【深度學習】BP反向傳播算法Python簡單實現

  個人覺得BP反向傳播是深度學習的一個基礎，所以很有必要把反向傳播算法好好學一下
  得益於一步一步弄懂反向傳播的例子這篇文章，給出一個例子來說明反向傳播
  不過是英文的，如果你感覺不好閱讀的話，優秀的國人已經把它翻譯出來了。
  一步一步弄懂反向傳播的例子（中文翻譯）

  然後我使用了那個博客的圖片。這次的目的主要是對那個博客的一個補充。但是首先我覺得先用面向過程的思想來實現一遍感覺會好一點。隨便把文中省略的公式給大家給寫出來。大家可以先看那篇博文。

∂Etotal∂w5=∂Etotal∂outo1×∂outo1∂neto1×∂neto1∂w5∂Etotal∂w6=∂Etotal∂outo1×∂outo1∂neto1×∂neto1∂w6∂Etotal∂w7=∂Etotal∂outo2×∂outo2∂neto2×∂neto2∂w7∂Etotal∂w8=∂Etotal∂outo2×∂outo2∂neto2×∂neto2∂w8∂Etotal∂w1=∂Etotal∂outh1×∂outh1∂neth1×∂neth1∂w1∂Etotal∂w2=∂Etotal∂outh1×∂outh1∂neth1×∂neth1∂w2∂Etotal∂w3=∂Etotal∂outh2×∂outh2∂neth2×∂neth2∂w3∂Etotal∂w4=∂Etotal∂outh2×∂outh2∂neth2×∂neth2∂w4(1)(2)(3)(4)(5)(6)(7)(8)$$\begin{array}{}\text{(1)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_5}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{o1}}\times \frac{\mathrm{\partial }ou{t}_{o1}}{\mathrm{\partial }ne{t}_{o1}}\times \frac{\mathrm{\partial }ne{t}_{o1}}{\mathrm{\partial }{w}_5}\text{(2)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_6}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{o1}}\times \frac{\mathrm{\partial }ou{t}_{o1}}{\mathrm{\partial }ne{t}_{o1}}\times \frac{\mathrm{\partial }ne{t}_{o1}}{\mathrm{\partial }{w}_6}\text{(3)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_7}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{o2}}\times \frac{\mathrm{\partial }ou{t}_{o2}}{\mathrm{\partial }ne{t}_{o2}}\times \frac{\mathrm{\partial }ne{t}_{o2}}{\mathrm{\partial }{w}_7}\text{(4)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_8}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{o2}}\times \frac{\mathrm{\partial }ou{t}_{o2}}{\mathrm{\partial }ne{t}_{o2}}\times \frac{\mathrm{\partial }ne{t}_{o2}}{\mathrm{\partial }{w}_8}\text{(5)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_1}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{h1}}\times \frac{\mathrm{\partial }ou{t}_{h1}}{\mathrm{\partial }ne{t}_{h1}}\times \frac{\mathrm{\partial }ne{t}_{h1}}{\mathrm{\partial }{w}_1}\text{(6)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_2}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{h1}}\times \frac{\mathrm{\partial }ou{t}_{h1}}{\mathrm{\partial }ne{t}_{h1}}\times \frac{\mathrm{\partial }ne{t}_{h1}}{\mathrm{\partial }{w}_2}\text{(7)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_3}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{h2}}\times \frac{\mathrm{\partial }ou{t}_{h2}}{\mathrm{\partial }ne{t}_{h2}}\times \frac{\mathrm{\partial }ne{t}_{h2}}{\mathrm{\partial }{w}_3}\text{(8)}& & \frac{\mathrm{\partial }{E_t}_{otal}}{\mathrm{\partial }{w}_4}=\frac{\mathrm{\partial }{E}_{total}}{\mathrm{\partial }ou{t}_{h2}}\times \frac{\mathrm{\partial }ou{t}_{h2}}{\mathrm{\partial }ne{t}_{h2}}\times \frac{\mathrm{\partial }ne{t}_{h2}}{\mathrm{\partial }{w}_4}\end{array}$$

