本文实例讲述了基于Python实现的ID3决策树功能。分享给大家供大家参考，具体如下：
ID3算法是决策树的一种，它是基于奥卡姆剃刀原理的，即用尽量用较少的东西做更多的事。ID3算法，即Iterative Dichotomiser 3，迭代二叉树3代，是Ross Quinlan发明的一种决策树算法，这个算法的基础就是上面提到的奥卡姆剃刀原理，越是小型的决策树越优于大的决策树，尽管如此，也不总是生成最小的树型结构，而是一个启发式算法。
如下示例是一个判断海洋生物数据是否是鱼类而构建的基于ID3思想的决策树
# coding=utf-8
import operator
from math import log
import time
def createDataSet():
dataSet = [[1, 1, 'yes'],
[1, 1, 'yes'],
[1, 0, 'no'],
[0, 1, 'no'],
[0, 1, 'no'],
[0,0,'maybe']]
labels = ['no surfaceing', 'flippers']
return dataSet, labels
# 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
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 # 因为数据集的最后一项是标签
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)
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()
运行结果如下：
{'no surfaceing': {0: {'flippers': {0: 'maybe', 1: 'no'}}, 1: {'flippers': {0: 'no', 1: 'yes'}}}}
execute for 0.0103958394532
最后我们测试一下这个脚本即可，如果想把这个生成的决策树用图像画出来，也只是在需要在脚本里面定义一个plottree的函数即可。
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希望本文所述对大家Python程序设计有所帮助。
