Python实现决策树C4.5算法的示例
为什么要改进成C4.5算法
原理
C4.5算法是在ID3算法上的一种改进,它与ID3算法最大的区别就是特征选择上有所不同,一个是基于信息增益比,一个是基于信息增益。
之所以这样做是因为信息增益倾向于选择取值比较多的特征(特征越多,条件熵(特征划分后的类别变量的熵)越小,信息增益就越大);因此在信息增益下面加一个分母,该分母是当前所选特征的熵,注意:这里而不是类别变量的熵了。
这样就构成了新的特征选择准则,叫做信息增益比。为什么加了这样一个分母就会消除ID3算法倾向于选择取值较多的特征呢?
因为特征取值越多,该特征的熵就越大,分母也就越大,所以信息增益比就会减小,而不是像信息增益那样增大了,一定程度消除了算法对特征取值范围的影响。
实现
在算法实现上,C4.5算法只是修改了信息增益计算的函数calcShannonEntOfFeature和最优特征选择函数chooseBestFeatureToSplit。
calcShannonEntOfFeature在ID3的calcShannonEnt函数上加了个参数feat,ID3中该函数只用计算类别变量的熵,而calcShannonEntOfFeature可以计算指定特征或者类别变量的熵。
chooseBestFeatureToSplit函数在计算好信息增益后,同时计算了当前特征的熵IV,然后相除得到信息增益比,以最大信息增益比作为最优特征。
在划分数据的时候,有可能出现特征取同一个值,那么该特征的熵为0,同时信息增益也为0(类别变量划分前后一样,因为特征只有一个取值),0/0没有意义,可以跳过该特征。
#coding=utf-8 import operator from math import log import time import os, sys import string def createDataSet(trainDataFile): print trainDataFile dataSet = [] try: fin = open(trainDataFile) for line in fin: line = line.strip() cols = line.split('\t') row = [cols[1], cols[2], cols[3], cols[4], cols[5], cols[6], cols[7], cols[8], cols[9], cols[10], cols[0]] dataSet.append(row) #print row except: print 'Usage xxx.py trainDataFilePath' sys.exit() labels = ['cip1', 'cip2', 'cip3', 'cip4', 'sip1', 'sip2', 'sip3', 'sip4', 'sport', 'domain'] print 'dataSetlen', len(dataSet) return dataSet, labels #calc shannon entropy of label or feature def calcShannonEntOfFeature(dataSet, feat): numEntries = len(dataSet) labelCounts = {} for feaVec in dataSet: currentLabel = feaVec[feat] if currentLabel not in labelCounts: labelCounts[currentLabel] = 0 labelCounts[currentLabel] += 1 shannonEnt = 0.0 for key in labelCounts: prob = float(labelCounts[key])/numEntries shannonEnt -= prob * log(prob, 2) return shannonEnt def splitDataSet(dataSet, axis, value): retDataSet = [] for featVec in dataSet: if featVec[axis] == value: reducedFeatVec = featVec[:axis] reducedFeatVec.extend(featVec[axis+1:]) retDataSet.append(reducedFeatVec) return retDataSet def chooseBestFeatureToSplit(dataSet): numFeatures = len(dataSet[0]) - 1 #last col is label baseEntropy = calcShannonEntOfFeature(dataSet, -1) bestInfoGainRate = 0.0 bestFeature = -1 for i in range(numFeatures): featList = [example[i] for example in dataSet] uniqueVals = set(featList) newEntropy = 0.0 for value in uniqueVals: subDataSet = splitDataSet(dataSet, i, value) prob = len(subDataSet) / float(len(dataSet)) newEntropy += prob *calcShannonEntOfFeature(subDataSet, -1) #calc conditional entropy infoGain = baseEntropy - newEntropy iv = calcShannonEntOfFeature(dataSet, i) if(iv == 0): #value of the feature is all same,infoGain and iv all equal 0, skip the feature continue infoGainRate = infoGain / iv if infoGainRate > bestInfoGainRate: bestInfoGainRate = infoGainRate bestFeature = i return bestFeature #feature is exhaustive, reture what you want label def majorityCnt(classList): classCount = {} for vote in classList: if vote not in classCount.keys(): classCount[vote] = 0 classCount[vote] += 1 return max(classCount) def createTree(dataSet, labels): classList = [example[-1] for example in dataSet] if classList.count(classList[0]) ==len(classList): #all data is the same label return classList[0] if len(dataSet[0]) == 1: #all feature is exhaustive return majorityCnt(classList) bestFeat = chooseBestFeatureToSplit(dataSet) bestFeatLabel = labels[bestFeat] if(bestFeat == -1): #特征一样,但类别不一样,即类别与特征不相关,随机选第一个类别做分类结果 return classList[0] myTree = {bestFeatLabel:{}} del(labels[bestFeat]) featValues = [example[bestFeat] for example in dataSet] uniqueVals = set(featValues) for value in uniqueVals: subLabels = labels[:] myTree[bestFeatLabel][value] = createTree(splitDataSet(dataSet, bestFeat, value),subLabels) return myTree def main(): if(len(sys.argv) < 3): print 'Usage xxx.py trainSet outputTreeFile' sys.exit() data,label = createDataSet(sys.argv[1]) t1 = time.clock() myTree = createTree(data,label) t2 = time.clock() fout = open(sys.argv[2], 'w') fout.write(str(myTree)) fout.close() print 'execute for ',t2-t1 if __name__=='__main__': main()
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