// HalfSquared Euclidean Distance
private double CalculateError(IMultilayerNeuralNetwork network, double[] expectedOutput)
{
double error = 0;
int layersCount = network.Layers.Length;
IActivationFunction function = network.Layers[0].Neurons[0].ActivationFunction;
ILayer layer = network.Layers.Last();
double[] errors = neuronErrors.Last();
for (int i = 0, n = layer.Neurons.Length; i < n; i++)
{
double output = layer.Neurons[i].Output;
double e = expectedOutput[i] - output;
errors[i] = e * function.ComputeDerivative2(output);
error += (e * e);
}
for (int j = layersCount - 2; j >= 0; j--)
{
layer = network.Layers[j];
ILayer layerNext = network.Layers[j + 1];
errors = neuronErrors[j];
double[] errorsNext = neuronErrors[j + 1];
for (int i = 0, n = layer.Neurons.Length; i < n; i++)
{
var sum = 0.0;
for (int k = 0; k < layerNext.Neurons.Length; k++)
{
sum += errorsNext[k] * layerNext.Neurons[k].Weights[i];
}
errors[i] = sum * function.ComputeDerivative2(layer.Neurons[i].Output);
}
}
return(error / 2.0);
}