# 朝思录

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Kernel-PCA是采用Kernel方法，在更高维的空间中做PCA分析，但是得到的主成分的个数却不能超过原始数据的维度。Kernel-PCA适用于非凸分布的数据的分析，但值得注意的是，因为Kernel-PCA是采用Kernel做隐式高维映射，所以只能进行信号的最小误差表示，不能进行信号重建操作。

$\tag{Katex}$

# Pearson 相关系数

$\rho_{X,Y}=\frac{cov(X,Y)}{\sigma_X\sigma_Y}$

$r = \frac{\sum_{i=1}^n(X_i-\overline X) (Y_i-\overline Y)} {\sqrt{\sum_{i=1}^n(X_i-\overline X)^2} \sqrt{\sum_{i=1}^n(Y_i-\overline Y)^2}}$

# Spearman 相关系数

Spearman相关系数是两个变量$X$,$Y$对应的等级变量$x$,$y$的pearson相关系数。

# 什么是环境

• Include - 头文件所在位置
• Lib - 库文件所在位置
• Scripts - python应用所在位置
• tcl - 目前我还不知道
• 还有其他文件夹我也不知道了

# 为什么需要虚拟环境

The basic problem being addressed is one of dependencies and versions, and indirectly permissions. Imagine you have an application that needs version 1 of LibFoo, but another application requires version 2. How can you use both these applications? If you install everything into /usr/lib/python2.7/site-packages (or whatever your platform’s standard location is), it’s easy to end up in a situation where you unintentionally upgrade an application that shouldn’t be upgraded.
Or more generally, what if you want to install an application and leave it be? If an application works, any change in its libraries or the versions of those libraries can break the application.

MATLAB支持两种类：

• Handle class
• Value class
这两种类的定义和区别如下：

# Handle class

Use a handle class when you want to create a reference to the data contained in an object of the class, and do not wnat copies of the object to make copies of the object data.