python - 尝试理解 3 层神经网络中的梯度检查错误

Question

我正在用 Python 构建一个基本的 3 层神经网络。编写梯度函数后，我继续使用数值梯度对其进行梯度检查。在获得较大的相对差异后，我展开权重矩阵的两个梯度并并排比较它们。

Function Gradient      Numerical Gradient
-0.000968788380809     0.0
 0.0153540197907       0.0153540197889
-0.00584391679274     -0.00584391679048
-0.00490359558077     -0.00490359558514
-0.00171892592537     -0.0017189259216
 0.00913024106334      0.00913024106319
-0.0182154767069      -0.0182154767092
 0.0152611324409       0.01526113244
-0.00373505297372     -0.00373505297135
-0.00513225994728     -0.00513225994814
-0.00531954399401     -0.00531954399641
-0.0185748801227      -0.0185748801163
 0.00745186105851      0.00745186105267
 0.0134566626927       0.0134566626908
 0.0251548691426       0.0251548691388
 0.00609388350562      0.00609388350226
-0.00471176815719     -0.00471176815564
 0.0113580721225       0.0113580721228
 0.00465172663488      0.00465172663944
-0.0221326283708      -0.02213262837
 0.300007655583       -0.300007655583 <-diverges, corresponding to theta2
 0.155638694282       -0.15345321819
 0.147747817305       -0.149026829224
 0.150703152382       -0.172330417252
 0.156307235611       -0.116975643856
 0.136898763375       -0.170081036297
 0.0621121242042      -0.0621121242372
 0.0442762464937      -0.0187338352431
 0.0489123689979      -0.00938236375481
 0.0244392582651      -0.0465061209964
 0.0237741996575      -0.028319115235
 0.0313594790974      -0.0330473942922
 0.106306327946       -0.106306327941
 0.0348751481828      -0.0704775747806
 0.0303373211657      -0.0756744476749
 0.0633094699759      -0.0461971224763
 0.0524239030728      -0.0477244101571
 0.0633274024777      -0.0397657392082

 Relative Difference:
 6.61473694017

每个列表中的前 20 个元素对应于第一个权重矩阵的梯度，其余 18 个元素对应于第二个权重矩阵的梯度。从我所看到的来看，错误似乎发生在列表中的最后 18 个元素(以及 theta2 矩阵梯度)中，其中函数梯度开始与“正确”的数值梯度不同。这也导致 scipy.optimize.fmin_cg 给我以下内容:

警告:由于精度损失，不一定能实现所需的误差。

任何帮助将不胜感激!相关代码如下:

def sigmoid(z):
    return 1 / (1+np.exp(z))

def sigmoid_gradient(z):
    return sigmoid(z)*(1-sigmoid(z))

def randInitializeWeights(layer_in, layer_out):
    matrix = np.zeros((layer_out, 1 + layer_in))
    epsilon_init = 0.12
    matrix = np.random.rand(layer_out, 1+layer_in) * 2 * epsilon_init -epsilon_init
    return matrix

def gradient(theta, *args):

    X, y, num_inputs, num_hidden_units, num_labels, lamb = args

    m = len(X)

    theta1 = np.reshape(theta[0:(num_hidden_units*(num_inputs+1))],(num_hidden_units, (num_inputs+1)))
    theta2 = np.reshape(theta[(num_hidden_units*(num_inputs+1)):],(num_labels, num_hidden_units+1)) 

    theta1_grad = np.zeros(theta1.shape)
    theta2_grad = np.zeros(theta2.shape)

    delta1 = np.zeros(theta1.shape)
    delta2 = np.zeros(theta2.shape)

    for t in range(0, m):
        vec_y = np.zeros(num_labels)
        vec_y[y[t]] = 1
        vec_y = vec_y[:, np.newaxis]    

        #feedforward to compute all the neuron activations
        a_1 = np.r_[[1], X[t]]
        a_1 = a_1[:, np.newaxis]
        z_2 = np.dot(theta1, a_1)   
        a_2 = np.vstack([1, sigmoid(z_2)])  
        z_3 = np.dot(theta2, a_2)   
        a_3 = sigmoid(z_3)  

        #error for output nodes
        del3 = a_3 - vec_y     
        #error for hidden nodes
        del2 = np.multiply(np.dot(theta2.T, del3), sigmoid_gradient(np.vstack([1, z_2])))
        #remove bias unit
        del2 = del2[1:] 

        #accumulate gradient
        delta1 = delta1 + del2*a_1.T
        delta2 = delta2 + del3*a_2.T

    #no need to regularize the first column
    theta1_grad[:, 0] = (1/m)*delta1[:, 0]
    theta2_grad[:, 0] = (1/m)*delta2[:, 0]

    #regularize the rest
    theta1_grad[:, 1:] = ((1/m) * delta1[:, 1:]) + (lamb/m)*theta1[:, 1:]
    theta2_grad[:, 1:] = ((1/m) * delta2[:, 1:]) + (lamb/m)*theta2[:, 1:]

