I am solving a Codeforces problem, and i ran into an interesting situation with doubles. When i add(d) to a HashSet the output is different from the output generated when i add(0.0) , although the value of d must be 0.0 since condition d == 0.0 must be true at the add operation. Why?
HashSet<Double> equations;
...
double d = (double)(y-y0)/(x-x0);
if(y == y0 && d == 0.0) {
equations.add(0.0); // if add(d) the output is different
}
Problem link: http://codeforces.com/contest/514/problem/B
Code:
int n;
int c=0;
int x0,y0;
HashSet<Double> equations;
Seqreader read = new Seqreader(" ");
n=read.nextInt();
x0 = read.nextInt();
y0 = read.nextInt();
equations = new HashSet<Double>();
for(int i=0;i<n;i++){
int x = read.nextInt();
int y = read.nextInt();
if (x == x0) {
equations.add(20000.0);
}else {
double d = (double)(y-y0)/(x-x0);
if(y == y0 && d == 0.0) {
equations.add(0.0); // if add(d) output is different
}
else
equations.add(d); // != 0
}
}
System.out.println(equations.size());
Answer
-0.0 == 0.0, but Double.valueOf(-0.0) does not equal Double.valueOf(0.0).
In your first case, when calling equations.add(d), one time it is adding 0.0, and another it is adding -0.0, which adds two different Double values. In your second case, when you are calling equations.add(0.0) it is adding the same Double value twice.
A small test for this:
public static void main(String[] args) throws Exception {
double pos = 0.0;
System.out.println("" + (pos == 0.0)); // true
double neg = -0.0;
System.out.println("" + (neg == 0.0)); // true
System.out.println("" + (neg == pos)); // true
HashSet<Double> set = new HashSet<Double>();
set.add(pos);
set.add(neg);
System.out.println("" + set.size()); // 2
}