Python神经网络 教你使用Python建立任意层数的深度神经网络

import numpy as np
import matplotlib.pyplot as plt
nn_architecture = [
{"layer_size": 4,"activation": "none"}, # input layer
{"layer_size": 5,"activation": "relu"},
{"layer_size": 4,"activation": "relu"},
{"layer_size": 3,"activation": "relu"},
{"layer_size": 1,"activation": "sigmoid"}
]
def initialize_parameters(nn_architecture, seed = 3):
np.random.seed(seed)
# python dictionary containingour parameters "W1", "b1", ..., "WL","bL"
parameters = {}
number_of_layers = len(nn_architecture)
for l in range(1,number_of_layers):
parameters['W' + str(l)] =np.random.randn(
nn_architecture[l]["layer_size"],
nn_architecture[l-1]["layer_size"]
) * 0.01
parameters['b' + str(l)] =np.zeros((nn_architecture[l]["layer_size"], 1))
return parameters

def sigmoid(Z):
    S = 1 / (1 + np.exp(-Z))
    return S

def relu(Z):
    R = np.maximum(0, Z)
    return R

def sigmoid_backward(dA, Z):
    S = sigmoid(Z)
    dS = S * (1 - S)
    return dA * dS

def relu_backward(dA, Z):
    dZ = np.array(dA, copy=True)
    dZ[Z <= 0] = 0
    return dZ

def L_model_forward(X, parameters, nn_architecture):
    forward_cache = {}
    A = X
    number_of_layers = len(nn_architecture)
    
for l in range(1, number_of_layers):
    A_prev = A
    W = parameters['W' + str(l)]
    b = parameters['b' + str(l)]
    activation = nn_architecture[l]["activation"]
    Z, A = linear_activation_forward(A_prev, W, b, activation)
    forward_cache['Z' + str(l)] = Z
    forward_cache['A' + str(l)] = A
    AL = A
return AL, forward_cache

def linear_activation_forward(A_prev, W, b, activation):
    if activation == "sigmoid":
        Z = linear_forward(A_prev, W, b)
        A = sigmoid(Z)
    elif activation == "relu":
        Z = linear_forward(A_prev, W, b)
        A = relu(Z)
    return Z, A

def linear_forward(A, W, b):
    Z = np.dot(W, A) + b
    return Z

def compute_cost(AL, Y):
    m = Y.shape[1]
    # Compute loss from AL and y
    logprobs = np.multiply(np.log(AL), Y) + np.multiply(1 - Y, np.log(1 - AL))
    # cross-entropy cost
    cost = - np.sum(logprobs) / m
    cost = np.squeeze(cost)
    return cost

def L_model_backward(AL, Y, parameters, forward_cache, nn_architecture):
    grads = {}
    number_of_layers =len(nn_architecture)
    m = AL.shape[1]
    Y = Y.reshape(AL.shape) # afterthis line, Y is the same shape as AL
    # Initializing thebackpropagation
    dAL = - (np.divide(Y, AL) -np.divide(1 - Y, 1 - AL))
    dA_prev = dAL
    for l in reversed(range(1,number_of_layers)):
        dA_curr = dA_prev
        activation =nn_architecture[l]["activation"]
        W_curr = parameters['W' +str(l)]
        Z_curr = forward_cache['Z' +str(l)]
        A_prev = forward_cache['A' +str(l-1)]
        dA_prev, dW_curr, db_curr =linear_activation_backward(dA_curr, Z_curr, A_prev, W_curr, activation)
        grads["dW" +str(l)] = dW_curr
        grads["db" +str(l)] = db_curr
    return grads
def linear_activation_backward(dA, Z, A_prev, W, activation):
    if activation =="relu":
        dZ = relu_backward(dA, Z)
        dA_prev, dW, db =linear_backward(dZ, A_prev, W)
    elif activation =="sigmoid":
        dZ = sigmoid_backward(dA, Z)
        dA_prev, dW, db =linear_backward(dZ, A_prev, W)
    return dA_prev, dW, db
def linear_backward(dZ, A_prev, W):
    m = A_prev.shape[1]
    dW = np.dot(dZ, A_prev.T) / m
    db = np.sum(dZ, axis=1,keepdims=True) / m
    dA_prev = np.dot(W.T, dZ)
    return dA_prev, dW, db

def update_parameters(parameters, grads, learning_rate):
    L = len(parameters)
    for l in range(1, L):
        parameters["W" +str(l)] = parameters["W" + str(l)] - learning_rate *grads["dW" + str(l)]
        parameters["b" +str(l)] = parameters["b" + str(l)] - learning_rate *grads["db" + str(l)]
    return parameters

def L_layer_model(X, Y, nn_architecture, learning_rate = 0.0075,num_iterations = 3000, print_cost=False):
    np.random.seed(1)
    # keep track of cost
    costs = []
    # Parameters initialization.
    parameters =initialize_parameters(nn_architecture)
    # Loop (gradient descent)
    for i in range(0,num_iterations):
    # Forward propagation:[LINEAR -> RELU]*(L-1) -> LINEAR -> SIGMOID.
        AL, forward_cache =L_model_forward(X, parameters, nn_architecture)
        # Compute cost.
        cost = compute_cost(AL, Y)
        # Backward propagation.
        grads = L_model_backward(AL,Y, parameters, forward_cache, nn_architecture)
        # Update parameters.
        parameters =update_parameters(parameters, grads, learning_rate)
        # Print the cost every 100training example
        if print_cost and i % 100 ==0:
            print("Cost afteriteration %i: %f" %(i, cost))
            costs.append(cost)
            # plot the cost
    plt.plot(np.squeeze(costs))
    plt.ylabel('cost')
    plt.xlabel('iterations (pertens)')
    plt.title("Learning rate=" + str(learning_rate))
    plt.show()
    return parameters