import numpy as np

# "pd" 偏導
def sigmoid(x):
    return 1 / (1 + np.exp(-x))
def sigmoidDerivationx(y):
    return y * (1 - y)

if __name__ == "__main__":
    #初始化
    bias = [0.35, 0.60]
    weight = [0.15, 0.2, 0.25, 0.3, 0.4, 0.45, 0.5, 0.55]
    output_layer_weights = [0.4, 0.45, 0.5, 0.55]
    i1 = 0.05
    i2 = 0.10
    target1 = 0.01
    target2 = 0.99
    alpha = 0.5 #學習速率
    numIter = 90000 #迭代次數
    for i in range(numIter):
        #正向傳播
        neth1 = i1*weight[1-1] + i2*weight[2-1] + bias[0]
        neth2 = i1*weight[3-1] + i2*weight[4-1] + bias[0]
        outh1 = sigmoid(neth1)
        outh2 = sigmoid(neth2)
        neto1 = outh1*weight[5-1] + outh2*weight[6-1] + bias[1]
        neto2 = outh2*weight[7-1] + outh2*weight[8-1] + bias[1]
        outo1 = sigmoid(neto1)
        outo2 = sigmoid(neto2)
        print(str(i) + ", target1 : " + str(target1-outo1) + ", target2 : " + str(target2-outo2))
        if i == numIter-1:
            print("lastst result : " + str(outo1) + " " + str(outo2))
        #反向傳播
        #計算w5-w8(輸出層權重)的誤差
        pdEOuto1 = - (target1 - outo1)
        pdOuto1Neto1 = sigmoidDerivationx(outo1)
        pdNeto1W5 = outh1
        pdEW5 = pdEOuto1 * pdOuto1Neto1 * pdNeto1W5
        pdNeto1W6 = outh2
        pdEW6 = pdEOuto1 * pdOuto1Neto1 * pdNeto1W6
        pdEOuto2 = - (target2 - outo2)
        pdOuto2Neto2 = sigmoidDerivationx(outo2)
        pdNeto1W7 = outh1
        pdEW7 = pdEOuto2 * pdOuto2Neto2 * pdNeto1W7
        pdNeto1W8 = outh2
        pdEW8 = pdEOuto2 * pdOuto2Neto2 * pdNeto1W8
        # 計算w1-w4(輸出層權重)的誤差
        pdEOuto1 = - (target1 - outo1) #之前算過
        pdEOuto2 = - (target2 - outo2)  #之前算過
        pdOuto1Neto1 = sigmoidDerivationx(outo1)    #之前算過
        pdOuto2Neto2 = sigmoidDerivationx(outo2)    #之前算過
        pdNeto1Outh1 = weight[5-1]
        pdNeto1Outh2 = weight[7-1]
        pdENeth1 = pdEOuto1 * pdOuto1Neto1 * pdNeto1Outh1 + pdEOuto2 * pdOuto2Neto2 * pdNeto1Outh2
        pdOuth1Neth1 = sigmoidDerivationx(outh1)
        pdNeth1W1 = i1
        pdNeth1W2 = i2
        pdEW1 = pdENeth1 * pdOuth1Neth1 * pdNeth1W1
        pdEW2 = pdENeth1 * pdOuth1Neth1 * pdNeth1W2
        pdNeto1Outh2 = weight[6-1]
        pdNeto2Outh2 = weight[8-1]
        pdOuth2Neth2 = sigmoidDerivationx(outh2)
        pdNeth1W3 = i1
        pdNeth1W4 = i2
        pdENeth2 = pdEOuto1 * pdOuto1Neto1 * pdNeto1Outh2 + pdEOuto2 * pdOuto2Neto2 * pdNeto2Outh2
        pdEW3 = pdENeth2 * pdOuth2Neth2 * pdNeth1W3
        pdEW4 = pdENeth2 * pdOuth2Neth2 * pdNeth1W4
        #權重更新
        weight[1-1] = weight[1-1] - alpha * pdEW1
        weight[2-1] = weight[2-1] - alpha * pdEW2
        weight[3-1] = weight[3-1] - alpha * pdEW3
        weight[4-1] = weight[4-1] - alpha * pdEW4
        weight[5-1] = weight[5-1] - alpha * pdEW5
        weight[6-1] = weight[6-1] - alpha * pdEW6
        weight[7-1] = weight[7-1] - alpha * pdEW7
        weight[8-1] = weight[8-1] - alpha * pdEW8
        # print(weight[1-1])
        # print(weight[2-1])
        # print(weight[3-1])
        # print(weight[4-1])
        # print(weight[5-1])
        # print(weight[6-1])
        # print(weight[7-1])
        # print(weight[8-1])

