python实现随机森林random forest的原理及方法
(编辑：jimmy 日期: 2024/11/21 浏览：3 次 )
引言
想通过随机森林来获取数据的主要特征
1、理论
随机森林是一个高度灵活的机器学习方法，拥有广泛的应用前景，从市场营销到医疗保健保险。 既可以用来做市场营销模拟的建模，统计客户来源，保留和流失。也可用来预测疾病的风险和病患者的易感性。
根据个体学习器的生成方式，目前的集成学习方法大致可分为两大类，即个体学习器之间存在强依赖关系，必须串行生成的序列化方法，以及个体学习器间不存在强依赖关系，可同时生成的并行化方法；
前者的代表是Boosting，后者的代表是Bagging和“随机森林”（Random
Forest）
随机森林在以决策树为基学习器构建Bagging集成的基础上，进一步在决策树的训练过程中引入了随机属性选择（即引入随机特征选择）。
简单来说，随机森林就是对决策树的集成，但有两点不同：
（2）特征选取的差异性：每个决策树的n个分类特征是在所有特征中随机选择的（n是一个需要我们自己调整的参数）
随机森林，简单理解， 比如预测salary，就是构建多个决策树job，age，house，然后根据要预测的量的各个特征（teacher，39，suburb）分别在对应决策树的目标值概率（salary<5000,salary>=5000），从而，确定预测量的发生概率（如，预测出P(salary<5000)=0.3）.
随机森林是一个可做能够回归和分类。 它具备处理大数据的特性，而且它有助于估计或变量是非常重要的基础数据建模。
参数说明：
最主要的两个参数是n_estimators和max_features。
n_estimators：表示森林里树的个数。理论上是越大越好。但是伴随着就是计算时间的增长。但是并不是取得越大就会越好，预测效果最好的将会出现在合理的树个数。
max_features：随机选择特征集合的子集合，并用来分割节点。子集合的个数越少，方差就会减少的越快，但同时偏差就会增加的越快。根据较好的实践经验。如果是回归问题则：
max_features＝n_features，如果是分类问题则max_features＝sqrt(n_features)。
如果想获取较好的结果，必须将max_depth＝None,同时min_sample_split=1。
同时还要记得进行cross_validated（交叉验证），除此之外记得在random forest中，bootstrap=True。但在extra-trees中，bootstrap=False。
2、随机森林python实现
2.1Demo1
实现随机森林基本功能
#随机森林
from sklearn.tree import DecisionTreeRegressor
from sklearn.ensemble import RandomForestRegressor
import numpy as np
from sklearn.datasets import load_iris
iris=load_iris()
#print iris#iris的４个属性是：萼片宽度 萼片长度 花瓣宽度 花瓣长度 标签是花的种类：setosa versicolour virginica
print(iris['target'].shape)
rf=RandomForestRegressor()#这里使用了默认的参数设置
rf.fit(iris.data[:150],iris.target[:150])#进行模型的训练
#随机挑选两个预测不相同的样本
instance=iris.data[[100,109]]
print(instance)
rf.predict(instance[[0]])
print('instance 0 prediction；',rf.predict(instance[[0]]))
print( 'instance 1 prediction；',rf.predict(instance[[1]]))
print(iris.target[100],iris.target[109])
运行结果
(150,)
[[ 6.3 3.3 6. 2.5]
[ 7.2 3.6 6.1 2.5]]
instance 0 prediction； [ 2.]
instance 1 prediction； [ 2.]