    #unroll
    grad = np.hstack([theta1_grad.ravel(), theta2_grad.ravel()])
    return grad


def gradientChecking(lamb):
    input_layer_size = 3
    hidden_layer_size = 5
    num_labels = 3
    m = 5

    theta1 = randInitializeWeights(input_layer_size, hidden_layer_size)
    theta2 = randInitializeWeights(hidden_layer_size, num_labels)

    X = np.random.rand(m, input_layer_size)
    y = np.array([1, 2, 0, 1, 2])

    nn_params = np.hstack([theta1.ravel(), theta2.ravel()])

    #calculate gradient with function
    grad = gradient(nn_params, X, y, input_layer_size, hidden_layer_size, num_labels, lamb)
    #calculate numerical gradient
    num_grad = computeNumericalGradient(lambda theta: computeCost(theta, X, y, input_layer_size, hidden_layer_size, num_labels, lamb), nn_params)

    print('Function Gradient', 'Numerical Gradient')
    for i in range(len(grad)):
        print(grad[i], num_grad[i])

    diff = np.linalg.norm(num_grad-grad)/np.linalg.norm(num_grad+grad)
    print('Relative Difference: ')
    print(diff)


def computeNumericalGradient(J, theta):
    numgrad = np.zeros(theta.shape)
    perturb = np.zeros(theta.shape)
    e = 0.0001

    for p in range(1, np.size(theta)):
        perturb[p] = e
        loss1 = J(theta - perturb)
        loss2 = J(theta + perturb)
        numgrad[p] = (loss2 - loss1) / (2*e)
        perturb[p] = 0
    return numgrad

Answer 1

您的 sigmoid 函数有错误。应该是这样的:

def sigmoid(z):
    return 1 / (1+np.exp(-z))

我对反向传播算法的实现有点困惑。我会在没有 for 循环的情况下完成此操作。

您没有发布您的computeCost，因此我对其进行了编程并检查了梯度。就我而言，两列是相等的:

('Function Gradient', 'Numerical Gradient')
(-0.0087363416123043425, 0.0)
(0.017468375248392107, 0.0174683752529603)
(-0.0016267134050363559, -0.0016267134039793518)
(0.0018882373947080224, 0.0018882373997719526)
(-0.0063531428795779391, -0.0063531428762253483)
(0.0029882213493977773, 0.0029882213481435826)
(0.014295787205089885, 0.014295787205131916)
(-0.026668095974979808, -0.026668095973736428)
(0.0043373799514851595, 0.0043373799440971084)
(0.0063740837472641377, 0.0063740837497050506)
(0.0027102260448642525, 0.0027102260435896142)
(0.0067009063282609839, 0.0067009063298151261)
(-0.0029645476578591843, -0.0029645476562478734)
(-0.012000477453137556, -0.012000477451756808)
(-0.020065071389262716, -0.020065071393293721)
(0.010308693441913186, 0.010308693438876304)
(-0.0015996484140612609, -0.0015996484115099463)
(-0.0086037766244218914, -0.0086037766244828617)
(-0.0099431361329477934, -0.0099431361344493041)
(0.0062574996404342166, 0.0062574996406716821)
(0.30213488769328123, 0.3021348876908192)
(0.14900524972537924, 0.14900524972549789)
(0.13305168538400619, 0.13305168538479961)
(0.16730920742910549, 0.16730920743279754)
(0.14245586995768528, 0.14245586995365045)
(0.15465244296463604, 0.15465244296519742)
(0.10813908901043021, 0.10813908900342284)
(0.040844058224880242, 0.04084405822446513)
(0.040566215206120269, 0.040566215204762557)
(0.036451467449020114, 0.036451467448905817)
(0.065664340475228455, 0.065664340476168093)
(0.070753692265581092, 0.07075369226283712)
(0.088651862157018618, 0.088651862166777562)
(0.028272897964677978, 0.028272897965031518)
(0.026876928049457398, 0.026876928049812676)
(0.056512225949437798, 0.056512225949933992)
(0.051775047342360533, 0.051775047342772496)
(0.025689087137289929, 0.025689087135294386)
Relative Difference: 
0.00878484310135

这是我的代码:

import numpy as np

def sigmoid(z):
    return 1 / (1+np.exp(-z))

def sigmoid_gradient(z):
    return sigmoid(z)*(1-sigmoid(z))

def randInitializeWeights(layer_in, layer_out):
    matrix = np.zeros((layer_out, 1 + layer_in))
    epsilon_init = 0.12
    matrix = np.random.rand(layer_out, 1+layer_in) * 2 * epsilon_init -epsilon_init
    return matrix

def gradient(theta, *args):

