So, I am trying to implement a neural network in Python by only using NumPy. I have tried to do this by following 3Blue1Brown's video's about the topic, however, when testing my implementation, the network does not seem to work fully. I am testing the implementation on the AND, OR, and XOR problems - the AND and OR problems run as expected, however, the XOR problem does not:

Input    Output:
0, 0     0.253
1, 0     0.793
0, 1     0.793
1, 1     0.793    <- WRONG!

These results have been generated with a sample size of 1000 samples chosen randomly, 10 epochs, and a neural network with one hidden layer of two nodes, and the sigmoid function in both the hidden layer and in the output layer. If I try a different model, for instance with two hidden layers with each three nodes, I get much worse results:

Input:    Output:
0, 0      0.712 <- Wrong
1, 0      0.712
0, 1      0.712
1, 1      0.712 <- Wrong

However, for some reason a model with no hidden layers does not as bad:

Input:    Output:
0, 0      0.025
1, 0      0.999
0, 1      0.984
1, 1      0.945 <- Still wrong tho

I have tried different amounts of testing data, epochs, dimensions in the hidden layers, and hidden layers, and nothing seems to work

So, my question is if anyone has any idea why i get the wrong result in the XOR problem but not in the AND and the OR problems? And if so, how to fix it?

My implementation is as follows:

import numpy as np
from HiddenLayer import HiddenLayer
from InputLayer import InputLayer

np.random.seed(1) # Picking seed for debugging

class NeuralNetwork:
    def __init__(self, input_dimensions):
        self.n_layers = 1
        self.layers = np.array([InputLayer(input_dimensions)])

    def add_layer(self, dimensions):
        self.layers = np.append(self.layers, HiddenLayer(dimensions, self.layers[self.n_layers - 1].dimensions))
        self.n_layers += 1

    def sigmoid_derived(x):
        return np.multiply(x, 1.0 - x)

    def train(self, X, t, n_epoch, learning_rate = 1.0):
        def update_weight(l, j, k):
            self.layers[l].weights[j, k] -= learning_rate * self.layers[l - 1].a[k] * NeuralNetwork.sigmoid_derived(self.layers[l].a[j]) * self.layers[l].grad[j]

        def update_bias(l, j):
            self.layers[l].b[j] -= learning_rate * NeuralNetwork.sigmoid_derived(self.layers[l].a[j]) * self.layers[l].grad[j]

        for _ in range(n_epoch):
            for x, _t in zip(X, t):

                # Forwardpropagation

                # Backpropagation
                for l in reversed(range(1, self.n_layers)): # Iterates through each layer (except input layer)
                    for j in range(self.layers[l].dimensions): # Iterates through each node of layer l
                        self.layers[l].grad[j] = 0

                        for k in range(self.layers[l - 1].dimensions):
                            if (l == self.n_layers - 1): # If layer l is the output layer
                                self.layers[l].grad[j] = 1/len(t[0]) * 2 * (self.layers[l].a[j] - _t[j])
                                for _j in range(self.layers[l + 1].dimensions):
                                    self.layers[l].grad[j] += self.layers[l + 1].weights[_j, k] * NeuralNetwork.sigmoid_derived(self.layers[l + 1].a[_j]) * self.layers[l + 1].grad[_j]
                            update_weight(l, j, k)                                
                        update_bias(l, j)

    def _forwardprop(self, x):
        for l in range(1, self.n_layers):
            self.layers[l].feedforward(self.layers[l].weights, self.layers[l - 1].a)

    def predict(self, x):

        return self.layers[self.n_layers - 1].a
import numpy as np

class HiddenLayer:
    def __init__(self, dimensions, dim_from):
        self.dimensions = dimensions
        self.a = np.zeros((dimensions, 1))
        self.b = np.random.rand(dimensions, 1)
        self.grad = np.zeros((dimensions, 1))
        self.weights = np.random.rand(dimensions, dim_from)

    def feedforward(self, input_weights, input_a):
        for i in range(self.dimensions):
            self.a[i] = 1/(1 + np.exp(-1 * (np.matmul(input_weights[i], input_a) + self.b[i])))
import numpy as np

class InputLayer:
    def __init__(self, dimensions):
        self.dimensions = dimensions
        self.a = np.zeros((dimensions, 1))

    def feedforward(self, x):
        self.a = x

I know it is quit a bit of code - please let me know if there is anything you find ambiguous :)


1 Answer 1


1) It appears to be nearly working on you exemple 3. Not reaching exactly 0 or 1 happens because of the sigmoid function, that can only reach 0 and 1 asymptotically. For binary prediction, it is quite usual to put a cut-off on the output, here 0.5 will do : X<0.5 => 0 and X>=0.5 => 1.

2) As far as I remember this is in line with the theory : you don't need much neurons to reproduce XOR.

3) As to why more complex networks fail, I don't really know. To me your sigmoid derivative looks off. But if it's due to that, it wouldn't explain why it converged on the simpler model.

As a general introduction to practical use of neural nets, I would recomend efficient back-prop from LeCun (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwi33Lfiut7nAhWxyIUKHSSLABIQFjAAegQIBBAB&url=http%3A%2F%2Fyann.lecun.com%2Fexdb%2Fpublis%2Fpdf%2Flecun-98b.pdf&usg=AOvVaw26Rjav-SyKYp0gX-SskHTa).

  • $\begingroup$ Thanks you so much for your help! Unfortunately, example 3 actually does not nearly work - I have accidentally written that an input of 1, 1 returns 0.095, where in reality it actually returns 0.945 :(. $\endgroup$
    – KeenBit
    Feb 19, 2020 at 20:46

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