决策树

冷不防 2021-09-30 04:16 524阅读 0赞

决策树对实例进行分类的树形结构，由节点和有向边组成。其实很像平时画的流程图。

学习决策树之前要搞懂几个概念：

熵：表示随机变量不确定性的度量，定义：H（p）=-![1409773-20180612174021427-268515251.png][]

信息增益：集合D的经验熵与特征A条件下D的经验条件熵H(D/A）之差（公式省略，自行查找）

信息增益比：信息增益g(D,A)与训练数据集D关于特征A的值得熵HA（D）之比（公式省略）

基尼系数：（公式省略）

以上几个公式要牢记并学会推到。

具体计算过程：

ID3算法：寻找信息增益最大的特征

![1409773-20180612174837178-1095952649.jpg][]![1409773-20180612174854269-1579627974.jpg][]

C4.5 寻找信息增益比最大的特征

![1409773-20180612175106368-939126747.jpg][]

另外有CART树算法，使用基尼系数来确定特征选择部分。

树的剪枝分为前剪枝和后剪枝。目的为防止过拟合。

前剪枝即为在树的构建过程中，若新增加的分支未使得准确率增加，则不进行该分支操作。

后剪枝即构建完决策树之后，从最底部的分支开始，若去掉该分支，分类准确性增加，则去掉该分支，否则保留。

def chooseBestFeatureToSplit(dataSet):#关键部分代码为寻找最优特征，寻找使得信息增益最大的特征
        numFeatures = len(dataSet[0]) - 1  # the last column is used for the labels
        baseEntropy = calcShannonEnt(dataSet)
        bestInfoGain = 0.0;
        bestFeature = -1
        for i in range(numFeatures):  # iterate over all the features
            featList = [example[i] for example in dataSet]  # create a list of all the examples of this feature
            uniqueVals = set(featList)  # get a set of unique values
            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  # calculate the info gain; ie reduction in entropy
            if (infoGain > bestInfoGain):  # compare this to the best gain so far
                bestInfoGain = infoGain  # if better than current best, set to best
                bestFeature = i
        return bestFeature  # returns an integer

def createTree(dataSet, labels): #构建决策树，寻找最优特征，做为根节点，之后根据特征分为几类，进行递归，最终形成完整决策树
        classList = [example[-1] for example in dataSet]
        if classList.count(classList[0]) == len(classList):
            return classList[0]  # stop splitting when all of the classes are equal
        if len(dataSet[0]) == 1:  # stop splitting when there are no more features in dataSet
            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[:]  # copy all of labels, so trees don't mess up existing labels
            myTree[bestFeatLabel][value] = createTree(splitDataSet(dataSet, bestFeat, value), subLabels)
        return myTree

对应SKlearn中的API接口：

`DecisionTreeClassifier 分类，注意此方法只有前剪枝选项。`

`DecisionTreeRegressor   回归。`

转载于:https://www.cnblogs.com/the-home-of-123/p/9174426.html

[1409773-20180612174021427-268515251.png]: /images/20210725/441b8154b7ef49a8af3ff9b8b9b137db.png
[1409773-20180612174837178-1095952649.jpg]: https://images2018.cnblogs.com/blog/1409773/201806/1409773-20180612174837178-1095952649.jpg
[1409773-20180612174854269-1579627974.jpg]: /images/20210725/e8adbea214694377b369bdf8f69204e5.png
[1409773-20180612175106368-939126747.jpg]: /images/20210725/ac2053c30bf34626b24469a5afcd2449.png