here... In the above example this becomes obvious for a big number.
Hope that helped
afotoglidis