Implementing an Artificial Neural Network in Pure Java (No external dependencies)

nn.java

 /**
 *
 * @author Deus Jeraldy
 * @Email: deusjeraldy@gmail.com
 * BSD License
 */

// np.java -> https://gist.github.com/Jeraldy/7d4262db0536d27906b1e397662512bc

import java.util.Arrays;

public class NN {
  
    public static void main(String[] args) {
  
        double[][] X = {{0, 0}, {0, 1}, {1, 0}, {1, 1}};
        double[][] Y = {{0}, {1}, {1}, {0}};

        int m = 4;
        int nodes = 400;

        X = np.T(X);
        Y = np.T(Y);

        double[][] W1 = np.random(nodes, 2);
        double[][] b1 = new double[nodes][m];

        double[][] W2 = np.random(1, nodes);
        double[][] b2 = new double[1][m];

        for (int i = 0; i < 4000; i++) {
            // Foward Prop
            // LAYER 1
            double[][] Z1 = np.add(np.dot(W1, X), b1);
            double[][] A1 = np.sigmoid(Z1);

            //LAYER 2
            double[][] Z2 = np.add(np.dot(W2, A1), b2);
            double[][] A2 = np.sigmoid(Z2);

            double cost = np.cross_entropy(m, Y, A2);
            //costs.getData().add(new XYChart.Data(i, cost));
         
            // Back Prop
            //LAYER 2
            double[][] dZ2 = np.subtract(A2, Y);
            double[][] dW2 = np.divide(np.dot(dZ2, np.T(A1)), m);
            double[][] db2 = np.divide(dZ2, m);

            //LAYER 1
            double[][] dZ1 = np.multiply(np.dot(np.T(W2), dZ2), np.subtract(1.0, np.power(A1, 2)));
            double[][] dW1 = np.divide(np.dot(dZ1, np.T(X)), m);
            double[][] db1 = np.divide(dZ1, m);

            // G.D
            W1 = np.subtract(W1, np.multiply(0.01, dW1));
            b1 = np.subtract(b1, np.multiply(0.01, db1));

            W2 = np.subtract(W2, np.multiply(0.01, dW2));
            b2 = np.subtract(b2, np.multiply(0.01, db2));

            if (i % 400 == 0) {
                print("==============");
                print("Cost = " + cost);
                print("Predictions = " + Arrays.deepToString(A2));
            }
        }
    }
}

However, what is happening, is that the NN hasn't learned what I thought it was going to learn. When I try four inputs, no matter what I give it, it outputs the learned pattern:

test input=[[1.0, 0.0, 1.0, 1.0], [0.0, 1.0, 1.0, 1.0]]

Cost = 0.02185745791739911
Test Prediction = [[0.003408446482782547, 0.9872463542830693, 0.9452257141301419, 0.014738669159170737]]

test input=[[1.0, 1.0, 0.0, 0.0], [0.0, 1.0, 0.0, 1.0]]

Cost = 0.03372416671312117
Test Prediction = [[0.002095908947609844, 0.9235729681325365, 0.998439571534914, 0.05041614843514693]]

So I think the NN has "learned" to output the static pattern from the training data rather than perform an XOR operation on a single pair.

When I test with 3 pairs, it outputs the first three static results of the training data:
test input=[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0]]

Cost = 0.0018916002564269888
Test Prediction = [[0.00336438364815533, 0.9975507610865536, 0.9982574181545878]]
I can make each prediction wrong by inverting the test data from the training data.

It doesn't seem like the X inputs are making a difference, only the Y.