无监督学习
无监督学习让我们能够在完全不知道结果的情况下处理问题，我们能够从不知道变量的情况下从数据当中得到结构
能够通过将数据中的变量进行聚类来获得结构
无监督学习没有基于预测结果的反馈（也就是没有训练集）
例子：
- 聚类：1，000，000个不同的基因找到一种方式自动将这些基因分成以不同变量分开的相似几组
- 非聚类：鸡尾酒派对算法，在混乱环境中找到结构，从各种不同声音中辨别独立的人声和音乐https://en.wikipedia.org/wiki/Cocktail_party_effect
常见Q&A
Q: Why do we have to use Matlab or Octave? Why not Clojure, Julia, Python, R or [Insert favourite language here]?A: As Prof. Ng explained in the 1st video of the Octave tutorial, he has tried teaching Machine Learning in a variety of languages, and found that students come up to speed faster with Matlab/Octave. Therefore the course was designed using Octave/Matlab, and the automatic submission grader uses those program interfaces. Octave and Matlab are optimized for rapid vectorized calculations, which is very useful in Machine Learning. R is a nice tool, but:
- It is a bit too high level. This course shows how to actually implement the algorithms of machine learning, while R already has them implemented. Since the focus of this course is to show you what happens in ML algorithms under the hood, you need to use Octave 2. This course offers some starter code in Octave/Matlab, which will really save you tons of time solving the tasks.
看来这门课程旨在揭开机器学习算法底下的面纱，Matlab和Octave在向量计算上非常快捷，并且在学习补充算法的时候比较方便，但是在R上使用这些算法，这些算法已经补充完全了，也就是说学不到任何东西，和刚才说的课程初衷相悖。
2.怎么输入答案？
回答：一维的矩阵不写括号，小数点最多写两位小数，分数只能写成小数形式，
3.Q: My quiz grade displayed is wrong or I have a verification issue or I cannot retake a quiz. What should I do? A: Contact Help Center. These queries can only be resolved by learner support and it is best if they are contacted directly.
