吴恩达机器学习练习:SVM支持向量机
1 Support Vector Machines
1.1 Example Dataset 1
%matplotlib inline
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sb
from scipy.io import loadmat
from sklearn import svm
大多数SVM的库会自动帮你添加额外的特征X₀已经θ₀，所以无需手动添加
mat = loadmat('./data/ex6data1.mat')
print(mat.keys())
# dict_keys(['__header__', '__version__', '__globals__', 'X', 'y'])
X = mat['X']
y = mat['y']
def plotData(X, y):
plt.figure(figsize=(8,5))
plt.scatter(X[:,0], X[:,1], c=y.flatten(), cmap='rainbow')
plt.xlabel('X1')
plt.ylabel('X2')
plt.legend()
plotData(X, y)
def plotBoundary(clf, X):
'''plot decision bondary'''
x_min, x_max = X[:,0].min()*1.2, X[:,0].max()*1.1
y_min, y_max = X[:,1].min()*1.1,X[:,1].max()*1.1
xx, yy = np.meshgrid(np.linspace(x_min, x_max, 500),
np.linspace(y_min, y_max, 500))
Z = clf.predict(np.c_[xx.ravel(), yy.ravel()])
Z = Z.reshape(xx.shape)
plt.contour(xx, yy, Z)
models = [svm.SVC(C, kernel='linear') for C in [1, 100]]
clfs = [model.fit(X, y.ravel()) for model in models]
title = ['SVM Decision Boundary with C = {} (Example Dataset 1'.format(C) for C in [1, 100]]
for model,title in zip(clfs,title):
plt.figure(figsize=(8,5))
plotData(X, y)
plotBoundary(model, X)
plt.title(title)
可以从上图看到，当C比较小时模型对误分类的惩罚增大，比较严格，误分类少，间隔比较狭窄。
当C比较大时模型对误分类的惩罚增大，比较宽松，允许一定的误分类存在，间隔较大。
1.2 SVM with Gaussian Kernels这部分，使用SVM做非线性分类。我们将使用高斯核函数。
为了用SVM找出一个非线性的决策边界，我们首先要实现高斯核函数。我可以把高斯核函数想象成一个相似度函数，用来测量一对样本的距离，(x ⁽ ʲ ⁾,y ⁽ ⁱ ⁾)
这里我们用sklearn自带的svm中的核函数即可。
1.2.1 Gaussian Kernel
def gaussKernel(x1, x2, sigma):
return np.exp(- ((x1 - x2) ** 2).sum() / (2 * sigma ** 2))
gaussKernel(np.array([1, 2, 1]),np.array([0, 4, -1]), 2.) # 0.32465246735834974
1.2.2 Example Dataset 2
mat = loadmat('./data/ex6data2.mat')
X2 = mat['X']
y2 = mat['y']
plotData(X2, y2)
sigma = 0.1
gamma = np.power(sigma,-2.)/2
clf = svm.SVC(C=1, kernel='rbf', gamma=gamma)
modle = clf.fit(X2, y2.flatten())
plotData(X2, y2)
plotBoundary(modle, X2)
mat3 = loadmat('data/ex6data3.mat')
X3, y3 = mat3['X'], mat3['y']
Xval, yval = mat3['Xval'], mat3['yval']
plotData(X3, y3)
Cvalues = (0.01, 0.03, 0.1, 0.3, 1., 3., 10., 30.)