    X, y, num_inputs, num_hidden_units, num_labels, lamb = args

    m = len(X)

    y_bin = np.zeros((m, num_labels))

    for i in range(m):
        y_bin[i, y[i]] = 1

    theta1 = np.reshape(theta[0:(num_hidden_units*(num_inputs+1))],(num_hidden_units, (num_inputs+1)))  #5x4
    theta2 = np.reshape(theta[(num_hidden_units*(num_inputs+1)):],(num_labels, num_hidden_units+1))     #3x6

    theta1_grad = np.zeros(theta1.shape)
    theta2_grad = np.zeros(theta2.shape)

    delta1 = np.zeros(theta1.shape)
    delta2 = np.zeros(theta2.shape)


    #forward

    a_1 = np.hstack((np.ones((m, 1)), X))   #5x4

    z_2 = np.dot(a_1, theta1.transpose())   #5x5
    a_2 = sigmoid(z_2)                      #5x5

    a_2 = np.hstack((np.ones((m, 1)), a_2)) #5x6
    z_3 = np.dot(a_2, theta2.transpose())   #5x3

    h = sigmoid(z_3)                        #5x3


    #backward

    delta3 = h - y_bin                      #5x3
    delta2 = np.dot(delta3, theta2[:, 1:num_hidden_units+1]) * sigmoid_gradient(z_2) #5x5

    D1 = np.dot(delta2.transpose(), a_1)    #5x4
    D2 = np.dot(delta3.transpose(), a_2)    #3x6

    theta1_grad = D1/m      #5x4
    theta2_grad = D2/m      #3x6

    #regularization
    theta1_grad[:, 1:num_inputs+1] = theta1_grad[:, 1:num_inputs+1] +lamb/m*  theta1[:, 1:num_inputs+1]
    theta2_grad[:, 1:num_hidden_units+1] = theta2_grad[:, 1:num_hidden_units+1] +lamb/m*  theta2[:, 1:num_hidden_units+1]

    #unroll
    grad = np.hstack([theta1_grad.ravel(), theta2_grad.ravel()])
    return grad

def gradientChecking(lamb):
    input_layer_size = 3
    hidden_layer_size = 5
    num_labels = 3
    m = 5

    theta1 = randInitializeWeights(input_layer_size, hidden_layer_size)
    theta2 = randInitializeWeights(hidden_layer_size, num_labels)

    X = np.random.rand(m, input_layer_size)
    y = np.array([1, 2, 0, 1, 2])

    nn_params = np.hstack([theta1.ravel(), theta2.ravel()])

    #calculate gradient with function
    grad = gradient(nn_params, X, y, input_layer_size, hidden_layer_size, num_labels, lamb)
    #calculate numerical gradient
    num_grad = computeNumericalGradient(lambda theta: computeCost(theta, X, y, input_layer_size, hidden_layer_size, num_labels, lamb), nn_params)

    print('Function Gradient', 'Numerical Gradient')
    for i in range(len(grad)):
        print(grad[i], num_grad[i])

    diff = np.linalg.norm(num_grad-grad)/np.linalg.norm(num_grad+grad)
    print('Relative Difference: ')
    print(diff)

def computeCost(theta, X, y, num_inputs, num_hidden_units, num_labels, lamb):

    m = len(X)

    y_bin = np.zeros((m, num_labels))

    for i in range(m):
        y_bin[i, y[i]] = 1

    theta1 = np.reshape(theta[0:(num_hidden_units*(num_inputs+1))],(num_hidden_units, (num_inputs+1))) #5x4
    theta2 = np.reshape(theta[(num_hidden_units*(num_inputs+1)):],(num_labels, num_hidden_units+1)) #3x6

    a_1 = np.hstack((np.ones((m, 1)), X))   #5x4

    z_2 = np.dot(a_1, theta1.transpose())   #5x5
    a_2 = sigmoid(z_2)                      #5x5

    a_2 = np.hstack((np.ones((m, 1)), a_2)) #5x6
    z_3 = np.dot(a_2, theta2.transpose())   #5x3

    h = sigmoid(z_3)

    cost = np.sum(-y_bin * np.log(h)    -    (1-y_bin) * np.log(1-h))/m


    #regularization

    theta1_sq = theta1[:, 1:num_inputs+1] * theta1[:, 1:num_inputs+1];
    theta2_sq = theta2[:, 1:num_hidden_units+1] * theta2[:, 1:num_hidden_units+1];

    cost = cost + lamb/(2.0*m)*(np.sum(theta1_sq) + np.sum(theta2_sq))

    return cost

def computeNumericalGradient(J, theta):
    numgrad = np.zeros(theta.shape)
    perturb = np.zeros(theta.shape)
    e = 0.0001

    for p in range(1, np.size(theta)):
        perturb[p] = e
        loss1 = J(theta - perturb)
        loss2 = J(theta + perturb)

        numgrad[p] = (loss2 - loss1) / (2*e)
        perturb[p] = 0
    return numgrad


gradientChecking(1.0)