Machine learning（4-Linear Regression with multiple variables ）

1、Multiple features

• 
• So what the form of the hypothesis should be ？

• For convenience， define x₀=1

• At this time, the parameter in the model is a ( + 1)-dimensional vector, and any training instance is also a ( + 1)-dimensional vector. The dimension of the feature matrix is { ∗ ( + 1)} , so the formula can be simplified to :

2、Gradient descent for multiple variables

• 
• Here is the gradient descent looks like

• Python code：

def computeCost(X, y, theta):
 inner = np.power(((X * theta.T) - y), 2)
 return np.sum(inner) / (2 * len(X)

3、Gradient descent in practice I ：Feature Scaling

• An idea about feature scaling（特征缩放） --- make sure features are on a similar scale and get every feature into approximately a -1≤x_i≤1 range

4、Gradient descent in practice II： Learning rate

• 
• 
• 

5、Features and Polynomial Regression

• Housing price prediction

• Linear regression is not suitable for all data, sometimes we need a curve to fit our data, such as a quadratic model :

• Or maybe a cubic model :

• 
• According to the graphical characteristics of the function, we can also use :

6、Normal Equation

• Normal equation ： method to solve for θ analytically
• 
• It is too long and involved
• 
• And now，I am going to take the dataset and add an extra column
• 
• Then construct a matrix X ：

• And construct a vector y ：

• Solve the vector using the normal equation ：

• We get ：

pinv(X'*X)*X'*y

• How to choose gradient descent or normal equation ？

• Use python to implement Normal Equation

import numpy as np

def normalEqn(X, y):
 theta = np.linalg.inv(X.T@X)@X.T@y  #X.T@X 等价于 X.T.dot(X)
 return theta

7、Normal Equation Non-invertibility

• 
• 

8、Supplement