Andrew Ng 's machine learning lecture note (6)

本文详细介绍了在解决分类问题时常用的逻辑回归方法。通过定义新的假设函数——逻辑函数,并解释了其与线性回归的区别。文中还探讨了决策边界的概念及其与参数θ的关系,以及如何使用梯度下降法来最小化成本函数。

Logistic function

When we are solving classifcation problem,we seldom use linear regression method.We should define a new function called logistic function to be our  hypothesis.
and we should notice that 

Decision Boundary

Decision boundary is a function of theta. It seperates the area of y=1 and y=0. 
theta'*X can be a non-linear .

Cost Function

We have the cost function in the form that : 

Gradient Decent

In order to minimize the cost function , we can use gradient decent ,and the same as linear regression problem , we should update theta simultaneously. 

Library Function

We can use OCTAVE's library function to calculate the gradient decent like library function 'fminunc'.

Pay attention

  (1) The hypothesis is not equal to theta*X' like linear regression
  (2)


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