Neural Networks and Deep Learning（week4）Building your Deep Neural Network: Step by Step

Building your Deep Neural Network: Step by Step

• 你将使用下面函数来构建一个深层神经网络来实现图像分类。
• 使用像relu这的非线性单元来改进你的模型
• 构建一个多隐藏层的神经网络（有超过一个隐藏层）

1 - Packages(导入的包)

• numpy：进行科学计算的包
• matplotlib ：绘图包
• dnn_utils：提供一些必要功能
• testCases 提供一些测试用例来评估函数的正确性
• np.random.seed(1) 设置随机数种子，易于测试。

import numpy as np

import h5py

import matplotlib.pyplot as plt

from testCases_v2 import *

from dnn_utils_v2 import sigmoid, sigmoid_backward, relu, relu_backward

%matplotlib inline

plt.rcParams['figure.figsize'] = (5.0, 4.0) # 设置最大图像大小

plt.rcParams['image.interpolation'] = 'nearest'

plt.rcParams['image.cmap'] = 'gray'

%load_ext autoreload

%autoreload 2

np.random.seed(1)

保存在本地

# TODO: 保存在dnn_utils.py

import numpy as np

def sigmoid(Z):

    """

    Implements the sigmoid activation in numpy

    Arguments:

    Z -- numpy array of any shape

    Returns:

    A -- output of sigmoid(z), same shape as Z

    cache -- returns Z as well, useful during backpropagation

    """

    A = 1/(1+np.exp(-Z))

    cache = Z

    return A, cache

def relu(Z):

    """

    Implement the RELU function.

    Arguments:

    Z -- Output of the linear layer, of any shape

    Returns:

    A -- Post-activation parameter, of the same shape as Z

    cache -- a python dictionary containing "A" ; stored for computing the backward pass efficiently

    """

    A = np.maximum(0,Z)

    assert(A.shape == Z.shape)

    cache = Z

    return A, cache

def relu_backward(dA, cache):

    """

    Implement the backward propagation for a single RELU unit.

    Arguments:

    dA -- post-activation gradient, of any shape

    cache -- 'Z' where we store for computing backward propagation efficiently

    Returns:

    dZ -- Gradient of the cost with respect to Z

    """

    Z = cache

    dZ = np.array(dA, copy=True) # just converting dz to a correct object.

    # When z <= 0, you should set dz to 0 as well.

    dZ[Z <= 0] = 0

    assert (dZ.shape == Z.shape)

    return dZ

def sigmoid_backward(dA, cache):

    """

    Implement the backward propagation for a single SIGMOID unit.

    Arguments:

    dA -- post-activation gradient, of any shape

    cache -- 'Z' where we store for computing backward propagation efficiently

    Returns:

    dZ -- Gradient of the cost with respect to Z

    """

    Z = cache

    s = 1/(1+np.exp(-Z))

    dZ = dA * s * (1-s)

    assert (dZ.shape == Z.shape)

    return dZ

# TODO: testCases.py

import numpy as np

def linear_forward_test_case():

    np.random.seed(1)

    """

    X = np.array([[-1.02387576, 1.12397796],

 [-1.62328545, 0.64667545],

 [-1.74314104, -0.59664964]])

    W = np.array([[ 0.74505627, 1.97611078, -1.24412333]])

    b = np.array([[1]])

    """

    A = np.random.randn(3,2)

    W = np.random.randn(1,3)

    b = np.random.randn(1,1)

    return A, W, b

def linear_activation_forward_test_case():

    """

    X = np.array([[-1.02387576, 1.12397796],

 [-1.62328545, 0.64667545],

 [-1.74314104, -0.59664964]])

    W = np.array([[ 0.74505627, 1.97611078, -1.24412333]])

    b = 5

    """

    np.random.seed(2)

    A_prev = np.random.randn(3,2)

    W = np.random.randn(1,3)

    b = np.random.randn(1,1)

    return A_prev, W, b

def L_model_forward_test_case():

    """

    X = np.array([[-1.02387576, 1.12397796],

 [-1.62328545, 0.64667545],

 [-1.74314104, -0.59664964]])

    parameters = {'W1': np.array([[ 1.62434536, -0.61175641, -0.52817175],

        [-1.07296862,  0.86540763, -2.3015387 ]]),

 'W2': np.array([[ 1.74481176, -0.7612069 ]]),

 'b1': np.array([[ 0.],

        [ 0.]]),

 'b2': np.array([[ 0.]])}

    """

    np.random.seed(1)

    X = np.random.randn(4,2)

    W1 = np.random.randn(3,4)

    b1 = np.random.randn(3,1)

    W2 = np.random.randn(1,3)

    b2 = np.random.randn(1,1)

    parameters = {"W1": W1,

                  "b1": b1,

                  "W2": W2,

                  "b2": b2}

    return X, parameters

def compute_cost_test_case():

