public static double CalculateEntropy(this Dataset dataset, int attributeIndex)
{
double entopy = 0;
if (dataset.Metadata.Attributes[attributeIndex] is NominalAttribute)
{
for (int classIndex = 0; classIndex < dataset.Metadata.Target.Values.Length; classIndex++)
{
double currentEntropy = 0;
for (int valueIndex = 0; valueIndex < ((NominalAttribute)dataset.Metadata.Attributes[attributeIndex]).Values.Length; valueIndex++)
{
int freq = dataset.Filter(attributeIndex, valueIndex, classIndex).Count;
if (freq != 0)
{
double prob = (double)freq / (double)dataset.Size;
currentEntropy -= prob * Math.Log(prob);
}
}
double classProb = (double)dataset.Metadata.Target.ValueCounts[classIndex] / (double)dataset.Size;
entopy += currentEntropy * classProb;
}
}
else
{
double stdv = dataset.CalculateStdValue(attributeIndex);
//ln(sigma*Sqrt(2*Pi*e))
double differentialEntropy = Math.Log(stdv * Math.Sqrt(2 * Math.PI * Math.E), Math.E);
entopy = differentialEntropy;
}
return(entopy);
}