2 2
2.2 Demo2
3种方法的比较
#random forest test from sklearn.model_selection import cross_val_score from sklearn.datasets import make_blobs from sklearn.ensemble import RandomForestClassifier from sklearn.ensemble import ExtraTreesClassifier from sklearn.tree import DecisionTreeClassifier X, y = make_blobs(n_samples=10000, n_features=10, centers=100,random_state=0) clf = DecisionTreeClassifier(max_depth=None, min_samples_split=2,random_state=0) scores = cross_val_score(clf, X, y) print(scores.mean()) clf = RandomForestClassifier(n_estimators=10, max_depth=None,min_samples_split=2, random_state=0) scores = cross_val_score(clf, X, y) print(scores.mean()) clf = ExtraTreesClassifier(n_estimators=10, max_depth=None,min_samples_split=2, random_state=0) scores = cross_val_score(clf, X, y) print(scores.mean())
运行结果：
0.979408793821
0.999607843137
0.999898989899
2.3 Demo3-实现特征选择
#随机森林2
from sklearn.tree import DecisionTreeRegressor
from sklearn.ensemble import RandomForestRegressor
import numpy as np
from sklearn.datasets import load_iris
iris=load_iris()
from sklearn.model_selection import cross_val_score, ShuffleSplit
X = iris["data"]
Y = iris["target"]
names = iris["feature_names"]
rf = RandomForestRegressor()
scores = []
for i in range(X.shape[1]):
score = cross_val_score(rf, X[:, i:i+1], Y, scoring="r2",
cv=ShuffleSplit(len(X), 3, .3))
scores.append((round(np.mean(score), 3), names[i]))
print(sorted(scores, reverse=True))
运行结果：
[(0.89300000000000002, 'petal width (cm)'), (0.82099999999999995, 'petal length
(cm)'), (0.13, 'sepal length (cm)'), (-0.79100000000000004, 'sepal width (cm)')]
2.4 demo4-随机森林
本来想利用以下代码来构建随机随机森林决策树，但是，遇到的问题是，程序一直在运行，无法响应，还需要调试。
#随机森林4
#coding:utf-8
import csv
from random import seed
from random import randrange
from math import sqrt
def loadCSV(filename):#加载数据，一行行的存入列表
dataSet = []
with open(filename, 'r') as file:
csvReader = csv.reader(file)
for line in csvReader:
dataSet.append(line)
return dataSet
# 除了标签列，其他列都转换为float类型
def column_to_float(dataSet):
featLen = len(dataSet[0]) - 1
for data in dataSet:
for column in range(featLen):
data[column] = float(data[column].strip())
# 将数据集随机分成N块，方便交叉验证，其中一块是测试集，其他四块是训练集
def spiltDataSet(dataSet, n_folds):
fold_size = int(len(dataSet) / n_folds)
dataSet_copy = list(dataSet)
dataSet_spilt = []
for i in range(n_folds):
fold = []
while len(fold) < fold_size: # 这里不能用if，if只是在第一次判断时起作用，while执行循环，直到条件不成立
index = randrange(len(dataSet_copy))
fold.append(dataSet_copy.pop(index)) # pop() 函数用于移除列表中的一个元素（默认最后一个元素），并且返回该元素的值。
dataSet_spilt.append(fold)
return dataSet_spilt
# 构造数据子集
def get_subsample(dataSet, ratio):
subdataSet = []
lenSubdata = round(len(dataSet) * ratio)#返回浮点数
while len(subdataSet) < lenSubdata:
index = randrange(len(dataSet) - 1)
subdataSet.append(dataSet[index])
# print len(subdataSet)
return subdataSet
# 分割数据集
def data_spilt(dataSet, index, value):
left = []
right = []
for row in dataSet:
if row[index] < value:
left.append(row)
else:
right.append(row)
return left, right
# 计算分割代价
def spilt_loss(left, right, class_values):
loss = 0.0
for class_value in class_values:
left_size = len(left)
if left_size != 0: # 防止除数为零
prop = [row[-1] for row in left].count(class_value) / float(left_size)
loss += (prop * (1.0 - prop))
right_size = len(right)
if right_size != 0:
prop = [row[-1] for row in right].count(class_value) / float(right_size)
loss += (prop * (1.0 - prop))
return loss
# 选取任意的n个特征，在这n个特征中，选取分割时的最优特征
def get_best_spilt(dataSet, n_features):
features = []
class_values = list(set(row[-1] for row in dataSet))
b_index, b_value, b_loss, b_left, b_right = 999, 999, 999, None, None
while len(features) < n_features:
index = randrange(len(dataSet[0]) - 1)
if index not in features:
features.append(index)
# print 'features:',features
for index in features:#找到列的最适合做节点的索引，（损失最小）
for row in dataSet:
left, right = data_spilt(dataSet, index, row[index])#以它为节点的，左右分支
loss = spilt_loss(left, right, class_values)
if loss < b_loss:#寻找最小分割代价
b_index, b_value, b_loss, b_left, b_right = index, row[index], loss, left, right
# print b_loss
# print type(b_index)
return {'index': b_index, 'value': b_value, 'left': b_left, 'right': b_right}
# 决定输出标签
def decide_label(data):
output = [row[-1] for row in data]
return max(set(output), key=output.count)
# 子分割，不断地构建叶节点的过程对对对
def sub_spilt(root, n_features, max_depth, min_size, depth):
