"""network2.py ~~~~~~~~~~~~~~ An improved version of network.py, implementing the stochastic gradient descent learning algorithm for a feedforward neural network. Improvements include the addition of the cross-entropy cost function, regularization, and better initialization of network weights. Note that I have focused on making the code simple, easily readable, and easily modifiable. It is not optimized, and omits many desirable features. """ #### Libraries # Standard library import json import random import sys # Third-party libraries import numpy as np #### Define the quadratic and cross-entropy cost functions class QuadraticCost(object): @staticmethod def fn(a, y): """Return the cost associated with an output ``a`` and desired output ``y``. """ return 0.5*np.linalg.norm(a-y)**2 @staticmethod def delta(z, a, y): """Return the error delta from the output layer.""" return (a-y) * sigmoid_prime(z) class CrossEntropyCost(object): @staticmethod def fn(a, y): """Return the cost associated with an output ``a`` and desired output ``y``. Note that np.nan_to_num is used to ensure numerical stability. In particular, if both ``a`` and ``y`` have a 1.0 in the same slot, then the expression (1-y)*np.log(1-a) returns nan. The np.nan_to_num ensures that that is converted to the correct value (0.0). """ return np.sum(np.nan_to_num(-y*np.log(a)-(1-y)*np.log(1-a))) @staticmethod def delta(z, a, y): """Return the error delta from the output layer. Note that the parameter ``z`` is not used by the method. It is included in the method's parameters in order to make the interface consistent with the delta method for other cost classes. """ return (a-y) #### Main Network class class Network(object): def __init__(self, sizes, cost=CrossEntropyCost): """The list ``sizes`` contains the number of neurons in the respective layers of the network. For example, if the list was [2, 3, 1] then it would be a three-layer network, with the first layer containing 2 neurons, the second layer 3 neurons, and the third layer 1 neuron. The biases and weights for the network are initialized randomly, using ``self.default_weight_initializer`` (see docstring for that method). """ self.num_layers = len(sizes) self.sizes = sizes self.default_weight_initializer() self.cost=cost def default_weight_initializer(self): """Initialize each weight using a Gaussian distribution with mean 0 and standard deviation 1 over the square root of the number of weights connecting to the same neuron. Initialize the biases using a Gaussian distribution with mean 0 and standard deviation 1. Note that the first layer is assumed to be an input layer, and by convention we won't set any biases for those neurons, since biases are only ever used in computing the outputs from later layers. """ self.biases = [np.random.randn(y, 1) for y in self.sizes[1:]] self.weights = [np.random.randn(y, x)/np.sqrt(x) for x, y in zip(self.sizes[:-1], self.sizes[1:])] def large_weight_initializer(self): """Initialize the weights using a Gaussian distribution with mean 0 and standard deviation 1. Initialize the biases using a Gaussian distribution with mean 0 and standard deviation 1. Note that the first layer is assumed to be an input layer, and by convention we won't set any biases for those neurons, since biases are only ever used in computing the outputs from later layers. This weight and bias initializer uses the same approach as in Chapter 1, and is included for purposes of comparison. It will usually be better to use the default weight initializer instead. """ self.biases = [np.random.randn(y, 1) for y in self.sizes[1:]] self.weights = [np.random.randn(y, x) for x, y in zip(self.sizes[:-1], self.sizes[1:])] def feedforward(self, a): """Return the output of the network if ``a`` is input.""" for b, w in zip(self.biases, self.weights): a = sigmoid(np.dot(w, a)+b) return a def SGD(self, training_data, epochs, mini_batch_size, eta, lmbda = 0.0, evaluation_data=None, monitor_evaluation_cost=False, monitor_evaluation_accuracy=False, monitor_training_cost=False, monitor_training_accuracy=False, early_stopping_n = 0): """Train the neural network using mini-batch stochastic gradient descent. The ``training_data`` is a list of tuples ``(x, y)`` representing the training inputs and the desired outputs. The other non-optional parameters are self-explanatory, as is the regularization parameter ``lmbda``. The method also accepts ``evaluation_data``, usually either the validation or test data. We can monitor the cost and accuracy on either the evaluation data or the training data, by setting the appropriate flags. The method returns a tuple containing four lists: the (per-epoch) costs on the evaluation data, the accuracies on the evaluation data, the costs on the training data, and the accuracies on the training data. All values are evaluated at the end of each training epoch. So, for example, if we train for 30 epochs, then the first element of the tuple will be a 30-element list containing the cost on the evaluation data at the end of each epoch. Note that the lists are empty if the corresponding flag is not set. """ # early stopping functionality: best_accuracy=1 training_data = list(training_data) n = len(training_data) if evaluation_data: evaluation_data = list(evaluation_data) n_data = len(evaluation_data) # early stopping functionality: best_accuracy=0 no_accuracy_change=0 evaluation_cost, evaluation_accuracy = [], [] training_cost, training_accuracy = [], [] for j in range(epochs): random.shuffle(training_data) mini_batches = [ training_data[k:k+mini_batch_size] for k in range(0, n, mini_batch_size)] for mini_batch in mini_batches: self.update_mini_batch( mini_batch, eta, lmbda, len(training_data)) print("Epoch %s training complete" % j) if monitor_training_cost: cost = self.total_cost(training_data, lmbda) training_cost.append(cost) print("Cost on training data: {}".format(cost)) if monitor_training_accuracy: accuracy = self.accuracy(training_data, convert=True) training_accuracy.append(accuracy) print("Accuracy on training data: {} / {}".format(accuracy, n)) if monitor_evaluation_cost: cost = self.total_cost(evaluation_data, lmbda, convert=True) evaluation_cost.append(cost) print("Cost on evaluation data: {}".format(cost)) if monitor_evaluation_accuracy: accuracy = self.accuracy(evaluation_data) evaluation_accuracy.append(accuracy) print("Accuracy on evaluation data: {} / {}".format(self.accuracy(evaluation_data), n_data)) # Early stopping: if early_stopping_n > 0: if accuracy > best_accuracy: best_accuracy = accuracy no_accuracy_change = 0 #print("Early-stopping: Best so far {}".format(best_accuracy)) else: no_accuracy_change += 1 if (no_accuracy_change == early_stopping_n): #print("Early-stopping: No accuracy change in last epochs: {}".format(early_stopping_n)) return evaluation_cost, evaluation_accuracy, training_cost, training_accuracy return evaluation_cost, evaluation_accuracy, \ training_cost, training_accuracy def update_mini_batch(self, mini_batch, eta, lmbda, n): """Update the network's weights and biases by applying gradient descent using backpropagation to a single mini batch. The ``mini_batch`` is a list of tuples ``(x, y)``, ``eta`` is the learning rate, ``lmbda`` is the regularization parameter, and ``n`` is the total size of the training data set. """ nabla_b = [np.zeros(b.shape) for b in self.biases] nabla_w = [np.zeros(w.shape) for w in self.weights] for x, y in mini_batch: delta_nabla_b, delta_nabla_w = self.backprop(x, y) nabla_b = [nb+dnb for nb, dnb in zip(nabla_b, delta_nabla_b)] nabla_w = [nw+dnw for nw, dnw in zip(nabla_w, delta_nabla_w)] self.weights = [(1-eta*(lmbda/n))*w-(eta/len(mini_batch))*nw for w, nw in zip(self.weights, nabla_w)] self.biases = [b-(eta/len(mini_batch))*nb for b, nb in zip(self.biases, nabla_b)] def backprop(self, x, y): """Return a tuple ``(nabla_b, nabla_w)`` representing the gradient for the cost function C_x. ``nabla_b`` and ``nabla_w`` are layer-by-layer lists of numpy arrays, similar to ``self.biases`` and ``self.weights``.""" nabla_b = [np.zeros(b.shape) for b in self.biases] nabla_w = [np.zeros(w.shape) for w in self.weights] # feedforward