public NumericalTest(NumericalAttribute attr, double thresh)
: base(attr)
{
if (attr == null)
{
throw new ArgumentNullException();
}
this._threshold = thresh;
}
/// <summary>
/// Finds the best splitting test involving a numerical attribute
/// </summary>
/// <param name="testAttr"></param>
/// <param name="goalAttr"></param>
/// <returns></returns>
private TestScore BestSplitTest(NumericalAttribute testAttr, SymbolicAttribute goalAttr)
{
int testIndex = _attributeSet.IndexOf(testAttr);
int goalNbVal = goalAttr.NumOfValues;
int goalIndex = _attributeSet.IndexOf(goalAttr);
//frequencyLower (frequencyHigher) counts the number of items lower
//(higher) than the threshold for each goal value. In the beginning,
//frequencyLower is zeroed because the threshold is chosen small.
double[] freqLower = new double[goalNbVal];
double[] freqHigher = new double[goalNbVal];
for (int gvi = 0; gvi < goalNbVal; gvi++)
{
SymbolicTest valTest = new SymbolicTest(goalAttr,
new KnownSymbolicValue[] { new KnownSymbolicValue(gvi) });
freqHigher[gvi] = Split(valTest).ElementAt(1).Size();
}
//Those two variables hold sum of the elements of the corresponding array.
double freqLowerSum = 0.0d;
double freqHigherSum = (double)_items.Count;
List <TestGoalValue> tgv = new List <TestGoalValue>();
for (int i = 0; i < _items.Count; i++)
{
double testVal = ((KnownNumericalValue)(this._items[i].ValueOf(testIndex))).Value;
int goalVal = ((KnownSymbolicValue)(this._items[i].ValueOf(goalIndex))).IntValue;
tgv.Add(new TestGoalValue(testVal, goalVal));
}
tgv.Sort();
int goalValue, goalValueNew = tgv[0].GoalValue;
double testValue, testValueNew = tgv[0].TestValue;
double bestScore = 0.0d;
double bestThreshold = testValueNew;
for (int i = 1; i < _items.Count; i++)
{
testValue = testValueNew;
goalValue = goalValueNew;
testValueNew = tgv[i].TestValue;
goalValueNew = tgv[i].GoalValue;
freqLower[goalValue]++;
freqLowerSum++;
freqHigher[goalValue]--;
freqHigherSum--;
if (testValue != testValueNew)
{
double score = CalEntropy(goalAttr)
- (freqLowerSum / _items.Count) * Entropy.CalEntropy(freqLower)
- (freqHigherSum / _items.Count) * Entropy.CalEntropy(freqHigher);
if (score > bestScore)
{
bestScore = score;
bestThreshold = (testValue + testValueNew) / 2.0d;
}
}
}
return(new TestScore(new NumericalTest(testAttr, bestThreshold), bestScore));
}