left = root['left']
# print left
right = root['right']
del (root['left'])
del (root['right'])
# print depth
if not left or not right:
root['left'] = root['right'] = decide_label(left + right)
# print 'testing'
return
if depth > max_depth:
root['left'] = decide_label(left)
root['right'] = decide_label(right)
return
if len(left) < min_size:
root['left'] = decide_label(left)
else:
root['left'] = get_best_spilt(left, n_features)
# print 'testing_left'
sub_spilt(root['left'], n_features, max_depth, min_size, depth + 1)
if len(right) < min_size:
root['right'] = decide_label(right)
else:
root['right'] = get_best_spilt(right, n_features)
# print 'testing_right'
sub_spilt(root['right'], n_features, max_depth, min_size, depth + 1)
# 构造决策树
def build_tree(dataSet, n_features, max_depth, min_size):
root = get_best_spilt(dataSet, n_features)
sub_spilt(root, n_features, max_depth, min_size, 1)
return root
# 预测测试集结果
def predict(tree, row):
predictions = []
if row[tree['index']] < tree['value']:
if isinstance(tree['left'], dict):
return predict(tree['left'], row)
else:
return tree['left']
else:
if isinstance(tree['right'], dict):
return predict(tree['right'], row)
else:
return tree['right']
# predictions=set(predictions)
def bagging_predict(trees, row):
predictions = [predict(tree, row) for tree in trees]
return max(set(predictions), key=predictions.count)
# 创建随机森林
def random_forest(train, test, ratio, n_feature, max_depth, min_size, n_trees):
trees = []
for i in range(n_trees):
train = get_subsample(train, ratio)#从切割的数据集中选取子集
tree = build_tree(train, n_features, max_depth, min_size)
# print 'tree %d: '%i,tree
trees.append(tree)
# predict_values = [predict(trees,row) for row in test]
predict_values = [bagging_predict(trees, row) for row in test]
return predict_values
# 计算准确率
def accuracy(predict_values, actual):
correct = 0
for i in range(len(actual)):
if actual[i] == predict_values[i]:
correct += 1
return correct / float(len(actual))
if __name__ == '__main__':
seed(1)
dataSet = loadCSV(r'G:\0研究生\tianchiCompetition\训练小样本2.csv')
column_to_float(dataSet)
n_folds = 5
max_depth = 15
min_size = 1
ratio = 1.0
# n_features=sqrt(len(dataSet)-1)
n_features = 15
n_trees = 10
folds = spiltDataSet(dataSet, n_folds)#先是切割数据集
scores = []
for fold in folds:
train_set = folds[
:] # 此处不能简单地用train_set=folds，这样用属于引用,那么当train_set的值改变的时候，folds的值也会改变，所以要用复制的形式。（L[:]）能够复制序列，D.copy() 能够复制字典，list能够生成拷贝 list(L)
train_set.remove(fold)#选好训练集
# print len(folds)
train_set = sum(train_set, []) # 将多个fold列表组合成一个train_set列表
# print len(train_set)
test_set = []
for row in fold:
row_copy = list(row)
row_copy[-1] = None
test_set.append(row_copy)
# for row in test_set:
# print row[-1]
actual = [row[-1] for row in fold]
predict_values = random_forest(train_set, test_set, ratio, n_features, max_depth, min_size, n_trees)
accur = accuracy(predict_values, actual)
scores.append(accur)
print ('Trees is %d' % n_trees)
print ('scores:%s' % scores)
print ('mean score:%s' % (sum(scores) / float(len(scores))))
2.5 随机森林分类sonic data
# CART on the Bank Note dataset
from random import seed
from random import randrange
from csv import reader
# Load a CSV file
def load_csv(filename):
file = open(filename, "r")
lines = reader(file)
dataset = list(lines)
return dataset
# Convert string column to float
def str_column_to_float(dataset, column):
for row in dataset:
row[column] = float(row[column].strip())
# Split a dataset into k folds
def cross_validation_split(dataset, n_folds):
dataset_split = list()
dataset_copy = list(dataset)
fold_size = int(len(dataset) / n_folds)
for i in range(n_folds):
fold = list()
while len(fold) < fold_size:
index = randrange(len(dataset_copy))
fold.append(dataset_copy.pop(index))
dataset_split.append(fold)
return dataset_split
# Calculate accuracy percentage
def accuracy_metric(actual, predicted):
correct = 0
for i in range(len(actual)):
if actual[i] == predicted[i]:
correct += 1
return correct / float(len(actual)) * 100.0
# Evaluate an algorithm using a cross validation split
def evaluate_algorithm(dataset, algorithm, n_folds, *args):
folds = cross_validation_split(dataset, n_folds)
scores = list()
for fold in folds:
train_set = list(folds)
train_set.remove(fold)
train_set = sum(train_set, [])
test_set = list()
for row in fold:
