private void ComputeGradients(FeedForwardData feedForwardData)
{
//output gradients
int numberOfOutputNeurons = LayerStructure.numberOfOutputNodes;
_outputGradientMatrix = new double[numberOfOutputNeurons];
int outputLayerIndex = feedForwardData.LayerInputs.Count - 1;
var outputLayerInputs = feedForwardData.LayerOutputs[outputLayerIndex];
for (int i = 0; i < numberOfOutputNeurons; i++)
{
_outputGradientMatrix[i] = DerivedErrorMatrix[i] * _outputActivationFunction.DerivedActivationFunctionOutput(outputLayerInputs[i]);
}
_gradientMatrices.Add(_outputGradientMatrix);
//hidden layer gradients
int numberOfHiddenLayers = LayerStructure.HiddenLayerList.Count;
var forwardLayerGradients = _outputGradientMatrix;
for (int i = numberOfHiddenLayers; i > 0; i--)
{
var currentWeightMatrix = Matrix.Transpose(_weights[i]); //this was transposed in the original... why?!
var currentHiddenLayerInputs = feedForwardData.LayerInputs[i]; //this is for regression
//var currentHiddenLayerInputs = feedForwardData.LayerOutputs[i]; //this is for classification
int numberOfNeuronsInCurrentLayer = feedForwardData.LayerOutputs[i].GetLength(0);
int numberOfNeuronsInForwardLayer = feedForwardData.LayerOutputs[i + 1].GetLength(0);
var hiddenGradientMatrix = new double[numberOfNeuronsInCurrentLayer];
for (int j = 0; j < numberOfNeuronsInCurrentLayer; j++)
{
double weightGradientSum = 0.0;
for (int k = 0; k < numberOfNeuronsInForwardLayer; k++)
{
weightGradientSum += currentWeightMatrix[j, k] * forwardLayerGradients[k];
}
hiddenGradientMatrix[j] = weightGradientSum * _hiddenActivationFunction.DerivedActivationFunctionOutput(currentHiddenLayerInputs[j]);
}
_gradientMatrices.Add(hiddenGradientMatrix);
forwardLayerGradients = hiddenGradientMatrix;
}
}