    Y = np.asarray([[1, 1, 1]])

    aL = np.array([[.8,.9,0.4]])

    return Y, aL

def linear_backward_test_case():

    """

    z, linear_cache = (np.array([[-0.8019545 ,  3.85763489]]), (np.array([[-1.02387576,  1.12397796],

       [-1.62328545,  0.64667545],

       [-1.74314104, -0.59664964]]), np.array([[ 0.74505627,  1.97611078, -1.24412333]]), np.array([[1]]))

    """

    np.random.seed(1)

    dZ = np.random.randn(1,2)

    A = np.random.randn(3,2)

    W = np.random.randn(1,3)

    b = np.random.randn(1,1)

    linear_cache = (A, W, b)

    return dZ, linear_cache

def linear_activation_backward_test_case():

    """

    aL, linear_activation_cache = (np.array([[ 3.1980455 ,  7.85763489]]), ((np.array([[-1.02387576,  1.12397796], [-1.62328545,  0.64667545], [-1.74314104, -0.59664964]]), np.array([[ 0.74505627,  1.97611078, -1.24412333]]), 5), np.array([[ 3.1980455 ,  7.85763489]])))

    """

    np.random.seed(2)

    dA = np.random.randn(1,2)

    A = np.random.randn(3,2)

    W = np.random.randn(1,3)

    b = np.random.randn(1,1)

    Z = np.random.randn(1,2)

    linear_cache = (A, W, b)

    activation_cache = Z

    linear_activation_cache = (linear_cache, activation_cache)

    return dA, linear_activation_cache

def L_model_backward_test_case():

    """

    X = np.random.rand(3,2)

    Y = np.array([[1, 1]])

    parameters = {'W1': np.array([[ 1.78862847,  0.43650985,  0.09649747]]), 'b1': np.array([[ 0.]])}

    aL, caches = (np.array([[ 0.60298372,  0.87182628]]), [((np.array([[ 0.20445225,  0.87811744],

           [ 0.02738759,  0.67046751],

           [ 0.4173048 ,  0.55868983]]),

    np.array([[ 1.78862847,  0.43650985,  0.09649747]]),

    np.array([[ 0.]])),

   np.array([[ 0.41791293,  1.91720367]]))])

   """

    np.random.seed(3)

    AL = np.random.randn(1, 2)

    Y = np.array([[1, 0]])

    A1 = np.random.randn(4,2)

    W1 = np.random.randn(3,4)

    b1 = np.random.randn(3,1)

    Z1 = np.random.randn(3,2)

    linear_cache_activation_1 = ((A1, W1, b1), Z1)

    A2 = np.random.randn(3,2)

    W2 = np.random.randn(1,3)

    b2 = np.random.randn(1,1)

    Z2 = np.random.randn(1,2)

    linear_cache_activation_2 = ( (A2, W2, b2), Z2)

    caches = (linear_cache_activation_1, linear_cache_activation_2)

    return AL, Y, caches

def update_parameters_test_case():

    """

    parameters = {'W1': np.array([[ 1.78862847,  0.43650985,  0.09649747],

        [-1.8634927 , -0.2773882 , -0.35475898],

        [-0.08274148, -0.62700068, -0.04381817],

        [-0.47721803, -1.31386475,  0.88462238]]),

 'W2': np.array([[ 0.88131804,  1.70957306,  0.05003364, -0.40467741],

        [-0.54535995, -1.54647732,  0.98236743, -1.10106763],

        [-1.18504653, -0.2056499 ,  1.48614836,  0.23671627]]),

 'W3': np.array([[-1.02378514, -0.7129932 ,  0.62524497],

        [-0.16051336, -0.76883635, -0.23003072]]),

 'b1': np.array([[ 0.],

        [ 0.],

        [ 0.],

        [ 0.]]),

 'b2': np.array([[ 0.],

        [ 0.],

        [ 0.]]),

 'b3': np.array([[ 0.],

        [ 0.]])}

    grads = {'dW1': np.array([[ 0.63070583,  0.66482653,  0.18308507],

        [ 0.        ,  0.        ,  0.        ],

        [ 0.        ,  0.        ,  0.        ],

        [ 0.        ,  0.        ,  0.        ]]),

 'dW2': np.array([[ 1.62934255,  0.        ,  0.        ,  0.        ],

        [ 0.        ,  0.        ,  0.        ,  0.        ],

        [ 0.        ,  0.        ,  0.        ,  0.        ]]),

 'dW3': np.array([[-1.40260776,  0.        ,  0.        ]]),

 'da1': np.array([[ 0.70760786,  0.65063504],

        [ 0.17268975,  0.15878569],

        [ 0.03817582,  0.03510211]]),

 'da2': np.array([[ 0.39561478,  0.36376198],

        [ 0.7674101 ,  0.70562233],

        [ 0.0224596 ,  0.02065127],

        [-0.18165561, -0.16702967]]),

 'da3': np.array([[ 0.44888991,  0.41274769],

        [ 0.31261975,  0.28744927],

        [-0.27414557, -0.25207283]]),

 'db1': 0.75937676204411464,

 'db2': 0.86163759922811056,

 'db3': -0.84161956022334572}

    """

    np.random.seed(2)