  不知道你是否對此感到熟悉一點了呢？反正我按照公式實現一遍之後深有體會，然後用向量的又寫了一次代碼。
  接下來我們要用向量來存儲這些權重，輸出結果等，因爲如果我們不這樣做，你看上面的例子就知道我們需要寫很多w1,w2等，這要是參數一多就很可怕。
  這些格式我是參考吳恩達的格式，相關課程資料->吳恩達深度學習視頻。

我將原文的圖片的變量名改成如上
然後正向傳播的公式如下：

z[1]1=w[1]1T⋅x+b1,a[1]1=σ(z[1]1)z[1]2=w[1]2T⋅x+b1,a[1]2=σ(z[1]2)z[2]1=w[2]1T⋅a1+b2,a[2]1=σ(z[2]1)z[2]2=w[2]2T⋅a1+b2,a[2]2=σ(z[2]2)(9)(10)(11)(12)(13)$$\begin{array}{}\text{(9)}& & z_1^{\left[1\right]}=w{{}_1^{[1]}}^T\cdot x+b_1,a_1^{[1]}=\sigma (z_1^{\left[1\right]})\text{(10)}& & z_2^{\left[1\right]}=w{{}_2^{[1]}}^T\cdot x+b_1,a_2^{[1]}=\sigma (z_2^{\left[1\right]})\text{(11)}& & z_1^{\left[2\right]}=w{{}_1^{[2]}}^T\cdot a_1+b_2,a_1^{[2]}=\sigma (z_1^{\left[2\right]})\text{(12)}& & z_2^{\left[2\right]}=w{{}_2^{[2]}}^T\cdot a_1+b_2,a_2^{[2]}=\sigma (z_2^{\left[2\right]})\text{(13)}& & \end{array}$$

其中

w[1]1T=(w1,w2)w[1]2T=(w3,w4)w[2]1T=(w5,w6)w[2]2T=(w7,w8)(14)(15)(16)(17)$$\begin{array}{}\text{(14)}& & w{{}_1^{[1]}}^T=(w_1,w_2)\text{(15)}& & w{{}_2^{[1]}}^T=(w_3,w_4)\text{(16)}& & w{{}_1^{[2]}}^T=(w_5,w_6)\text{(17)}& & w{{}_2^{[2]}}^T=(w_7,w_8)\end{array}$$

然後反向傳播的公式如下：

∂E∂w[2]1=∂E∂a[2]1⋅∂a[2]1∂z[2]1⋅∂z[2]1∂w[2]1∂E∂w[2]1=∂E∂a[2]2⋅∂a[2]2∂z[2]2⋅∂z[2]2∂w[2]2∂E∂w[1]1=∂E∂a[1]1⋅∂a[1]1∂z[1]1⋅∂z[1]1∂w[1]1∂E∂w[1]2=∂E∂a[1]2⋅∂a[1]2∂z[1]2⋅∂z[1]2∂w[1]2(18)(19)(20)(21)$$\begin{array}{}\text{(18)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{w}_1^{[2]}}=\frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[2]}}\cdot \frac{\mathrm{\partial }{a}_1^{[2]}}{\mathrm{\partial }{z}_1^{[2]}}\cdot \frac{\mathrm{\partial }{z}_1^{[2]}}{\mathrm{\partial }{w}_1^{[2]}}\text{(19)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{w}_1^{[2]}}=\frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_2^{[2]}}\cdot \frac{\mathrm{\partial }{a}_2^{[2]}}{\mathrm{\partial }{z}_2^{[2]}}\cdot \frac{\mathrm{\partial }{z}_2^{[2]}}{\mathrm{\partial }{w}_2^{[2]}}\text{(20)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{w}_1^{[1]}}=\frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[1]}}\cdot \frac{\mathrm{\partial }{a}_1^{[1]}}{\mathrm{\partial }{z}_1^{[1]}}\cdot \frac{\mathrm{\partial }{z}_1^{[1]}}{\mathrm{\partial }{w}_1^{[1]}}\text{(21)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{w}_2^{[1]}}=\frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_2^{[1]}}\cdot \frac{\mathrm{\partial }{a}_2^{[1]}}{\mathrm{\partial }{z}_2^{[1]}}\cdot \frac{\mathrm{\partial }{z}_2^{[1]}}{\mathrm{\partial }{w}_2^{[1]}}\end{array}$$

