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)
1.2.3 Example Dataset 3
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 = {} num 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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原文链接:https://blog.csdn.net/Cowry5/article/details/80465922