private bool ShouldSplitOnAttributeAccordingToChiSquared(DataSetAttributeWithCounts attributeToSplitOn)
{
int positiveValuesGlobal = attributeToSplitOn.PossibleValueCounts.Values.Select(s => s.AppearWhenTrueCount).Sum();
int negativeValuesGlobal = attributeToSplitOn.PossibleValueCounts.Values.Select(s => s.AppearWhenFalseCount).Sum();
double chiTestValue = 0;
foreach (var possibleValueCounts in attributeToSplitOn.PossibleValueCounts)
{
string valueKey = possibleValueCounts.Key;
int positiveValuesLocal = possibleValueCounts.Value.AppearWhenTrueCount;
int negativeValuesLocal = possibleValueCounts.Value.AppearWhenFalseCount;
double weightFactor = 1.0 * (positiveValuesLocal + negativeValuesLocal) / (positiveValuesGlobal + negativeValuesGlobal);
double pExpected = positiveValuesGlobal * weightFactor;
double nExpected = negativeValuesGlobal * weightFactor;
double pActual = positiveValuesLocal;
double nActual = negativeValuesLocal;
double diffP = (pExpected - pActual);
double diffN = (nExpected - nActual);
double localChiTestValue = diffP * diffP / pExpected + diffN * diffN / nExpected;
chiTestValue += localChiTestValue;
}
double chiQuareCumulative = ChiSquaredUtils.CalculateChiSquareCDT(attributeToSplitOn.PossibleValueCounts.Count - 1, chiTestValue);
return(chiQuareCumulative >= ChiTestLimit);
}
public void D3(List <DataSetAttribute> attributes, List <DataSetValue> values)
{
// Check whether we even need to split or not
int totalTrueValues = values.Count(v => v.Output);
int totalFalseValues = values.Count(v => !v.Output);
if (totalFalseValues == 0 && totalTrueValues > 0)
{
_localValue = true;
return;
}
if (totalTrueValues == 0 && totalFalseValues > 0)
{
_localValue = false;
return;
}
// Can we split on attributes?
if (attributes.Count == 0)
{
// Can't split anymore. We'll decide on the most prevalent value
_localValue = totalTrueValues > totalFalseValues;
return;
}
// First, find the attribute with the highest "E"
List <DataSetAttributeWithCounts> e = CalculateEForAllAttributes(attributes, values);
DataSetAttributeWithCounts attributeWithMinEntropy = FindAttributeWithMinEntropy(e);
_attributeToSplitOn = attributeWithMinEntropy;
// Is it worth it to split on attributes
if (!ShouldSplitOnAttributeAccordingToChiSquared(attributeWithMinEntropy))
{
// Not worth it to split. We'll decide on the most prevalent value
_localValue = totalTrueValues > totalFalseValues;
return;
}
// Remove this attribute from the list of new attributes to create new subtrees
List <DataSetAttribute> newAttributes = attributes.Where(a => a.Name != attributeWithMinEntropy.Name).ToList();
// Split the values in many sets
_dictionaryOfSubTrees = new Dictionary <string, DecisionTreeLevel>(attributeWithMinEntropy.PossibleValues.Count);
var dictionaryOfValues = new Dictionary <string, List <DataSetValue> >();
foreach (var dataSetValue in values)
{
string value = dataSetValue.Values[attributeWithMinEntropy.ValueIndex];
DecisionTreeLevel localTreeLevel;
List <DataSetValue> localValues;
if (!_dictionaryOfSubTrees.TryGetValue(value, out localTreeLevel))
{
localTreeLevel = new DecisionTreeLevel(ChiTestLimit);
_dictionaryOfSubTrees[value] = localTreeLevel;
localValues = new List <DataSetValue>();
dictionaryOfValues[value] = localValues;
}
else
{
localValues = dictionaryOfValues[value];
}
localValues.Add(dataSetValue);
}
// Recursively run D3 on them
foreach (var decisionTree in _dictionaryOfSubTrees)
{
var localValues = dictionaryOfValues[decisionTree.Key];
decisionTree.Value.D3(newAttributes, localValues);
}
}