activation = x activations = [x] # list to store all the activations, layer by layer zs = [] # list to store all the z vectors, layer by layer for b, w in zip(self.biases, self.weights): z = np.dot(w, activation)+b zs.append(z) activation = sigmoid(z) activations.append(activation) # backward pass delta = (self.cost).delta(zs[-1], activations[-1], y) nabla_b[-1] = delta nabla_w[-1] = np.dot(delta, activations[-2].transpose()) # Note that the variable l in the loop below is used a little # differently to the notation in Chapter 2 of the book. Here, # l = 1 means the last layer of neurons, l = 2 is the # second-last layer, and so on. It's a renumbering of the # scheme in the book, used here to take advantage of the fact # that Python can use negative indices in lists. for l in range(2, self.num_layers): z = zs[-l] sp = sigmoid_prime(z) delta = np.dot(self.weights[-l+1].transpose(), delta) * sp nabla_b[-l] = delta nabla_w[-l] = np.dot(delta, activations[-l-1].transpose()) return (nabla_b, nabla_w) def accuracy(self, data, convert=False): """Return the number of inputs in ``data`` for which the neural network outputs the correct result. The neural network's output is assumed to be the index of whichever neuron in the final layer has the highest activation. The flag ``convert`` should be set to False if the data set is validation or test data (the usual case), and to True if the data set is the training data. The need for this flag arises due to differences in the way the results ``y`` are represented in the different data sets. In particular, it flags whether we need to convert between the different representations. It may seem strange to use different representations for the different data sets. Why not use the same representation for all three data sets? It's done for efficiency reasons -- the program usually evaluates the cost on the training data and the accuracy on other data sets. These are different types of computations, and using different representations speeds things up. More details on the representations can be found in mnist_loader.load_data_wrapper. """ if convert: results = [(np.argmax(self.feedforward(x)), np.argmax(y)) for (x, y) in data] else: results = [(np.argmax(self.feedforward(x)), y) for (x, y) in data] result_accuracy = sum(int(x == y) for (x, y) in results) return result_accuracy def total_cost(self, data, lmbda, convert=False): """Return the total cost for the data set ``data``. The flag ``convert`` should be set to False if the data set is the training data (the usual case), and to True if the data set is the validation or test data. See comments on the similar (but reversed) convention for the ``accuracy`` method, above. """ cost = 0.0 for x, y in data: a = self.feedforward(x) if convert: y = vectorized_result(y) cost += self.cost.fn(a, y)/len(data) cost += 0.5*(lmbda/len(data))*sum(np.linalg.norm(w)**2 for w in self.weights) # '**' - to the power of. return cost def save(self, filename): """Save the neural network to the file ``filename``.""" data = {"sizes": self.sizes, "weights": [w.tolist() for w in self.weights], "biases": [b.tolist() for b in self.biases], "cost": str(self.cost.__name__)} f = open(filename, "w") json.dump(data, f) f.close() #### Loading a Network def load(filename): """Load a neural network from the file ``filename``. Returns an instance of Network. """ f = open(filename, "r") data = json.load(f) f.close() cost = getattr(sys.modules[__name__], data["cost"]) net = Network(data["sizes"], cost=cost) net.weights = [np.array(w) for w in data["weights"]] net.biases = [np.array(b) for b in data["biases"]] return net #### Miscellaneous functions def vectorized_result(j): """Return a 10-dimensional unit vector with a 1.0 in the j'th position and zeroes elsewhere. This is used to convert a digit (0...9) into a corresponding desired output from the neural network. """ e = np.zeros((10, 1)) e[j] = 1.0 return e def sigmoid(z): """The sigmoid function.""" return 1.0/(1.0+np.exp(-z)) def sigmoid_prime(z): """Derivative of the sigmoid function.""" return sigmoid(z)*(1-sigmoid(z))