row_copy = list(row)
test_set.append(row_copy)
row_copy[-1] = None
predicted = algorithm(train_set, test_set, *args)
actual = [row[-1] for row in fold]
accuracy = accuracy_metric(actual, predicted)
scores.append(accuracy)
return scores
# Split a data set based on an attribute and an attribute value
def test_split(index, value, dataset):
left, right = list(), list()
for row in dataset:
if row[index] < value:
left.append(row)
else:
right.append(row)
return left, right
# Calculate the Gini index for a split dataset
def gini_index(groups, class_values):
gini = 0.0
for class_value in class_values:
for group in groups:
size = len(group)
if size == 0:
continue
proportion = [row[-1] for row in group].count(class_value) / float(size)
gini += (proportion * (1.0 - proportion))
return gini
# Select the best split point for a dataset
def get_split(dataset):
class_values = list(set(row[-1] for row in dataset))
b_index, b_value, b_score, b_groups = 999, 999, 999, None
for index in range(len(dataset[0])-1):
for row in dataset:
groups = test_split(index, row[index], dataset)
gini = gini_index(groups, class_values)
if gini < b_score:
b_index, b_value, b_score, b_groups = index, row[index], gini, groups
print ({'index':b_index, 'value':b_value})
return {'index':b_index, 'value':b_value, 'groups':b_groups}
# Create a terminal node value
def to_terminal(group):
outcomes = [row[-1] for row in group]
return max(set(outcomes), key=outcomes.count)
# Create child splits for a node or make terminal
def split(node, max_depth, min_size, depth):
left, right = node['groups']
del(node['groups'])
# check for a no split
if not left or not right:
node['left'] = node['right'] = to_terminal(left + right)
return
# check for max depth
if depth >= max_depth:
node['left'], node['right'] = to_terminal(left), to_terminal(right)
return
# process left child
if len(left) <= min_size:
node['left'] = to_terminal(left)
else:
node['left'] = get_split(left)
split(node['left'], max_depth, min_size, depth+1)
# process right child
if len(right) <= min_size:
node['right'] = to_terminal(right)
else:
node['right'] = get_split(right)
split(node['right'], max_depth, min_size, depth+1)
# Build a decision tree
def build_tree(train, max_depth, min_size):
root = get_split(train)
split(root, max_depth, min_size, 1)
return root
# Make a prediction with a decision tree
def predict(node, row):
if row[node['index']] < node['value']:
if isinstance(node['left'], dict):
return predict(node['left'], row)
else:
return node['left']
else:
if isinstance(node['right'], dict):
return predict(node['right'], row)
else:
return node['right']
# Classification and Regression Tree Algorithm
def decision_tree(train, test, max_depth, min_size):
tree = build_tree(train, max_depth, min_size)
predictions = list()
for row in test:
prediction = predict(tree, row)
predictions.append(prediction)
return(predictions)
# Test CART on Bank Note dataset
seed(1)
# load and prepare data
filename = r'G:\0pythonstudy\决策树\sonar.all-data.csv'
dataset = load_csv(filename)
# convert string attributes to integers
for i in range(len(dataset[0])-1):
str_column_to_float(dataset, i)
# evaluate algorithm
n_folds = 5
max_depth = 5
min_size = 10
scores = evaluate_algorithm(dataset, decision_tree, n_folds, max_depth, min_size)
print('Scores: %s' % scores)
print('Mean Accuracy: %.3f%%' % (sum(scores)/float(len(scores))))
运行结果：
{'index': 38, 'value': 0.0894}
{'index': 36, 'value': 0.8459}
{'index': 50, 'value': 0.0024}
{'index': 15, 'value': 0.0906}
{'index': 16, 'value': 0.9819}
{'index': 10, 'value': 0.0785}
{'index': 16, 'value': 0.0886}
{'index': 38, 'value': 0.0621}
{'index': 5, 'value': 0.0226}
{'index': 8, 'value': 0.0368}
{'index': 11, 'value': 0.0754}
{'index': 0, 'value': 0.0239}
{'index': 8, 'value': 0.0368}
{'index': 29, 'value': 0.1671}
{'index': 46, 'value': 0.0237}
{'index': 38, 'value': 0.0621}
{'index': 14, 'value': 0.0668}
{'index': 4, 'value': 0.0167}
{'index': 37, 'value': 0.0836}
{'index': 12, 'value': 0.0616}
{'index': 7, 'value': 0.0333}
{'index': 33, 'value': 0.8741}
{'index': 16, 'value': 0.0886}
{'index': 8, 'value': 0.0368}
{'index': 33, 'value': 0.0798}
{'index': 44, 'value': 0.0298}
Scores: [48.78048780487805, 70.73170731707317, 58.536585365853654, 51.2195121951
2195, 39.02439024390244]
Mean Accuracy: 53.659%
请按任意键继续. . .
