// This method is called by multiple threads to modify the same list of elements and is not synchronized.
// However, it is thread safe due to mutation slicing: each invocation will modify exactly one index of the values
public static void AssignProbabilities(DiscreteAttribute attribute, List <Record> elements, List <DiscreteAttribute> outliers = null)
{
var counts = new Dictionary <string /*value*/, double /*count*/>();
Array.ForEach(attribute.Values, str => counts.Add(str, 0));
foreach (var element in elements)
{
double count;
if (counts.TryGetValue(element[attribute], out count))
{
counts[element[attribute]] = count + 1;
}
}
var picker = new ProbabilityPicker(ConvertToProbabilities(counts));
if (!picker.IsSignificant() && outliers != null)
{
lock (outliers)
outliers.Add(attribute);
}
foreach (var element in elements)
{
if (!counts.ContainsKey(element[attribute]))
{
element[attribute] = picker.Pick();
}
}
}
public static void AssignProbabilitiesByClass(DiscreteAttribute attribute, List <Record> trainingSet)
{
var groups = trainingSet.GroupBy(elem => elem.IsPositive);
foreach (var group in groups)
{
AssignProbabilities(attribute, group.ToList(), new List <DiscreteAttribute>());
}
}
static double CalculateRatio(List <Record> recordsSet, DiscreteAttribute header)
{
double result = 0.0d;
foreach (var grouping in recordsSet.GroupBy(record => record[header]))
{
result = result + CalcPartEntropy(grouping.Count(), recordsSet.Count);
}
return(result);
}
public static DiscreteAttribute GetBestAttribute(List <Record> recordsSet, List <DiscreteAttribute> headersSet)
{
if (!headersSet.Any())
{
return(null);
}
double bestGain = 0;
DiscreteAttribute bestAttribute = null;
Parallel.ForEach(headersSet.ToArray(), header =>
{
var gain = CalculateGain(recordsSet, header);
//double ratio = CalculateRatio(recordsSet, header);
if (gain.Item1)
//if (true)
{
lock (typeof(DecisionTree))
{
if (gain.Item2 > bestGain)
{
bestGain = gain.Item2;
bestAttribute = header;
}
}
}
else
{
lock (headersSet)
{
headersSet.Remove(header);
}
}
});
return(bestAttribute);
}
public TreeNode(TreeNode parent, string value, List <DiscreteAttribute> attributes, List <Record> records, double fracCertainty)
{
this.parent = parent;
Value = value;
children = new ConcurrentDictionary <string, TreeNode>();
if (DecideTrue(records))
{
return;
}
if (DecideFalse(records))
{
return;
}
DecideWithProbability(records);
splitAttribute = DecisionTree.GetBestAttribute(records, attributes);
if (IsLeafNode())
{
return;
}
BuildChildNodes(attributes, records, fracCertainty);
}
public string this[DiscreteAttribute attr]
{
get { return(this[attr.Index]); }
set { this[attr.Index] = value; }
}
static Tuple <bool /*significant*/, double> CalculateGain(List <Record> samplesSet, DiscreteAttribute attribute)
{
double entropyAfter = 0;
foreach (var @group in samplesSet.GroupBy(sample => sample[attribute]))
{
//if (attribute.Values.Contains(@group.Key))
{
int groupTotals = @group.Count();
int groupPositives = CountPositiveExamples(@group);
entropyAfter += CalculateEntropy(groupPositives, groupTotals - groupPositives) * groupTotals /
samplesSet.Count;
}
}
int positives = CountPositiveExamples(samplesSet);
double entropyBefore = CalculateEntropy(positives, samplesSet.Count - positives);
return(new Tuple <bool, double>(entropyAfter < entropyBefore, entropyBefore - entropyAfter));
}