sigmavalues = Cvalues
best_pair, best_score = (0, 0), 0
for C in Cvalues:
for sigma in sigmavalues:
gamma = np.power(sigma,-2.)/2
model = svm.SVC(C=C,kernel='rbf',gamma=gamma)
model.fit(X3, y3.flatten())
this_score = model.score(Xval, yval)
if this_score > best_score:
best_score = this_score
best_pair = (C, sigma)
print('best_pair={}, best_score={}'.format(best_pair, best_score))
# best_pair=(1.0, 0.1), best_score=0.965
model = svm.SVC(C=1., kernel='rbf', gamma = np.power(.1, -2.)/2)
model.fit(X3, y3.flatten())
plotData(X3, y3)
plotBoundary(model, X3)
# 这我的一个练习画图的，和作业无关，给个画图的参考。
import numpy as np
import matplotlib.pyplot as plt
from sklearn import svm
# we create 40 separable points
np.random.seed(0)
X = np.array([[3,3],[4,3],[1,1]])
Y = np.array([1,1,-1])
# fit the model
clf = svm.SVC(kernel='linear')
clf.fit(X, Y)
# get the separating hyperplane
w = clf.coef_[0]
a = -w[0] / w[1]
xx = np.linspace(-5, 5)
yy = a * xx - (clf.intercept_[0]) / w[1]
# plot the parallels to the separating hyperplane that pass through the
# support vectors
b = clf.support_vectors_[0]
yy_down = a * xx + (b[1] - a * b[0])
b = clf.support_vectors_[-1]
yy_up = a * xx + (b[1] - a * b[0])
# plot the line, the points, and the nearest vectors to the plane
plt.figure(figsize=(8,5))
plt.plot(xx, yy, 'k-')
plt.plot(xx, yy_down, 'k--')
plt.plot(xx, yy_up, 'k--')
# 圈出支持向量
plt.scatter(clf.support_vectors_[:, 0], clf.support_vectors_[:, 1],
s=150, facecolors='none', edgecolors='k', linewidths=1.5)
plt.scatter(X[:, 0], X[:, 1], c=Y, cmap=plt.cm.rainbow)
plt.axis('tight')
plt.show()
print(clf.decision_function(X))
[ 1. 1.5 -1. ]
2 Spam Classification
2.1 Preprocessing Emails
这部分用SVM建立一个垃圾邮件分类器。你需要将每个email变成一个n维的特征向量，这个分类器将判断给定一个邮件x是垃圾邮件(y=1)或不是垃圾邮件(y=0)。
take a look at examples from the dataset
with open('data/emailSample1.txt', 'r') as f:
email = f.read()
print(email)
> Anyone knows how much it costs to host a web portal ?
>
Well, it depends on how many visitors you're expecting.
This can be anywhere from less than 10 bucks a month to a couple of $100.
You should checkout http://www.rackspace.com/ or perhaps Amazon EC2
if youre running something big..
To unsubscribe yourself from this mailing list, send an email to:
groupname-unsubscribe@egroups.com
可以看到，邮件内容包含 a URL, an email address(at the end), numbers, and dollar amounts. 很多邮件都会包含这些元素，但是每封邮件的具体内容可能会不一样。因此，处理邮件经常采用的方法是标准化这些数据，把所有URL当作一样，所有数字看作一样。
例如，我们用唯一的一个字符串‘httpaddr'来替换所有的URL，来表示邮件包含URL，而不要求具体的URL内容。这通常会提高垃圾邮件分类器的性能，因为垃圾邮件发送者通常会随机化URL，因此在新的垃圾邮件中再次看到任何特定URL的几率非常小。
