随机森林算法实现

关键词：随机森林算法原理、随机森林算法 python、随机森林算法

随机森林是一个包含多个决策树的分类器，该分类器的输出结果是由所有决策树输出结果的众数而定，每个决策树都生长于一个被称为bootstrap样本的训练数据集之上，所谓“bootstrap样本”的构造原理为：对于容量为n原始训练数据，采取重复抽样的方式抽样n次，形成一组新的训练数据，被称为原始数据的一个bootstrap样本。在bootstrap样本上构造决策树的原理与传统的构造决策树的方法大致相同，但也存在一些差别，不同之处在于：在对非叶节点分裂时，需要从所有的m个属性中随机地选取p个属性(一般建议取p的值为m的平方根)，再从这p个属性中选取最佳分裂属性以及相应的最佳分裂阀值，在该bootstrap样本上最终生成的决策树不需要剪枝。在原始训练数据上生成多个bootstrap样本，利用上述方法就会得到多颗完全生长的决策树，因为取样过程以及变量选取过程都是随机的，因此被形象地称为随机森林。

算法的为代码如下：

本文利用Python语言实现随机森林算法，构建的每颗决策树都基于CART算法，所有代码都位于一个文件

randomforests.py中，

[python]

from __future__ import division
import numpy as np
import math
class node:
def __init__(self, col=-1, value=None, results=None, trueBranch=None, falseBranch=None):
self.col = col
self.value = value
self.results = results
self.trueBranch = trueBranch
self.falseBranch = falseBranch
def getLabel(self):
if self.results == None:
return None
else:
max_counts = 0
for key in self.results.keys():
if self.results[key] > max_counts:
label = key
max_counts = self.results[key]
return label
class RandomForestsClassifier:
def __init__(self, n_bootstrapSamples=20):
self.n_bootstrapSamples = n_bootstrapSamples
self.list_tree = []
def divideSet(self, samples, column, value):
splitFunction = None
if isinstance(value,int) or isinstance(value,float):
splitFunction = lambda row: row[column] >= value
else:
splitFunction = lambda row: row[column] == value
set1 = [row for row in samples if splitFunction(row)]
set2 = [row for row in samples if not splitFunction(row)]
return (set1,set2)
def uniqueCounts(self, samples):
results = {}
for row in samples:
r = row[len(row)-1]
if r not in results:
results[r] = 0
results[r] = results[r]+1
return results
def giniEstimate(self, samples):
if len(samples)==0: return 0
total = len(samples)
counts = self.uniqueCounts(samples)
gini = 0
for target in counts:
gini = gini + pow(counts[target],2)
gini = 1 – gini / pow(total,2)
return gini
def buildTree(self, samples):#构造CART决策树
if len(samples) == 0:
return node()
currentGini = self.giniEstimate(samples)
bestGain = 0
bestCriteria = None
bestSets = None
colCount = len(samples[0]) – 1
colRange = range(0,colCount)
np.random.shuffle(colRange)
for col in colRange[0:int(math.ceil(math.sqrt(colCount)))]:
colValues = {}
for row in samples:
colValues[row[col]] = 1
for value in colValues.keys():
(set1,set2) = self.divideSet(samples,col,value)
gain = currentGini – (len(set1)*self.giniEstimate(set1) + len(set2)*self.giniEstimate(set2)) / len(samples)
if gain > bestGain and len(set1) > 0 and len(set2) > 0:
bestGain = gain
bestCriteria = (col,value)
bestSets = (set1,set2)
if bestGain > 0:
trueBranch = self.buildTree(bestSets[0])
falseBranch = self.buildTree(bestSets[1])
return node(col=bestCriteria[0],value=bestCriteria[1],trueBranch=trueBranch,falseBranch=falseBranch)
else:
return node(results=self.uniqueCounts(samples))
def printTree(self, tree,indent=‘ ‘):#以文本形式显示决策树
if tree.results != None:
print str(tree.results)
else:
print str(tree.col)+‘:’+str(tree.value)+‘?’
print indent+‘T->’,
self.printTree(tree.trueBranch,indent+‘ ‘)
print indent+‘F->’,
self.printTree(tree.falseBranch,indent+‘ ‘)
def predict_tree(self, observation, tree):#利用决策树进行分类
if tree.results != None:
return tree.getLabel()
else:
v = observation[tree.col]
branch = None
if isinstance(v,int) or isinstance(v,float):
if v >= tree.value: branch = tree.trueBranch
else: branch = tree.falseBranch
else:
if v == tree.value: branch = tree.trueBranch
else: branch = tree.falseBranch
return self.predict_tree(observation,branch)
def generateBootstrapSamples(self, data):#构造bootstrap样本
samples = []
for i in range(len(data)):
samples.append(data[np.random.randint(len(data))])
return samples
def fit(self, data):#构造随机森林
for i in range(self.n_bootstrapSamples):
samples = self.generateBootstrapSamples(data)
currentTree = self.buildTree(samples)
self.list_tree.append(currentTree)
def predict_randomForests(self, observation):#利用随机森林对给定观测数据进行分类
results = {}
for i in range(len(self.list_tree)):
currentResult = self.predict_tree(observation, self.list_tree[i])
if currentResult not in results:
results[currentResult] = 0
results[currentResult] = results[currentResult] + 1
max_counts = 0
for key in results.keys():
if results[key] > max_counts:
finalResult = key
max_counts = results[key]
return finalResult

接下来使用经典的鸢尾花数据集来检验随机森林的分类效果，代码如下：

from sklearn.datasets import load_iris
iris = load_iris()
X = iris.data
y = iris.target
temp_data = np.concatenate([X, y.reshape((150,1))], axis=1)
#由于上述代码要求输入的观测数据存储在二维列表中，需将numpy二维数组转换成列表
data = []
for i in range(temp_data.shape[0]):
temp = []
for j in range(temp_data.shape[1]):
temp.append(temp_data[i][j])
data.append(temp)
rowRange = range(150)
np.random.shuffle(rowRange)
#从鸢尾花数据集(容量为150)按照随机均匀抽样的原则选取70%的数据作为训练数据
training_data = [data[i] for i in rowRange[0:105]]
#按照随机均匀抽样的原则选取30%的数据作为检验数据
testing_data = [data[i] for i in rowRange[105:150]]
classifier = randomforest.RandomForestsClassifier(n_bootstrapSamples=10)#初始化随机森林
classifier.fit(training_data)#利用训练数据进行拟合
finalResults = []
for row in testing_data:
finalResult = classifier3.predict_randomForests(row[0:4])#对检验数据集进行分类
finalResults.append(finalResult)
errorVector = np.zeros((45,1))
errorVector[np.array(finalResults) != (np.array(testing_data))[:,4]] =1
errorVector.sum()/45#计算错判率

运行上述代码，可以计算出随机森林分类器classifier在检验数据集上的错判率为11.11%，取得不错的分类效果，实际上如果将随机森林中决策树的个数设置为20(n_bootstrapSamples=10)，再重复运行上述代码，会发现错判率几乎接近0，这些都表明随机森林作为分类器的效果是非常好的。

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