What is machine learning?

1、What is machine learning

• Arthur Samuel(1959): Machine Learning is a field of study that gives computers the ability to learn without being explicitly programmed.
• Tom Mitchell(1998): Well-posed Learning Problem: A computer program is said to learn from experience E with respect to some task T and some performance measure P,if its performance on T,as measured by P,improves with experience E.（一个适当的学习问题定义如下，一个计算机程序从经验E中学习，解决某一任务T，进行某一性能度量P，通过P测定在T上的表现因经验E而提高）
  For example: your email program watches which emails you do or do not mark as spam (垃圾邮件),and based on that learns how to better filter spam.
  In this example
  "Classifying emails as spam or not spam" is the task T;
  "Watching you label emails as spam or not spam" is the experience E;
  "The number (or fraction) of emails correctly classified as spam/not spam" is the performance measure P.

2、Machine learning algorithms

• Supervised learning (监督学习): our human to teach the computer how to do something.
• Unsupervised learning (无监督学习): to let computer learn it by itself.
• Others: Reinforcement learning (强化学习),recommender systems.