    W1 = np.random.randn(3,4)

    b1 = np.random.randn(3,1)

    W2 = np.random.randn(1,3)

    b2 = np.random.randn(1,1)

    parameters = {"W1": W1,

                  "b1": b1,

                  "W2": W2,

                  "b2": b2}

    np.random.seed(3)

    dW1 = np.random.randn(3,4)

    db1 = np.random.randn(3,1)

    dW2 = np.random.randn(1,3)

    db2 = np.random.randn(1,1)

    grads = {"dW1": dW1,

             "db1": db1,

             "dW2": dW2,

             "db2": db2}

    return parameters, grads

2 - 任务概要

• 双隐藏层 和 L层神经网络 的 参数初始化。
• 实现前向传播操作（forward propagation） 。计算 损失函数。
• 完成 层的 前向传播 的 线性部分。(计算出 Z = WX + b) 。
• 使用 relu 和 sigmod 激活函数计算结果值。
• 将前两个步骤组合成一个新的前向函数（线性->激活） [LINEAR->ACTIVATION]
• 对输出层之前的 L-1 层，做 L-1 次 的 前向传播 [LINEAR->RELU] ，L层输出层的 激活函数 为 sigmod
• 实现 后向传播操作 模块（在下图中用红色表示）。最后更新参数。
• 计算神经网络 反向传播的 LINEAR 部分。
• 计算 激活函数 （Relu 或者 sigmod）的 梯度。
• 综合前两个步骤，产生一个新的后向函数【Liner --> Activation】
• 更新参数

注意，前向函数和反向函数相对应。前向传播的每一步都将反向传播用的到值存储在cache。cache中值对于计算梯度非常有用。

3 - Initialization（初始化）

为你的模型编写函数初始化参数。第一个函数将用于 初始化两层模型 的参数。第二个函数用于 初始化 L层模型 的参数。

3.1 - 2-layer Neural Network (双隐藏层神经网络)

Exercise: 创建和初始化 2层神经网络 的参数.

Instructions:

• 模型结果： LINEAR -> RELU -> LINEAR -> SIGMOID.
• 使用 随机初始化 权重矩阵。用 np.random.randn(shape)*0.01 用正确的shape。
• 使用 0 初始化偏差。用 np.zeros(shape)。

# GRADED FUNCTION: initialize_parameters

def initialize_parameters(n_x, n_h, n_y):

    """

    Argument:

    n_x -- size of the input layer

    n_h -- size of the hidden layer

    n_y -- size of the output layer

    Returns:

    parameters -- python dictionary containing your parameters:

                    W1 -- weight matrix of shape (n_h, n_x)

                    b1 -- bias vector of shape (n_h, 1)

                    W2 -- weight matrix of shape (n_y, n_h)

                    b2 -- bias vector of shape (n_y, 1)

    """

    np.random.seed(1)

    ### START CODE HERE ### (≈ 4 lines of code)

    W1 = np.random.randn(n_h, n_x)*0.01

    b1 = np.zeros((n_h, 1))

    W2 = np.random.randn(n_y, n_h)*0.01

    b2 = np.zeros((n_y, 1))

    ### END CODE HERE ###

    assert(W1.shape == (n_h, n_x))

    assert(b1.shape == (n_h, 1))

    assert(W2.shape == (n_y, n_h))

    assert(b2.shape == (n_y, 1))

    parameters = {"W1": W1,

                  "b1": b1,

                  "W2": W2,

                  "b2": b2}

    return parameters    

parameters = initialize_parameters(3,2,1)

print("W1 = " + str(parameters["W1"]))

print("b1 = " + str(parameters["b1"]))

print("W2 = " + str(parameters["W2"]))

print("b2 = " + str(parameters["b2"]))

W1 = [[ 0.01624345 -0.00611756 -0.00528172]

 [-0.01072969  0.00865408 -0.02301539]]

b1 = [[ 0.]

 [ 0.]]

W2 = [[ 0.01744812 -0.00761207]]

b2 = [[ 0.]]

Expected output:

W1	[[ 0.01624345 -0.00611756 -0.00528172] [-0.01072969 0.00865408 -0.02301539]]
b1	[[ 0.] [ 0.]]
W2	[[ 0.01744812 -0.00761207]]
b2	[[ 0.]]

