Create Buckets ith Linq [duplicate]

Question

I would like to create buckets on List<double> such as divided in n groups such as:

List<double> list = new List<double>() { 0, 0.1, 1.1, 2.2, 3.3, 4.1, 5.6, 6.3, 7.1, 8.9, 9.8, 9.9, 10 }; n = 5

I want to obtain something like this

 bucket values --------------------------------- [0 .. 2] -> {0, 0.1, 1.1} [2 .. 4] -> {2.2, 3.3} ... [8 .. 10] -> {8.9, 9.8, 9.9, 10}

The problem is if I do a GroupBy using:

return items .Select((item, inx) => new { item, inx }) .GroupBy(x => Math.Floor(x.item / step)) .Select(g => g.Select(x => x.item));

I always get unwanted first or last bucket such as [10 .. 12] (note that all the values are in [0 .. 10] range) or [0 .. 0] (note the wrong range of the bucket) which contains extreme values only (0 or 10 in the example above).

any Help ?

So you have a list and a dictionary? How are these two things related? What is step? I don't understand this question at all. — John Wu
– John Wu, Commented Mar 16, 2021 at 15:40
could you please be more elaborate what you mean by "extreme values"? if you have any bugs that only happen in edge cases, you should include at least one of those edge cases in your question. additionally, your question is pretty unclear. please try to bring more detail in your description — Franz Gleichmann
– Franz Gleichmann, Commented Mar 16, 2021 at 15:41
With what input do you get an unexpected output? What is that output and what did you expect? — Magnus
– Magnus, Commented Mar 16, 2021 at 15:43
Are you looking for cluster ?en.wikipedia.org/wiki/K-means_clustering. Can you covers the basic edge case with your minimal reproducible example? Do we groupe {3.3, 4.1} because there is less than 1 between those 2? — Drag and Drop
– Drag and Drop, Commented Mar 16, 2021 at 15:45

Dmitrii Bychenko · Accepted Answer · 2021-03-16 16:08:15Z

Well, for arbitrary list you have to compute range: [min..max] and then

 step = (max - min) / 2;

Code:

 // Given List<double> list = new List<double>() { 0, 0.1, 1.1, 2.2, 3.3, 4.1, 5.6, 6.3, 7.1, 8.9, 9.8, 9.9, 10 }; int n = 5; // We compute step double min = list.Min(); double max = list.Max(); double step = (max - min) / 5; // And, finally, group by: double[][] result = list .GroupBy(item => (int)Math.Clamp((item - min) / step, 0, n - 1)) .OrderBy(group => group.Key) .Select(group => group.ToArray()) .ToArray(); // Let's have a look: string report = string.Join(Environment.NewLine, result .Select((array, i) => $"[{min + i * step} .. {min + i * step + step,2}) : {{{string.Join("; ", array)}}}")); Console.WriteLine(report);

Outcome:

[0 .. 2) : {0; 0.1; 1.1} [2 .. 4) : {2.2; 3.3} [4 .. 6) : {4.1; 5.6} [6 .. 8) : {6.3; 7.1} [8 .. 10) : {8.9; 9.8; 9.9; 10}

Please, note Math.Clamp method to ensure [0..n-1] range for groups keys. If you want a Dictionary<int, double[]> where Key is index of bucket:

 Dictionary<int, double[]> buckets = list .GroupBy(item => (int)Math.Clamp((item - min) / step, 0, n - 1)) .ToDictionary(group => group.Key, group => group.ToArray());

Thanks! first time using the Math.Clamp method, very helopful

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