机器学习算法—决策树应用

def createDataSet():
dataSet=[[1,1,'yes'],
[1,1,'yes'],
[1,0,'no'],
[0,1,'no'],
[0,1,'no']]
labels = ['no surfaceing','flippers']
return dataSet, labels

#计算香农熵,分两步，第一步计算频率，第二步根据公式计算香农熵
def calcShannonEnt(dataSet):
numEntries = len(dataSet)
labelCounts = {}
for feaVec in dataSet:
currentLabel = feaVec[-1]
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

#划分数据集，将满足X[axis]==value的值都划分到一起，返回一个划分好的集合（不包括用来划分的axis属性）
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

#选择最好的属性进行划分，思路就是对每个属性都划分下，看哪个好。这里使用到了一个set来选取列表中唯一的元素
def chooseBestFeatureToSplit(dataSet):
numFeatures = len(dataSet[0]) - 1#因为数据集的最后一项是标签yes/no
baseEntropy = calcShannonEnt(dataSet)
bestInfoGain = 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 * calcShannonEnt(subDataSet)
infoGain = baseEntropy -newEntropy
if infoGain > bestInfoGain:
bestInfoGain = infoGain
bestFeature = i
return bestFeature

#因为我们递归构建决策树是根据属性的消耗进行计算的，所以可能会存在最后属性用完了，但是分类还是没有算完，这时候就会采用多数表决的方式计算节点分类
def majorityCnt(classList):
classCount = {}
for vote in classList:
if vote not in classCount.keys():
classCount[vote] = 0
classCount[vote] += 1
return max(classCount)

#构建决策树
def createTree(dataSet, labels):
classList = [example[-1] for example in dataSet]
if classList.count(classList[0]) ==len(classList):#类别相同则停止划分
return classList[0]
if len(dataSet[0]) == 1:#所有特征已经用完
return majorityCnt(classList)
bestFeat = chooseBestFeatureToSplit(dataSet)
bestFeatLabel = labels[bestFeat]
myTree = {bestFeatLabel:{}}
del(labels[bestFeat])
featValues = [example[bestFeat] for example in dataSet]
uniqueVals = set(featValues)
for value in uniqueVals:
subLabels = labels[:]#为了不改变原始列表的内容复制了一下
myTree[bestFeatLabel][value] = createTree(splitDataSet(dataSet, bestFeat,value),subLabels)
return myTree

def main():
data,label = createDataSet()
t1 = time.clock()
myTree = createTree(data,label)
t2 = time.clock()
print (myTree)
print ('execute for ',t2-t1)
if __name__=='__main__':
main()