3.2 - L-layer Neural Network（L-层隐藏层神经网络）

当完成 initialize_parameters_deep 时，你应该确保每个层之间的维度匹配。n^l 是 L层中单位数。如，输入X，size = (12288, 209)（有m=209个样本)：

Exercise: 实现 L层神经网络的 初始化。

Instructions:

• 模型结构：[LINEAR -> RELU] × (L-1) --> LINEAR -> SIGMOID. , 所以 L-1 层是需要用 ReLu激活函数，输出层是用 sigmod函数。
• 权重矩阵采用 随机初始化的 方式：用 np.random.randn(shape) * 0.01.
• 偏移矩阵仍是 0 矩阵进行初始化：用 np.zeros(shape).
• 我们将每层神经元数量信息进行存储，layer_dims。例如，在平面数据分类模型中 layer_dims 的值是 [2, 4, 1]：
  • 其中 输入层的神经元个数是2，一个隐藏层的神经元个数是 4，输出层的神经元个数是1。
  • 对应 W1.shape = (4, 2),  b1.shape = (1, 4)， W2.shape = (1, 4),  b2.shape = (1, 1)。
• 下面是实现 L=1 层神经网络：

  if L == 1:

      parameters["W" + str(L)] = np.random.randn(layer_dims[1], layer_dims[0]) * 0.01

      parameters["b" + str(L)] = np.zeros((layer_dims[1], 1))

• L 层神经网络实现方式（参数初始化）：

# GRADED FUNCTION: initialize_parameters_deep

def initialize_parameters_deep(layer_dims):

    """

    Arguments:

    layer_dims -- python array (list) containing the dimensions of each layer in our network

    Returns:

    parameters -- python dictionary containing your parameters "W1", "b1", ..., "WL", "bL":

                    Wl -- weight matrix of shape (layer_dims[l], layer_dims[l-1])

                    bl -- bias vector of shape (layer_dims[l], 1)

    """

    np.random.seed(3)

    parameters = {}

    L = len(layer_dims)            # number of layers in the network

    for l in range(1, L):

        ### START CODE HERE ### (≈ 2 lines of code)

        parameters['W' + str(l)] = np.random.randn(layer_dims[l], layer_dims[l - 1]) * 0.01

        parameters['b' + str(l)] = np.zeros((layer_dims[l], 1))

        ### END CODE HERE ###

        assert(parameters['W' + str(l)].shape == (layer_dims[l], layer_dims[l-1]))

        assert(parameters['b' + str(l)].shape == (layer_dims[l], 1))

    return parameters

parameters = initialize_parameters_deep([5,4,3])

print("W1 = " + str(parameters["W1"]))

print("b1 = " + str(parameters["b1"]))

print("W2 = " + str(parameters["W2"]))

print("b2 = " + str(parameters["b2"]))

Expected output:

W1	[[ 0.01788628 0.0043651 0.00096497 -0.01863493 -0.00277388] [-0.00354759 -0.00082741 -0.00627001 -0.00043818 -0.00477218] [-0.01313865 0.00884622 0.00881318 0.01709573 0.00050034] [-0.00404677 -0.0054536 -0.01546477 0.00982367 -0.01101068]]
b1	[[ 0.] [ 0.] [ 0.] [ 0.]]
W2	[[-0.01185047 -0.0020565 0.01486148 0.00236716] [-0.01023785 -0.00712993 0.00625245 -0.00160513] [-0.00768836 -0.00230031 0.00745056 0.01976111]]
b2	[[ 0.] [ 0.] [ 0.]]

4 - Forward propagation module(前向传播模型)

4.1 - Linear Forward

前向传播的过程，先计算如下的线性部分：。其中，

Exercise: 建立前向传播的线性部分。

# GRADED FUNCTION: linear_forward

def linear_forward(A, W, b):

    """

    Implement the linear part of a layer's forward propagation.

    Arguments:

    A -- activations from previous layer (or input data): (size of previous layer, number of examples)

    W -- weights matrix: numpy array of shape (size of current layer, size of previous layer)

    b -- bias vector, numpy array of shape (size of the current layer, 1)

    Returns:

    Z -- the input of the activation function, also called pre-activation parameter

    cache -- a python dictionary containing "A", "W" and "b" ; stored for computing the backward pass efficiently

    """

    ### START CODE HERE ### (≈ 1 line of code)

    Z = np.dot(W, A) + b

#     print("W: ", W.shape)

#     print("A: ", A.shape)

#     print("b: ", b.shape)

    ### END CODE HERE ###

    assert(Z.shape == (W.shape[0], A.shape[1]))

    cache = (A, W, b)

    return Z, cache

A, W, b = linear_forward_test_case()

Z, linear_cache = linear_forward(A, W, b)

print("Z = " + str(Z))

