Grouping relatively close values using LINQ or loop

Question

I guess this would be more maths than C#. I've got an array of float values, where most values belong to one of the few tightly packed ranges. Here's an example (Lower Limit=0,Upper Limit=612):

3.4,5.0,6.1, 
144.0,144.14,145.0,147.0, 
273.77,275.19,279.0, 
399.4,399.91,401.45, 
533.26,537.0,538.9

This is a single array of 16 values, just separated them to show those "groups". What I need to do is to somehow group them, either using Linq, or a manual loop or whatever, so that those close values fall in a single group.

A simple math operation like dividing by 10 (or 100) won't work, because 399 would fall in a different group than 401 (4th group in the above example). Another approach would be to create a histogram of some kind, but I'm looking for something simple here. Any help would be greatly appreciated.

Answer 1

Just an another idea of a clustering with using GroupBy with a custom comparer

var numbers = new float[]
    {
       3.4f, 5.0f, 6.1f, 144.0f, 144.14f, 145.0f, 
       147.0f, 273.77f, 275.19f, 279.0f, 399.4f, 399.91f, 401.45f,
       49, 50, 51,
       533.26f, 537.0f, 538.9f
    };

foreach (var group in numbers.GroupBy(i => i, new ClosenessComparer(4f)))
    Console.WriteLine(string.Join(", ", group));

And the custom ClosenessComparer:

public class ClosenessComparer : IEqualityComparer<float>
{
    private readonly float delta;

    public ClosenessComparer(float delta)
    {
        this.delta = delta;
    }

    public bool Equals(float x, float y)
    {
        return Math.Abs((x + y)/ 2f - y) < delta;
    }

    public int GetHashCode(float obj)
    {
        return 0;
    }
}

And the output:

1: 3,4 5 6,1
2: 144 144,14 145 147
3: 273,77 275,19 279
4: 399,4 399,91 401,45
6: 49 50 51
5: 533,26 537 538,9

Answer 2

Here's a method that groups elements if they're within a certain delta (4 by default) of the previous value:

IEnumerable<IEnumerable<double>> GetClusters(IEnumerable<double> data,
                                             double delta = 4.0)
{
    var cluster = new List<double>();
    foreach (var item in data.OrderBy(x=>x))
    {
        if (cluster.Count > 0 && item > cluster[cluster.Count - 1] + delta)
        {
            yield return cluster;
            cluster = new List<double>();
        }
        cluster.Add(item);
    }
    if (cluster.Count > 0)
        yield return cluster;
}

You can get tweak the algorithm by changing what you use for cluster[cluster.Count - 1] + delta. For example, you might use

• cluster[0] + delta - delta from first element in the cluster
• cluster.Average() + delta - delta from the mean of the cluster so far
• cluster[cluster.Count / 2] + delta - delta from the median of the cluster so far