知识点：
1.load CSV file
from csv import reader # Load a CSV file def load_csv(filename): file = open(filename, "r") lines = reader(file) dataset = list(lines) return dataset filename = r'G:\0pythonstudy\决策树\sonar.all-data.csv' dataset=load_csv(filename) print(dataset)
2.把数据转化成float格式
# Convert string column to float def str_column_to_float(dataset, column): for row in dataset: row[column] = float(row[column].strip()) # print(row[column]) # convert string attributes to integers for i in range(len(dataset[0])-1): str_column_to_float(dataset, i)
3.把最后一列的分类字符串转化成0、1整数
def str_column_to_int(dataset, column): class_values = [row[column] for row in dataset]#生成一个class label的list # print(class_values) unique = set(class_values)#set 获得list的不同元素 print(unique) lookup = dict()#定义一个字典 # print(enumerate(unique)) for i, value in enumerate(unique): lookup[value] = i # print(lookup) for row in dataset: row[column] = lookup[row[column]] print(lookup['M'])
4、把数据集分割成K份
# Split a dataset into k folds def cross_validation_split(dataset, n_folds): dataset_split = list()#生成空列表 dataset_copy = list(dataset) print(len(dataset_copy)) print(len(dataset)) #print(dataset_copy) fold_size = int(len(dataset) / n_folds) for i in range(n_folds): fold = list() while len(fold) < fold_size: index = randrange(len(dataset_copy)) # print(index) fold.append(dataset_copy.pop(index))#使用.pop()把里边的元素都删除（相当于转移），这k份元素各不相同。 dataset_split.append(fold) return dataset_split n_folds=5 folds = cross_validation_split(dataset, n_folds)#k份元素各不相同的训练集
5.计算正确率
# Calculate accuracy percentage def accuracy_metric(actual, predicted): correct = 0 for i in range(len(actual)): if actual[i] == predicted[i]: correct += 1 return correct / float(len(actual)) * 100.0#这个是二值分类正确性的表达式
6.二分类每列
# Split a data set based on an attribute and an attribute value def test_split(index, value, dataset): left, right = list(), list()#初始化两个空列表 for row in dataset: if row[index] < value: left.append(row) else: right.append(row) return left, right #返回两个列表，每个列表以value为界限对指定行（index）进行二分类。
7.使用gini系数来获得最佳分割点
# Calculate the Gini index for a split dataset
def gini_index(groups, class_values):
gini = 0.0
for class_value in class_values:
for group in groups:
size = len(group)
if size == 0:
continue
proportion = [row[-1] for row in group].count(class_value) / float(size)
gini += (proportion * (1.0 - proportion))
return gini
# Select the best split point for a dataset
def get_split(dataset):
class_values = list(set(row[-1] for row in dataset))
b_index, b_value, b_score, b_groups = 999, 999, 999, None
for index in range(len(dataset[0])-1):
for row in dataset:
groups = test_split(index, row[index], dataset)
gini = gini_index(groups, class_values)
if gini < b_score:
b_index, b_value, b_score, b_groups = index, row[index], gini, groups
# print(groups)
print ({'index':b_index, 'value':b_value,'score':gini})
return {'index':b_index, 'value':b_value, 'groups':b_groups}
这段代码，在求gini指数，直接应用定义式，不难理解。获得最佳分割点可能比较难看懂，这里用了两层迭代，一层是对不同列的迭代，一层是对不同行的迭代。并且，每次迭代，都对gini系数进行更新。
8、决策树生成
# Create child splits for a node or make terminal def split(node, max_depth, min_size, depth): left, right = node['groups'] del(node['groups']) # check for a no split if not left or not right: node['left'] = node['right'] = to_terminal(left + right) return # check for max depth if depth >= max_depth: node['left'], node['right'] = to_terminal(left), to_terminal(right) return # process left child if len(left) <= min_size: node['left'] = to_terminal(left) else: node['left'] = get_split(left) split(node['left'], max_depth, min_size, depth+1) # process right child if len(right) <= min_size: node['right'] = to_terminal(right) else: node['right'] = get_split(right) split(node['right'], max_depth, min_size, depth+1)
这里使用了递归编程，不断生成左叉树和右叉树。
9.构建决策树
# Build a decision tree def build_tree(train, max_depth, min_size): root = get_split(train) split(root, max_depth, min_size, 1) return root tree=build_tree(train_set, max_depth, min_size) print(tree)
10、预测test集
# