Question

I’m looking to produce a data histogram from a given dataset. I’ve read about different options for constructing the histogram and I’m most interested in a method based on the work of

Shimazaki, H.; Shinomoto, S. (2007). “A method for selecting the bin
size of a time histogram”

The above method uses estimation to determine the optimal bin width and distribution, which is needed in my case because the sample data will vary in distribution and hard to determine the bin count and width in advance.

Can someone recommend a good source or a starting point for writing such a function in c# or have a close enough c# histogram code.

Many thanks.

Answer 1

The following is a port I wrote of the Python version of this algorithm from here. I know the API could do with some work, but this should be enough to get you started. The results of this code are identical to those produced by the Python code for the same input data.

public class HistSample
{
    public static void CalculateOptimalBinWidth(double[] x)
    {
        double xMax = x.Max(), xMin = x.Min();
        int minBins = 4, maxBins = 50;
        double[] N = Enumerable.Range(minBins, maxBins - minBins)
            .Select(v => (double)v).ToArray();
        double[] D = N.Select(v => (xMax - xMin) / v).ToArray();
        double[] C = new double[D.Length];

        for (int i = 0; i < N.Length; i++)
        {
            double[] binIntervals = LinearSpace(xMin, xMax, (int)N[i] + 1);
            double[] ki = Histogram(x, binIntervals);
            ki = ki.Skip(1).Take(ki.Length - 2).ToArray();

            double mean = ki.Average();
            double variance = ki.Select(v => Math.Pow(v - mean, 2)).Sum() / N[i];

            C[i] = (2 * mean - variance) / (Math.Pow(D[i], 2));
        }

        double minC = C.Min();
        int index = C.Select((c, ix) => new { Value = c, Index = ix })
            .Where(c => c.Value == minC).First().Index;
        double optimalBinWidth = D[index];
    }

    public static double[] Histogram(double[] data, double[] binEdges)
    {
        double[] counts = new double[binEdges.Length - 1];

        for (int i = 0; i < binEdges.Length - 1; i++)
        {
            double lower = binEdges[i], upper = binEdges[i + 1];

            for (int j = 0; j < data.Length; j++)
            {
                if (data[j] >= lower && data[j] <= upper)
                {
                    counts[i]++;
                }
            }
        }

        return counts;
    }

    public static double[] LinearSpace(double a, double b, int count)
    {
        double[] output = new double[count];

        for (int i = 0; i < count; i++)
        {
            output[i] = a + ((i * (b - a)) / (count - 1));
        }

        return output;
    }
}

Run it like this:

double[] x =
{
    4.37, 3.87, 4.00, 4.03, 3.50, 4.08, 2.25, 4.70, 1.73,
    4.93, 1.73, 4.62, 3.43, 4.25, 1.68, 3.92, 3.68, 3.10,
    4.03, 1.77, 4.08, 1.75, 3.20, 1.85, 4.62, 1.97, 4.50,
    3.92, 4.35, 2.33, 3.83, 1.88, 4.60, 1.80, 4.73, 1.77,
    4.57, 1.85, 3.52, 4.00, 3.70, 3.72, 4.25, 3.58, 3.80,
    3.77, 3.75, 2.50, 4.50, 4.10, 3.70, 3.80, 3.43, 4.00,
    2.27, 4.40, 4.05, 4.25, 3.33, 2.00, 4.33, 2.93, 4.58,
    1.90, 3.58, 3.73, 3.73, 1.82, 4.63, 3.50, 4.00, 3.67,
    1.67, 4.60, 1.67, 4.00, 1.80, 4.42, 1.90, 4.63, 2.93,
    3.50, 1.97, 4.28, 1.83, 4.13, 1.83, 4.65, 4.20, 3.93,
    4.33, 1.83, 4.53, 2.03, 4.18, 4.43, 4.07, 4.13, 3.95,
    4.10, 2.27, 4.58, 1.90, 4.50, 1.95, 4.83, 4.12
};

HistSample.CalculateOptimalBinWidth(x);