Expected output:

Z	[[ 3.26295337 -1.23429987]]

4.2 - 激活函数（相邻两层的激活实现）

你要使用的两个激励函数：

Exercise: 实现前向传播（LINEAR->ACTIVATION layer）。数学公式是：，激励函数“g”是 sigmod 或者 relu()。使用 linear_forward()  和 正确的 激励函数。

//预先实现的 sigmod 和 relu 函数

import numpy as np

def sigmoid(Z):

    """n

    Implements the sigmoid activation in numpy

    Arguments:

    Z -- numpy array of any shape

    Returns:

    A -- output of sigmoid(z), same shape as Z

    cache -- returns Z as well, useful during backpropagation

    """

    A = 1/(1+np.exp(-Z))

    cache = Z

    return A, cache

def relu(Z):

    """

    Implement the RELU function.

    Arguments:

    Z -- Output of the linear layer, of any shape

    Returns:

    A -- Post-activation parameter, of the same shape as Z

    cache -- a python dictionary containing "A" ; stored for computing the backward pass efficiently

    """

    A = np.maximum(0,Z)

    assert(A.shape == Z.shape)

    cache = Z

    return A, cache

//linear_activation_forward()

# GRADED FUNCTION: linear_activation_forward

def linear_activation_forward(A_prev, W, b, activation):

    """

    Implement the forward propagation for the LINEAR->ACTIVATION layer

    Arguments:

    A_prev -- activations from previous layer (or input data): (size of previous layer, number of examples)

    W -- weights matrix: numpy array of shape (size of current layer, size of previous layer)

    b -- bias vector, numpy array of shape (size of the current layer, 1)

    activation -- the activation to be used in this layer, stored as a text string: "sigmoid" or "relu"

    Returns:

    A -- the output of the activation function, also called the post-activation value

    cache -- a python dictionary containing "linear_cache" and "activation_cache";

             stored for computing the backward pass efficiently

    """

    if activation == "sigmoid":

        # Inputs: "A_prev, W, b". Outputs: "A, activation_cache".

        ### START CODE HERE ### (≈ 2 lines of code)

        Z, linear_cache = linear_forward(A_prev, W, b)   # linear_cache：A_prev, W, b

        A, activation_cache = sigmoid(Z)                 # activation_cache：Z

        ### END CODE HERE ###

    elif activation == "relu":

        # Inputs: "A_prev, W, b". Outputs: "A, activation_cache".

        ### START CODE HERE ### (≈ 2 lines of code)

        Z, linear_cache = linear_forward(A_prev, W, b)

        A, activation_cache = relu(Z)

        ### END CODE HERE ###

    assert (A.shape == (W.shape[0], A_prev.shape[1]))

    cache = (linear_cache, activation_cache)

    return A, cache

A_prev, W, b = linear_activation_forward_test_case()

A, linear_activation_cache = linear_activation_forward(A_prev, W, b, activation = "sigmoid")

print("With sigmoid: A = " + str(A))

A, linear_activation_cache = linear_activation_forward(A_prev, W, b, activation = "relu")

print("With ReLU: A = " + str(A))

Expected output:

,

With sigmoid: A	[[ 0.96890023 0.11013289]]
With ReLU: A	[[ 3.43896131 0. ]]

d) L-Layer Model （L层模型）

[Linear -> Relu] x (L - 1) --> Linear--> Sigmod model

Exercise: 实现以上 前向传播模型

Instruction: AL：，AL有时候叫做：

Tips:

• 使用之前用的函数
• 使用循环重复 【Linear --> Relu】（L-1）次
• 不要忘记跟踪"cache"列表中的cache。添加 c 到 list。用 list.append(c).

# GRADED FUNCTION: L_model_forward

def L_model_forward(X, parameters):

    """

    Implement forward propagation for the [LINEAR->RELU]*(L-1)->LINEAR->SIGMOID computation

    Arguments:

    X -- data, numpy array of shape (input size, number of examples)

    parameters -- output of initialize_parameters_deep()

    Returns:

    AL -- last post-activation value

    caches -- list of caches containing:

                every cache of linear_activation_forward() (there are L-1 of them, indexed from 0 to L-1)

    """

    caches = []

    A = X

    L = len(parameters) // 2                  # number of layers in the neural network

    # Implement [LINEAR -> RELU]*(L-1). Add "cache" to the "caches" list.

    for l in range(1, L):

        A_prev = A

        ### START CODE HERE ### (≈ 2 lines of code)

        A, cache = linear_activation_forward(A_prev,

                                                     parameters["W" + str(l)],

                                                     parameters["b" + str(l)],

                                                     activation='relu')        # cache = (A W b, Z)

        caches.append(cache)

