吴恩达机器学习笔记：手搓线性回归(梯度下降寻优)

概念就不介绍了，记录下公式推导和代码实现，以及与最小二乘的比较。

吴恩达老师课程中使用一个参数theta保存两个变量，不过我好像没把中间变量的形状对齐，所以最后实现了两个参数的版本。

 

 

代码：

import time
import numpy as np
import pandas
import matplotlib.pyplot as plt

#随机种子
rd = np.random.RandomState(round(time.time())) 

#输出一个5*5的单位矩阵、随机矩阵
arr1 = np.ones((5,5))
arr2 = rd.randint(-10,10,(2,3))
print('5*5单位矩阵:')
print(arr1)
print('2*3随机int矩阵:')
print(arr2)


#随机生成散点数据
Population = rd.uniform(1,50,100) #均匀分布
Profit = rd.rand(100)*1000  #高斯分布


#排序(模拟线性数据) 如果是numpy array,则要使用numpy.sort()
Population.sort()
Profit.sort()


#绘制散点图
fig = plt.figure(figsize=(16, 9), dpi=40)
plt.scatter(Population, Profit) #散点图
plt.grid(color='r', linestyle='--', linewidth=1,alpha=0.3)
plt.show()


#构造代价函数Cost，使用MSE估计误差,为方便求导将目标函数设置为MSE/2m (m为样本数量)
#对于线性回归y=wx+b，回归参数有两个：w与b，我们将打包其放入“theta”中
def costFunction(x,y,theta):
    tmp = np.matmul(x,theta)
    cost = np.power(tmp-y, 2)
    return np.sum(cost)/(2*len(x))


#梯度下降
def gradientDescent(x, y, theta, rate, iters):
    X = np.array(x.reshape(len(x),1))
    Y = np.array(y.reshape(len(y),1))
    theta = theta.reshape(2,1)
    #X添加一列ones，以便与theta相乘
    X = np.insert(X, 1, np.array(np.ones(len(x))), axis=1)
    
    temp = np.matrix(np.zeros(theta.shape))
    cost = np.zeros(iters)
    
    i = 0
    cost[i] = 1e-4
    
    while i < iters and cost[i]>1e-5 :
        error = np.dot(X , theta) - Y
        print(error.shape)
        print(X[:,0].shape)
        term = error * X[:,0]
        print(term.shape)
        theta[0,0] = theta[0,0] - ((rate /(len(x))) * (( np.sum(term) )))
        theta[1,0] = theta[1,0] - ((rate /(len(x))) * ((np.sum(error))) )
    
        cost[i] = costFunction(X, Y, theta)
        
    return theta, cost


#两个参数版本的costFuction与gradientDescent
def costFun(x,y,theta_w,theta_b):
    x = x.reshape(len(x),1)
    y = y.reshape(len(y),1)
    cost = (np.dot(x,theta_w) + theta_b) - y
    return np.sum(np.power(cost,2))/(len(x)*2)


def gradientDescent2(x,y,rate,iters,theta_w,theta_b):
    x = np.array(x.reshape(len(x),1))
    y = np.array(y.reshape(len(y),1))
    
    y_hat = np.dot(x,theta_w) + theta_b
    error = np.array(y_hat - y)
    
    dw = 2*(np.vdot(x,error))
    db = sum(2*error)
    
    i = 0
    cost = np.zeros(iters)
    cost[i] = 1e-4
    while i  1e-5:
        y_hat = np.dot(x,theta_w) + theta_b
        error = y_hat - y
    
        dw = 2*np.vdot(x,error)
        db = sum(2*error)
        
        #更新参数
        theta_w = theta_w - rate / (len(x)*2)*dw
        theta_b = theta_b - rate / (len(x)*2)*db
        
        cost[i] = costFun(x,y,theta_w,theta_b) #打印出每一轮迭代的cost，查看是否一直在变小
        i+=1
        
    Theta = list([theta_w,theta_b])
    
    return Theta,cost
    
    
#初始化,需保证输入的数据为numpy matrix
#x = np.matrix(Population.values)
#y = np.matrix(Profit.values)


#超参数
rate = 0.001  #学习率
iters = 1500  #迭代次数
theta = np.matrix(np.array([10,5])) # a与b的初始值为0


result2 = gradientDescent2(Population, Profit, rate,iters,1,1)
print(result2)

#训练结果可视化
plt.title('Linear Regression with 1D Feature')

X = np.linspace(np.min(Population),np.max(Population*1.1),len(Population)*100)
Y = X*result2[0][0] + result2[0][1]
ax1 = plt.scatter(Population, Profit, marker= 'o', s=50)
plt.plot(X,Y,'r', label='Gradient Descending')
plt.grid(color='r', linestyle='--', linewidth=1,alpha=0.3)
plt.xlabel('Input_value(x)')
plt.ylabel('Output_value(y)')
plt.show()



# 最小二乘回归拟合对比梯度下降=============================================================================
from sklearn.model_selection import train_test_split
from sklearn.linear_model import LinearRegression
from sklearn.datasets import make_regression
# 生产一元随机回归数据集
# X,Y = make_regression(n_samples=10,n_features=1,n_informative=1,\
#                       n_targets=1,noise=0.1,random_state=0)

Population = Population.reshape(np.size(Population),1)
Profit = Profit.reshape(np.size(Profit),1)

# 数据分割
X_train,X_test,Y_train,Y_test = train_test_split(Population, Profit,test_size=1/4,\
                                random_state=0)


# 最小二乘
linreg = LinearRegression().fit(X_train,Y_train)
print('模型系数w：{}\n模型截距b：{}\n训练集R^2得分：{}\n测试集R^2得分：{:.3f}'\
      .format(linreg.coef_, linreg.intercept_,linreg.score(X_train,Y_train)\
      ,linreg.score(X_test,Y_test)))

ax2 = plt.scatter(Population, Profit, marker= 'o', s=10,label='Samples')

X_ols = np.linspace(np.min(Population),np.max(Population*1.5),len(Population)*100)
Y_ols = X_ols * linreg.coef_+linreg.intercept_
Y_ols = Y_ols.reshape(np.size(Y_ols),1)

print(Y_ols.shape)
ax3 = plt.plot(X_ols,Y_ols,'g',label='Least Sum of Squares')

print("ax3:")
print(ax3)
#梯度下降
result = gradientDescent2(np.array(X_train), np.array(Y_train), rate,iters,1,1)
print(result)


X_d = np.linspace(np.min(X),np.max(X*1.5),len(X)*100)
Y_d = X_d*result[0][0] + result[0][1]


ax4 = plt.plot(X_d,Y_d,'b',label='gradientDescent')
plt.legend()


#形状对不上..
# result1 = gradientDescent(Population, Profit, theta, rate, iters)
# print(result1)

输出结果：

 

 

纸上习得终觉浅..

Reference：

吴恩达机器学习ex1 - Heywhale.com

一文看懂简单线性回归：梯度下降法和最小二乘法（代码实现及数学公式详解）_Maxxi Chen的博客-CSDN博客_最小二乘法和梯度下降法实现线性模型的过程