具體地

∂E∂a[2]1=−(y1−a[2]1)∂a[2]1∂z[2]1=a[2]1(1−a[2]1)∂z[2]1∂w[2]1=a[2]1∂E∂a[2]2=−(y2−a[2]2)∂a[2]1∂z[2]1=a[2]2(1−a[2]2)∂z[2]1∂w[2]2=a[2]2∂E∂a[1]1=w[2]1Tδ2∂a[1]1∂z[1]1=a[1]1⋅(1−a[1]1)∂z[1]1∂w[1]1=a[1]1∂E∂a[1]1=w[2]2Tδ2∂a[1]2∂z[1]1=a[1]2⋅(1−a[1]2)∂z[1]1∂w[1]2=a[1]2(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)$$\begin{array}{}\text{(22)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[2]}}=-(y_1-a_{{}_1}^{[2]})\text{(23)}& & \frac{\mathrm{\partial }{a}_1^{[2]}}{\mathrm{\partial }{z}_1^{[2]}}=a_{{}_1}^{[2]}(1-a_{{}_1}^{[2]})\text{(24)}& & \frac{\mathrm{\partial }{z}_1^{[2]}}{\mathrm{\partial }{w}_1^{[2]}}=a_1^{[2]}\text{(25)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_2^{[2]}}=-(y_2-a_2^{[2]})\text{(26)}& & \frac{\mathrm{\partial }{a}_1^{[2]}}{\mathrm{\partial }{z}_1^{[2]}}=a_2^{[2]}(1-a_2^{[2]})\text{(27)}& & \frac{\mathrm{\partial }{z}_1^{[2]}}{\mathrm{\partial }{w}_2^{[2]}}=a_2^{[2]}\text{(28)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[1]}}=w{{}_1^{[2]}}^T{\delta }^2\text{(29)}& & \frac{\mathrm{\partial }{a}_1^{[1]}}{\mathrm{\partial }{z}_1^{[1]}}=a_1^{[1]}\cdot (1-a_1^{[1]})\text{(30)}& & \frac{\mathrm{\partial }{z}_1^{[1]}}{\mathrm{\partial }{w}_1^{[1]}}=a_1^{[1]}\text{(31)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[1]}}=w{{}_2^{[2]}}^T{\delta }^2\text{(32)}& & \frac{\mathrm{\partial }{a}_2^{[1]}}{\mathrm{\partial }{z}_1^{[1]}}=a_2^{[1]}\cdot (1-a_2^{[1]})\text{(33)}& & \frac{\mathrm{\partial }{z}_1^{[1]}}{\mathrm{\partial }{w}_2^{[1]}}=a_2^{[1]}\end{array}$$

