说明：  在matlab2019B中，已经将princomp函数改为了pca函数，功能相同，函数名改变，所以如果要使用，记得将老版本的代码中的princomp改为pca。
(In MATLAB 2019b, the principal function has been changed to PCA function. The functions are the same, and the function name changes. So if you want to use it, remember to change the principal function in the old version of the code to PCA.)