截断 SVD 和 PCA

Question

理论上，如果特征均值为0，PCA和SVD的投影结果是相同的。所以我在python上尝试了。

from sklearn import datasets
cancer = datasets.load_breast_cancer()

from sklearn.preprocessing import StandardScaler
# we can set our feature to have mean 0 by setting with_mean=False
scaler = StandardScaler(with_mean=False,with_std=False)
scaler.fit(cancer.data)
X_scaled = scaler.transform(cancer.data)

from sklearn.decomposition import PCA
pca=PCA(n_components=3,svd_solver='randomized') 
pca.fit(X_scaled) 
X_pca=pca.transform(X_scaled) 

from sklearn.decomposition import TruncatedSVD
svdm=TruncatedSVD(n_components=3,algorithm='randomized') 
svdm.fit(X_scaled) 
X_svdm=svdm.transform(X_scaled)

但是当我打印结果时，情况就不同了。为什么会这样？

print(X_pca)
print(X_svdm)
>>>[[1160.1425737  -293.91754364   48.57839763]
 [1269.12244319   15.63018184  -35.39453423]
 [ 995.79388896   39.15674324   -1.70975298]
 ...
 [ 314.50175618   47.55352518  -10.44240718]
 [1124.85811531   34.12922497  -19.74208742]
 [-771.52762188  -88.64310636   23.88903189]]
>>>[[2241.97427647  347.71556015  -27.53741942]
 [2372.40840267   56.90166991   23.86316187]
 [2101.8402797    11.94762737   30.41138602]
 ...
 [1424.53280954  -55.0217124    -3.5794351 ]
 [2231.65579282   19.99439854    3.31619182]
 [ 331.69302638   -5.29733966  -39.12136435]]

我应该解决什么问题才能获得两种算法的相同结果？

Answer 1

来自help page for scaler：

with_mean bool, default=True 如果为 True，则在缩放之前将数据居中。 尝试时这不起作用（并且会引发异常） 稀疏矩阵，因为使它们居中需要建立一个密集的矩阵 在常见用例中可能太大而无法放入的矩阵 记忆。

要使 PCA 和 SVD 提供相同的输出，您需要对数据进行居中和缩放，另请参阅 this post for details，所以如果您这样做：

# which is also the default
scaler = StandardScaler(with_mean=True, with_std=True)
X_scaled = scaler.fit_transform(cancer.data)

pca=PCA(n_components=3,svd_solver='randomized')
pca.fit(X_scaled)
X_pca=pca.transform(X_scaled)

svdm=TruncatedSVD(n_components=3,algorithm='randomized')
svdm.fit(X_scaled)
X_svdm=svdm.transform(X_scaled)

X_pca
array([[ 9.19283683,  1.94858306, -1.12316567],
       [ 2.3878018 , -3.76817175, -0.52929196],
       [ 5.73389628, -1.0751738 , -0.55174751],
       ...,
       [ 1.25617928, -1.90229671,  0.56273027],
       [10.37479406,  1.67201011, -1.87702986],
       [-5.4752433 , -0.6706368 ,  1.49044385]])

X_svdm
array([[ 9.19283683,  1.94858307, -1.12316615],
       [ 2.3878018 , -3.76817174, -0.52929266],
       [ 5.73389628, -1.0751738 , -0.55174759],
       ...,
       [ 1.25617928, -1.90229671,  0.56273052],
       [10.37479406,  1.67201011, -1.87702935],
       [-5.4752433 , -0.67063679,  1.49044309]])