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这是一个单变量特征选择的例子。这个例子将无信息的噪因特征添加到iris数据集里。对于每一个特征，我们画出单变量特征选择的p-value和一个SVM对应的权重。结果显示：有信息的特征具有更大的权值。

在整个的特征集里，仅仅有4个是显著的，它们具有最高的单变量特征选择分数。SVM同时也选择了很多无信息的特征。在SVM之前应用单变量特征选择，能够增加显著的特征的权值，这样也改善了分类的效果。

实例代码

import numpy as np
import matplotlib.pyplot as plt

from sklearn import datasets, svm
from sklearn.feature_selection import SelectPercentile, f_classif

# #############################################################################
# Import some data to play with

# The iris dataset
iris = datasets.load_iris()

# Some noisy data not correlated
E = np.random.uniform(0, 0.1, size=(len(iris.data), 20))

# Add the noisy data to the informative features
X = np.hstack((iris.data, E))
y = iris.target

plt.figure(1)
plt.clf()

X_indices = np.arange(X.shape[-1])

# #############################################################################
# Univariate feature selection with F-test for feature scoring
# We use the default selection function: the 10% most significant features
selector = SelectPercentile(f_classif, percentile=10)
selector.fit(X, y)
scores = -np.log10(selector.pvalues_)
scores /= scores.max()
plt.bar(X_indices - .45, scores, width=.2,
        label=r'Univariate score ($-Log(p_{value})$)', color='darkorange',
        edgecolor='black')

# #############################################################################
# Compare to the weights of an SVM
clf = svm.SVC(kernel='linear')
clf.fit(X, y)

svm_weights = (clf.coef_ ** 2).sum(axis=0)
svm_weights /= svm_weights.max()

plt.bar(X_indices - .25, svm_weights, width=.2, label='SVM weight',
        color='navy', edgecolor='black')

clf_selected = svm.SVC(kernel='linear')
clf_selected.fit(selector.transform(X), y)

svm_weights_selected = (clf_selected.coef_ ** 2).sum(axis=0)
svm_weights_selected /= svm_weights_selected.max()

plt.bar(X_indices[selector.get_support()] - .05, svm_weights_selected,
        width=.2, label='SVM weights after selection', color='c',
        edgecolor='black')


plt.title("Comparing feature selection")
plt.xlabel('Feature number')
plt.yticks(())
plt.axis('tight')
plt.legend(loc='upper right')
plt.show()

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