我们可以做如下处理：
1. Lower-casing: 把整封邮件转化为小写。
2. Stripping HTML: 移除所有HTML标签，只保留内容。
3. Normalizing URLs: 将所有的URL替换为字符串 “httpaddr”.
4. Normalizing Email Addresses: 所有的地址替换为 “emailaddr”
5. Normalizing Dollars: 所有dollar符号($)替换为“dollar”.
6. Normalizing Numbers: 所有数字替换为“number”
7. Word Stemming(词干提取): 将所有单词还原为词源。例如，“discount”, “discounts”, “discounted” and “discounting”都替换为“discount”。
8. Removal of non-words: 移除所有非文字类型，所有的空格(tabs, newlines, spaces)调整为一个空格.
%matplotlib inline
import numpy as np
import matplotlib.pyplot as plt
from scipy.io import loadmat
from sklearn import svm
import re #regular expression for e-mail processing
# 这是一个可用的英文分词算法(Porter stemmer)
from stemming.porter2 import stem
# 这个英文算法似乎更符合作业里面所用的代码，与上面效果差不多
import nltk, nltk.stem.porter
def processEmail(email):
"""做除了Word Stemming和Removal of non-words的所有处理"""
email = email.lower()
email = re.sub('<[^<>]>', ' ', email) # 匹配<开头，然后所有不是< ,> 的内容，知道>结尾，相当于匹配<...>
email = re.sub('(http|https)://[^\s]*', 'httpaddr', email ) # 匹配//后面不是空白字符的内容，遇到空白字符则停止
email = re.sub('[^\s]+@[^\s]+', 'emailaddr', email)
email = re.sub('[\$]+', 'dollar', email)
email = re.sub('[\d]+', 'number', email)
return email
接下来就是提取词干，以及去除非字符内容。
def email2TokenList(email):
"""预处理数据，返回一个干净的单词列表"""
# I'll use the NLTK stemmer because it more accurately duplicates the
# performance of the OCTAVE implementation in the assignment
stemmer = nltk.stem.porter.PorterStemmer()
email = preProcess(email)
# 将邮件分割为单个单词，re.split() 可以设置多种分隔符
tokens = re.split('[ \@\$\/\#\.\-\:\&\*\+\=\[\]\?\!\(\)\{\}\,\'\"\>\_\<\;\%]', email)
# 遍历每个分割出来的内容
tokenlist = []
for token in tokens:
# 删除任何非字母数字的字符
token = re.sub('[^a-zA-Z0-9]', '', token);
# Use the Porter stemmer to 提取词根
stemmed = stemmer.stem(token)
# 去除空字符串‘'，里面不含任何字符
if not len(token): continue
tokenlist.append(stemmed)
return tokenlist
2.1.1 Vocabulary List(词汇表)
在对邮件进行预处理之后，我们有一个处理后的单词列表。下一步是选择我们想在分类器中使用哪些词，我们需要去除哪些词。
我们有一个词汇表vocab.txt，里面存储了在实际中经常使用的单词，共1899个。
我们要算出处理后的email中含有多少vocab.txt中的单词，并返回在vocab.txt中的index，这就我们想要的训练单词的索引。
def email2VocabIndices(email, vocab):
"""提取存在单词的索引"""
token = email2TokenList(email)
index = [i for i in range(len(vocab)) if vocab[i] in token ]
return index
2.2 Extracting Features from Emails
def email2FeatureVector(email):
"""
将email转化为词向量，n是vocab的长度。存在单词的相应位置的值置为1，其余为0
"""
df = pd.read_table('data/vocab.txt',names=['words'])
vocab = df.as_matrix() # return array
vector = np.zeros(len(vocab)) # init vector
vocab_indices = email2VocabIndices(email, vocab) # 返回含有单词的索引
# 将有单词的索引置为1
for i in vocab_indices:
vector[i] = 1
return vector
vector = email2FeatureVector(email)
print('length of vector = {}\nnum of non-zero = {}'.format(len(vector), int(vector.sum())))
length of vector = 1899
num of non-zero = 45
2.3 Training SVM for Spam Classification
读取已经训提取好的特征向量以及相应的标签。分训练集和测试集。
# Training set
mat1 = loadmat('data/spamTrain.mat')
X, y = mat1['X'], mat1['y']
# Test set
mat2 = scipy.io.loadmat('data/spamTest.mat')
Xtest, ytest = mat2['Xtest'], mat2['ytest']
clf = svm.SVC(C=0.1, kernel='linear')
clf.fit(X, y)
2.4 Top Predictors for Spam
predTrain = clf.score(X, y)
predTest = clf.score(Xtest, ytest)
predTrain, predTest
(0.99825, 0.989)
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