Python使用三种方法实现PCA算法

import numpy as np
from sklearn.decomposition import PCA
import sys
#returns choosing how many main factors
def index_lst(lst, component=0, rate=0):
  #component: numbers of main factors
  #rate: rate of sum(main factors)/sum(all factors)
  #rate range suggest: (0.8,1)
  #if you choose rate parameter, return index = 0 or less than len(lst)
  if component and rate:
    print('Component and rate must choose only one!')
    sys.exit(0)
  if not component and not rate:
    print('Invalid parameter for numbers of components!')
    sys.exit(0)
  elif component:
    print('Choosing by component, components are %s......'%component)
    return component
  else:
    print('Choosing by rate, rate is %s ......'%rate)
    for i in range(1, len(lst)):
      if sum(lst[:i])/sum(lst) >= rate:
        return i
    return 0

def main():
  # test data
  mat = [[-1,-1,0,2,1],[2,0,0,-1,-1],[2,0,1,1,0]]
  
  # simple transform of test data
  Mat = np.array(mat, dtype='float64')
  print('Before PCA transforMation, data is:\n', Mat)
  print('\nMethod 1: PCA by original algorithm:')
  p,n = np.shape(Mat) # shape of Mat 
  t = np.mean(Mat, 0) # mean of each column
  
  # substract the mean of each column
  for i in range(p):
    for j in range(n):
      Mat[i,j] = float(Mat[i,j]-t[j])
      
  # covariance Matrix
  cov_Mat = np.dot(Mat.T, Mat)/(p-1)
  
  # PCA by original algorithm
  # eigvalues and eigenvectors of covariance Matrix with eigvalues descending
  U,V = np.linalg.eigh(cov_Mat) 
  # Rearrange the eigenvectors and eigenvalues
  U = U[::-1]
  for i in range(n):
    V[i,:] = V[i,:][::-1]
  # choose eigenvalue by component or rate, not both of them euqal to 0
  Index = index_lst(U, component=2) # choose how many main factors
  if Index:
    v = V[:,:Index] # subset of Unitary matrix
  else: # improper rate choice may return Index=0
    print('Invalid rate choice.\nPlease adjust the rate.')
    print('Rate distribute follows:')
    print([sum(U[:i])/sum(U) for i in range(1, len(U)+1)])
    sys.exit(0)
  # data transformation
  T1 = np.dot(Mat, v)
  # print the transformed data
  print('We choose %d main factors.'%Index)
  print('After PCA transformation, data becomes:\n',T1)
  
  # PCA by original algorithm using SVD
  print('\nMethod 2: PCA by original algorithm using SVD:')
  # u: Unitary matrix, eigenvectors in columns 
  # d: list of the singular values, sorted in descending order
  u,d,v = np.linalg.svd(cov_Mat)
  Index = index_lst(d, rate=0.95) # choose how many main factors
  T2 = np.dot(Mat, u[:,:Index]) # transformed data
  print('We choose %d main factors.'%Index)
  print('After PCA transformation, data becomes:\n',T2)
  
  # PCA by Scikit-learn
  pca = PCA(n_components=2) # n_components can be integer or float in (0,1)
  pca.fit(mat) # fit the model
  print('\nMethod 3: PCA by Scikit-learn:')
  print('After PCA transformation, data becomes:')
  print(pca.fit_transform(mat)) # transformed data      
main()