Avoid duplicates in HashSet<double> for slightly different values

Question

I'm trying to skip duplicates for the list of values below. The result I'm trying to achieve is -112.94487230674, -49.47838592529, -89.9999574979198, I'm using the HashSet class. I've implemented the IEqualityComparer below but it's not working.

What am I doing wrong?

    class HeightEqualityComparer : IEqualityComparer<double>
    {
        public bool Equals(double a, double b)
        {
            return a - b < 1e-3;
        }

        public int GetHashCode(double value)
        {
            return value.GetHashCode();
        }
    }

Here is the list of values:

    [0] -112.94487230674    double
    [1] -112.94487230674001 double
    [2] -49.478385925290006 double
    [3] -49.47838592529     double
    [4] -49.478385925289992 double
    [5] -89.9999574979198   double
    [6] -89.99995749791978  double
    [7] -49.478385925289984 double
    [8] -89.999957497919809 double
    [9] -49.478385925290013 double

Answer 1

Well, the current implementation of Equals

   return a - b < 1e-3;

is incorrect one. Equals must be

1. Equals(a, a) == true;
2. Symmetric: Equals(a, b) == Equals(b, a);
3. Transitive Equals(a, b) && Equals(b, c) leads to Equals(a, c);

Conditions 2 and 3 are not hold in the current implementation. It's easy to repair the second condition with a help of Math.Abs; the third one is real difficulty: for arbitrary positive tolerance (which is 1e-3 in your case) we have

   a == a + tolerance == a + 2 * tolerance == ... == a + n * tolerance  

which means

   a == a + n * tolerance

for abitrary large n; thus a == b for all a and b (all numbers are equal).

As a partial (but valid) solution you can try rounding the values:

   class HeightEqualityComparer : IEqualityComparer<double>
   {
       public bool Equals(double a, double b)
       {
           return Math.Round(a, 3) == Math.Round(b, 3);
       }

       public int GetHashCode(double value)
       {
           return Math.Round(value, 3).GetHashCode();
       }
   } 

Note, that we have to change GetHashCode

Answer 2

You'd need to round your values in GetHashCode to the same precision you are eliminating in the equality.

Answer 3

Check John Skeets' answer here: How does HashSet compare elements for equality?

When you add an element to the set, it will find the hash code using IEqualityComparer.GetHashCode, and store both the hash code and the element (after checking whether the element is already in the set, of course).

In your code, all the values return different hash codes. You need to make sure that close values return the same.