        ### END CODE HERE ###

    # Implement LINEAR -> SIGMOID. Add "cache" to the "caches" list.

    ### START CODE HERE ### (≈ 2 lines of code)

    AL, cache = linear_activation_forward(A,

                                              parameters["W" + str(L)],

                                              parameters["b" + str(L)],

                                              activation="sigmoid")

    caches.append(cache)

    ### END CODE HERE ###

    assert(AL.shape == (1,X.shape[1]))

    return AL, caches

X, parameters = L_model_forward_test_case()

AL, caches = L_model_forward(X, parameters)

print("AL = " + str(AL))

print("Length of caches list = " + str(len(caches)))

AL = [[ 0.17007265 0.2524272 ]]

Length of caches list = 2

5 - Cost function(代价函数)

Exercise: 计算交叉熵成本 cost J, 使用下面公式：

# GRADED FUNCTION: compute_cost

def compute_cost(AL, Y):

    """

    Implement the cost function defined by equation (7).

    Arguments:

    AL -- probability vector corresponding to your label predictions, shape (1, number of examples)

    Y -- true "label" vector (for example: containing 0 if non-cat, 1 if cat), shape (1, number of examples)

    Returns:

    cost -- cross-entropy cost

    """

    m = Y.shape[1]

    # Compute loss from aL and y.

    ### START CODE HERE ### (≈ 1 lines of code)

    cost = - (1 / m) * np.sum(np.multiply(Y, np.log(AL)) + np.multiply(1 - Y, np.log(1 - AL)))

    ### END CODE HERE ###

    cost = np.squeeze(cost)      # To make sure your cost's shape is what we expect (e.g. this turns [[17]] into 17).

    assert(cost.shape == ())

    return cost

Y, AL = compute_cost_test_case()

print("cost = " + str(compute_cost(AL, Y)))

Expected Output:

cost	0.41493159961539694

6 - Backward propagation module(反向传播模型)

• 反向传播用于计算损失函数相对于参数的梯度

Figure3：紫色部分：前向传播；红色部分：反向传播；

建立反向传播3个步骤：

• Linear backward
• Linear--> Activation backward (activation 计算Relu 或者sigmod的导数) 
• [Linear-->Relu] x (L-1) --> Linear --> Sigmod backward (整个模型)

6.1 - Linear backward (反向传播线性部分)

• 对 层，线性部分是：

（cache提供 tuple值 -- (A_prev, W, b）

Exercise: 使用上面三个公式实现反向传播的线性部分： linear_backward().

# GRADED FUNCTION: linear_backward

def linear_backward(dZ, cache):

    """

    Implement the linear portion of backward propagation for a single layer (layer l)

    Arguments:

    dZ -- Gradient of the cost with respect to the linear output (of current layer l)

    cache -- tuple of values (A_prev, W, b) coming from the forward propagation in the current layer

    Returns:

    dA_prev -- Gradient of the cost with respect to the activation (of the previous layer l-1), same shape as A_prev

    dW -- Gradient of the cost with respect to W (current layer l), same shape as W

    db -- Gradient of the cost with respect to b (current layer l), same shape as b

    """

    A_prev, W, b = cache

    m = A_prev.shape[1]

    ### START CODE HERE ### (≈ 3 lines of code)

    dW = (1 / m) * np.dot(dZ, A_prev.T)

    db = (1 / m ) * np.sum(dZ, axis=1, keepdims=True)

    dA_prev = np.dot(W.T, dZ)

    ### END CODE HERE ###

    assert (dA_prev.shape == A_prev.shape)

    assert (dW.shape == W.shape)

    assert (db.shape == b.shape)

    return dA_prev, dW, db

# Set up some test inputs

dZ, linear_cache = linear_backward_test_case()

dA_prev, dW, db = linear_backward(dZ, linear_cache)

print ("dA_prev = "+ str(dA_prev))

print ("dW = " + str(dW))

print ("db = " + str(db))

Expected Output:

dA_prev	[[ 0.51822968 -0.19517421] [-0.40506361 0.15255393] [ 2.37496825 -0.89445391]]
dW	[[-0.10076895 1.40685096 1.64992505]]
db	[[ 0.50629448]]

6.2 - Linear-Activation backward (求 dz；相邻两层的梯度实现dA_prev, dW, db)

使用: linear_backward 和.用于激励的后向步骤 linear_activation_backward.

为帮助你实现 linear_activation_backward, 我们提供两个反向函数：

• sigmoid_backward: 实现反向传播的sigmod单元。你可以使用：

dZ = sigmoid_backward(dA, activation_cache)  # activation_cache就是Z

• relu_backward: 实现反向传播的relu单元。你可以使用：

dZ = relu_backward(dA, activation_cache)

如果g(.) 是激励函数，sigmod_backward和relu_backward用来计算 。

Exercise: 实现反向传播( for the LINEAR->ACTIVATION layer.)的求导部分

//预先实现的sigmoid_backward和relu_backward

def relu_backward(dA, cache):

    """

    Implement the backward propagation for a single RELU unit.

