/// <summary>
/// Calculates segmented conditional impurity of y | x When stipulating ranges (r), X is broken
/// up into
/// |r| many segments therefore P(X=x_r) becomes a range probability
/// rather than a fixed probability. In essence the average over H(Y|X = x) becomes SUM_s [ p_r *
/// H(Y|X = x_r) ]. The values that were used to do the split are stored in the Splits member.
/// </summary>
/// <exception cref="InvalidOperationException">Thrown when the requested operation is invalid.</exception>
/// <param name="y">Target impurity.</param>
/// <param name="x">Conditioned impurity.</param>
/// <param name="ranges">Number of segments over x to condition upon.</param>
/// <returns>Segmented conditional impurity measure.</returns>
public double SegmentedConditional(Vector y, Vector x, IEnumerable<Range> ranges)
{
if (x == null && y == null)
throw new InvalidOperationException("x and y do not exist!");
double p = 0, // probability of slice
h = 0, // impurity of y | x_i : ith slice
result = 0, // aggregated sum
count = x.Count(); // total items in list
Segments = ranges.OrderBy(r => r.Min).ToArray();
Discrete = false;
// for each range calculate
// conditional impurity and
// aggregate results
foreach (Range range in Segments)
{
// get slice
var s = x.Indices(d => d >= range.Min && d < range.Max);
// slice probability
p = (double)s.Count() / (double)count;
// impurity of (y | x_i)
h = Calculate(y.Slice(s));
// sum up
result += p * h;
}
return result;
}
/// <summary>
/// Calculates conditional impurity of y | x
/// R(Y|X) is the average of H(Y|X = x) over all possible values
/// X may take.
/// </summary>
/// <param name="y">Target impurity</param>
/// <param name="x">Conditioned impurity</param>
/// <param name="width">Split of values over x to condition upon</param>
/// <returns>Conditional impurity measure</returns>
public double Conditional(Vector y, Vector x)
{
if (x == null && y == null)
throw new InvalidOperationException("x and y do not exist!");
double p = 0, // probability of slice
h = 0, // impurity of y | x_i : ith slice
result = 0, // aggregated sum
count = x.Count(); // total items in list
var values = x.Distinct().OrderBy(z => z); // distinct values to split on
Segments = values.Select(z => Range.Make(z, z)).ToArray();
Discrete = true;
// for each distinct value
// calculate conditional impurity
// and aggregate results
foreach (var i in values)
{
// get slice
var s = x.Indices(d => d == i);
// slice probability
p = (double)s.Count() / (double)count;
// impurity of (y | x_i)
h = Calculate(y.Slice(s));
// sum up
result += p * h;
}
return result;
}