其中

δ2=⎛⎝⎜⎜⎜⎜⎜⎜∂E∂a[2]1⋅∂a[2]1∂z[2]1∂E∂a[2]2⋅∂a[2]2∂z[2]2(34)(35)⎞⎠⎟⎟⎟⎟⎟⎟=(∂E∂a[2]⋅∂a[2]∂z[2])$${\delta }^2=\left(\begin{array}{}\text{(34)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[2]}}\cdot \frac{\mathrm{\partial }{a}_1^{[2]}}{\mathrm{\partial }{z}_1^{[2]}}\text{(35)}& & \frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_2^{[2]}}\cdot \frac{\mathrm{\partial }{a}_2^{[2]}}{\mathrm{\partial }{z}_2^{[2]}}\end{array}\right)=\left(\frac{\mathrm{\partial }E}{\mathrm{\partial }{a}^{[2]}}\cdot \frac{\mathrm{\partial }{a}^{[2]}}{\mathrm{\partial }{z}^{[2]}}\right)$$

爲啥這樣寫呢，一開始我也沒明白，後來看到∂E∂a[2]1⋅∂a[2]1∂z[2]1$\frac{\mathrm{\partial }E}{\mathrm{\partial }{a}_1^{[2]}}\cdot \frac{\mathrm{\partial }{a}_1^{[2]}}{\mathrm{\partial }{z}_1^{[2]}}$ 有好幾次重複，且也便於梯度公式的書寫。

import numpy as np

def sigmoid(x):
    return 1 / (1 + np.exp(-x))
def sigmoidDerivationx(y):
    return y * (1 - y)

if __name__ == '__main__':
    #初始化一些參數
    alpha = 0.5
    w1 = [[0.15, 0.20], [0.25, 0.30]] #Weight of input layer
    w2 = [[0.40, 0.45], [0.50, 0.55]]
    b1 = 0.35
    b2 = 0.60
    x = [0.05, 0.10]
    y = [0.01, 0.99]
    #前向傳播
    z1 = np.dot(w1, x) + b1
    a1 = sigmoid(z1)
    z2 = np.dot(w2, a1) + b2
    a2 = sigmoid(z2)
    for n in range(10000):
        #反向傳播 使用代價函數爲C=1 / (2n) * sum[(y-a2)^2]
        #分爲兩次
        # 一次是最後一層對前面一層的錯誤
        delta2 = np.multiply(-(y-a2), np.multiply(a2, 1-a2))
        # for i in range(len(w2)):
        #     print(w2[i] - alpha * delta2[i] * a1)
        #計算非最後一層的錯誤
        # print(delta2)
        delta1 = np.multiply(np.dot(w1, delta2), np.multiply(a1, 1-a1))
        # print(delta1)
        # for i in range(len(w1)):
            # print(w1[i] - alpha * delta1[i] * np.array(x))
        #更新權重
        for i in range(len(w2)):
            w2[i] = w2[i] - alpha * delta2[i] * a1
        for i in range(len(w1)):
            w1[i] - alpha * delta1[i] * np.array(x)
        #繼續前向傳播，算出誤差值
        z1 = np.dot(w1, x) + b1
        a1 = sigmoid(z1)
        z2 = np.dot(w2, a1) + b2
        a2 = sigmoid(z2)
        print(str(n) + " result:" + str(a2[0]) + ", result:" +str(a2[1]))
        # print(str(n) + "  error1:" + str(y[0] - a2[0]) + ", error2:" +str(y[1] - a2[1]))

可以看到，用向量來表示的話代碼就簡短了非常多。但是用了向量化等的方法，如果不太熟，去看吳恩達深度學習的第一部分，再返過來看就能懂了。
下面，來看一個例子。用神經網絡實現XOR（01=1，10=1，00=0，11=0）。我們都知道感知機是沒法實現異或的，原因是線性不可分。
接下里的這個例子，我是用2個輸入結點，3個隱層結點，1個輸出結點來實現的。

讓我們以一個輸入爲例。
前向傳播：

⎛⎝⎜⎜⎜w[1]11w[1]12w[1]21w[1]22w[1]31w[1]32⎞⎠⎟⎟⎟⋅(x1x2)=⎛⎝⎜⎜⎜z[1]1z[1]2z[1]3⎞⎠⎟⎟⎟$$\left(\begin{array}{l}{w}_{11}^{[1]}{w}_{12}^{[1]}\\ w_{21}^{[1]}{w}_{22}^{[1]}\\ w_{31}^{[1]}{w}_{32}^{[1]}\end{array}\right)\cdot \left(\begin{array}{l}{x}_1\\ x_2\end{array}\right)=\left(\begin{array}{l}{z}_1^{[1]}\\ z_2^{[1]}\\ z_3^{[1]}\end{array}\right)$$