    Arguments:

    dA -- post-activation gradient, of any shape

    cache -- 'Z' where we store for computing backward propagation efficiently

    Returns:

    dZ -- Gradient of the cost with respect to Z

    """

    Z = cache

    dZ = np.array(dA, copy=True) # just converting dz to a correct object. g'(z) = 1

    # When z <= 0, you should set dz to 0 as well.

    dZ[Z <= 0] = 0

    assert (dZ.shape == Z.shape)

    return dZ

def sigmoid_backward(dA, cache):

    """

    Implement the backward propagation for a single SIGMOID unit.

    Arguments:

    dA -- post-activation gradient, of any shape

    cache -- 'Z' where we store for computing backward propagation efficiently

    Returns:

    dZ -- Gradient of the cost with respect to Z

    """

    Z = cache

    s = 1/(1+np.exp(-Z))

    dZ = dA * s * (1-s)     # g'(z) = s * (1 - s)

    assert (dZ.shape == Z.shape)

    return dZ

 综合求 dz, dA_prev, dW, db

# GRADED FUNCTION: linear_activation_backward

def linear_activation_backward(dA, cache, activation):

    """

    Implement the backward propagation for the LINEAR->ACTIVATION layer.

    Arguments:

    dA -- post-activation gradient for current layer l

    cache -- tuple of values (linear_cache, activation_cache) we store for computing backward propagation efficiently

    activation -- the activation to be used in this layer, stored as a text string: "sigmoid" or "relu"

    Returns:

    dA_prev -- Gradient of the cost with respect to the activation (of the previous layer l-1), same shape as A_prev

    dW -- Gradient of the cost with respect to W (current layer l), same shape as W

    db -- Gradient of the cost with respect to b (current layer l), same shape as b

    """

    linear_cache, activation_cache = cache    # A_prev W b, Z

    if activation == "relu":

        ### START CODE HERE ### (≈ 2 lines of code)

        dZ = relu_backward(dA, activation_cache)             # activation_cache： Z

        dA_prev, dW, db = linear_backward(dZ, linear_cache)  # linear_cache： A_prev, W, b

        ### END CODE HERE ###

    elif activation == "sigmoid":

        ### START CODE HERE ### (≈ 2 lines of code)

        dZ = sigmoid_backward(dA, activation_cache)

        dA_prev, dW, db = linear_backward(dZ, linear_cache)

        ### END CODE HERE ###

    return dA_prev, dW, db

dAL, linear_activation_cache = linear_activation_backward_test_case()

dA_prev, dW, db = linear_activation_backward(dAL, linear_activation_cache, activation = "sigmoid")

print ("sigmoid:")

print ("dA_prev = "+ str(dA_prev))

print ("dW = " + str(dW))

print ("db = " + str(db) + "\n")

dA_prev, dW, db = linear_activation_backward(dAL, linear_activation_cache, activation = "relu")

print ("relu:")

print ("dA_prev = "+ str(dA_prev))

print ("dW = " + str(dW))

print ("db = " + str(db))

Expected output with sigmoid:

dA_prev	[[ 0.11017994 0.01105339] [ 0.09466817 0.00949723] [-0.05743092 -0.00576154]]
dW	[[ 0.10266786 0.09778551 -0.01968084]]
db	[[-0.05729622]]

 

Expected output with relu:

dA_prev	[[ 0.44090989 0. ] [ 0.37883606 0. ] [-0.2298228 0. ]]
dW	[[ 0.44513824 0.37371418 -0.10478989]]
db	[[-0.20837892]]

6.3 - L-Model Backward(L层模型)

在L_model_forward函数中每次迭代都存储了一个cache--(X, W, b, Z). 在 后向传播中，你将用到这些变量来计算 梯度。

在L_model_backward函数中，将遍历所有隐藏层，从L层开始。每一步中，你将使用 l 层的cache值中进行反向传播。如图：

初始化反向传播: 要通过这个网络进行反向传播，要知道输出是：。你的代码需要计算：。

使用下面公式：

dAL = - (np.divide(Y, AL) - np.divide(1 - Y, 1 - AL)) # derivative of cost with respect to AL.  -（Y / AL - (1 - Y) / (1 - AL)）

推导如下： 

前向传播：

反向传播：

然后，你可以用这个 dAL 来保持进行后向传播。现在，你可以使用 dAL 在 Linear-->Sigmod后向传播函数中（使用由L_model_forward函数产生的cache值）。然后，你不得不使用 一个循环来迭代每一层，使用Linear-->Relu后向传播函数。你应该存储每一个 dA, dW, db在 grad字典中，用下列公式：

例如：对于 ， 存储在 grad["dW3"].

Exercise: 实现后向传播 （[LINEAR->RELU] × (L-1) -> LINEAR -> SIGMOID model）

def L_model_backward(AL, Y, caches):

    """

    Implement the backward propagation for the [LINEAR->RELU] * (L-1) -> LINEAR -> SIGMOID group

    Arguments:

    AL -- probability vector, output of the forward propagation (L_model_forward())

    Y -- true "label" vector (containing 0 if non-cat, 1 if cat)

    caches -- list of caches containing:

                every cache of linear_activation_forward() with "relu" (there are (L-1) or them, indexes from 0 to L-2)

                the cache of linear_activation_forward() with "sigmoid" (there is one, index L-1)

    Returns:

    grads -- A dictionary with the gradients

             grads["dA" + str(l)] = ...

             grads["dW" + str(l)] = ...

             grads["db" + str(l)] = ...

    """

    grads = {}

    L = len(caches) # the number of layers

    m = AL.shape[1]

    Y = Y.reshape(AL.shape) # after this line, Y is the same shape as AL

    # Initializing the backpropagation

    dAL = - (np.divide(Y, AL) - np.divide(1 - Y, 1 - AL))

    # Lth layer (SIGMOID -> LINEAR) gradients. Inputs: "AL, Y, caches". Outputs: "grads["dAL"], grads["dWL"], grads["dbL"]

    current_cache = caches[L-1]

    grads["dA" + str(L-1)], grads["dW" + str(L)], grads["db" + str(L)] = linear_activation_backward(dAL, current_cache, activation = "sigmoid")

    for l in reversed(range(L-1)):

        # lth layer: (RELU -> LINEAR) gradients.

        current_cache = caches[l]

        dA_prev_temp, dW_temp, db_temp = linear_activation_backward(grads["dA" + str(l + 1)], current_cache, activation = "relu")

        grads["dA" + str(l)] = dA_prev_temp

        grads["dW" + str(l + 1)] = dW_temp

        grads["db" + str(l + 1)] = db_temp

    return grads

AL, Y_assess, caches = L_model_backward_test_case()

grads = L_model_backward(AL, Y_assess, caches)

print ("dW1 = "+ str(grads["dW1"]))

print ("db1 = "+ str(grads["db1"]))

print ("dA1 = "+ str(grads["dA1"]))

Expected Output

dW1	[[ 0.41010002 0.07807203 0.13798444 0.10502167] [ 0. 0. 0. 0. ] [ 0.05283652 0.01005865 0.01777766 0.0135308 ]]
db1	[[-0.22007063] [ 0. ] [-0.02835349]]
dA1	[[ 0.12913162 -0.44014127] [-0.14175655 0.48317296] [ 0.01663708 -0.05670698]]

6.4 - Update Parameters（更新参数）

在这个任务，使用梯度下降来更新参数：

（α是学习率，在更新参数后，存储他们在参数字典中。）

Exercise: 实现 update_parameters() 来更新参数。

Instructions： 在每一个 ，使用梯度下降来更新参数

# GRADED FUNCTION: update_parameters

def update_parameters(parameters, grads, learning_rate):

    """

    Update parameters using gradient descent

    Arguments:

    parameters -- python dictionary containing your parameters

    grads -- python dictionary containing your gradients, output of L_model_backward

    Returns:

    parameters -- python dictionary containing your updated parameters

                  parameters["W" + str(l)] = ...

                  parameters["b" + str(l)] = ...

    """

    L = len(parameters) // 2 # number of layers in the neural network

    # Update rule for each parameter. Use a for loop.

    ### START CODE HERE ### (≈ 3 lines of code)

    for l in range(L):

        parameters["W" + str(l+1)] = parameters["W" + str(l + 1)] - grads["dW" + str(l + 1)] * learning_rate

        parameters["b" + str(l+1)] = parameters["b" + str(l + 1)] - grads["db" + str(l + 1)] * learning_rate

    ### END CODE HERE ###

    return parameters

parameters, grads = update_parameters_test_case()

parameters = update_parameters(parameters, grads, 0.1)

print ("W1 = "+ str(parameters["W1"]))

print ("b1 = "+ str(parameters["b1"]))

print ("W2 = "+ str(parameters["W2"]))

print ("b2 = "+ str(parameters["b2"]))

Expected Output:

W1	[[-0.59562069 -0.09991781 -2.14584584 1.82662008] [-1.76569676 -0.80627147 0.51115557 -1.18258802] [-1.0535704 -0.86128581 0.68284052 2.20374577]]
b1	[[-0.04659241] [-1.28888275] [ 0.53405496]]
W2	[[-0.55569196 0.0354055 1.32964895]]
b2	[[-0.84610769]]