⎛⎝⎜⎜⎜a[1]1a[1]2a[1]3⎞⎠⎟⎟⎟=⎛⎝⎜⎜⎜σ(z[1]1)σ(z[1]2)σ(z[1]3)⎞⎠⎟⎟⎟$$\left(\begin{array}{l}{a}_1^{[1]}\\ a_2^{[1]}\\ a_3^{[1]}\end{array}\right)=\left(\begin{array}{l}\sigma (z_1^{[1]})\\ \sigma (z_2^{[1]})\\ \sigma (z_3^{[1]})\end{array}\right)$$

(w[2]11w[2]12w[2]13)⎛⎝⎜⎜⎜a[1]1a[1]2a[1]3⎞⎠⎟⎟⎟=(z[2]1)$$\left(w_{11}^{[2]}{w}_{12}^{[2]}{w}_{13}^{[2]}\right)\left(\begin{array}{l}{a}_1^{[1]}\\ a_2^{[1]}\\ a_3^{[1]}\end{array}\right)=\left(z_1^{[2]}\right)$$

(a[2]1)=(σ(z[2]1))$$\left(a_1^{[2]}\right)=\left(\sigma (z_1^{[2]})\right)$$

反向傳播：
主要是有2個公式比較重要

δL=∇aC⊙σ′(aL)=−(y−aL)⊙(aL(1−aL))$${\delta }^L={\mathrm{\nabla }}_{a}C\odot {\sigma }^{\prime }(a^L)=-(y-a^L)\odot (a^L(1-a^L))$$

原理同上

δl=((wl+1)T)⊙σ′(al)$${\delta }^l=((w^{l+1}{)}^T)\odot {\sigma }^{\prime }(a^l)$$

wl=wl−ηδl(al−1)T$$w^l=w^l-\eta {\delta }^l(a^{l-1}{)}^T$$

這次省略了偏導，代碼如下

import numpy as np
# sigmoid function
def sigmoid(x):
    return 1 / (1 + np.exp(-x))
def sigmoidDerivationx(y):
    return y * (1 - y)

if __name__ == '__main__':
    alpha = 1
    input_dim = 2
    hidden_dim = 3
    output_dim = 1
    synapse_0 = 2 * np.random.random((hidden_dim, input_dim)) - 1  #(2, 3)
    # synapse_0 = np.ones((hidden_dim, input_dim)) * 0.5
    synapse_1 = 2 * np.random.random((output_dim, hidden_dim)) - 1  #(2, 2)
    # synapse_1 = np.ones((output_dim, hidden_dim)) * 0.5
    x = np.array([[0, 1], [1, 0], [0, 0], [1, 1]]).T #(2, 4)
    # x = np.array([[0, 1]]).T #(3, 1)
    y = np.array([[1], [1], [0], [0]]).T    #(1, 4)
    # y = np.array([[1]]).T    #(2, 1)
    for i in range(2000000):
        z1 = np.dot(synapse_0, x)   #(3, 4)
        a1 = sigmoid(z1)    #(3, 4)
        z2 = np.dot(synapse_1, a1)  #(1, 4)
        a2 = sigmoid(z2)    #(1, 4)
        error = -(y - a2) #(1, 4)
        delta2 = np.multiply(-(y - a2) / x.shape[1], sigmoidDerivationx(a2))  #(1, 4)
        delta1 = np.multiply(np.dot(synapse_1.T, delta2), sigmoidDerivationx(a1))  #(3, 4)
        synapse_1 = synapse_1 - alpha * np.dot(delta2, a1.T) #(1, 3)
        synapse_0 = synapse_0 - alpha * np.dot(delta1, x.T) #(3, 2)
        print(str(i) + ":", end=' ')
        print(a2)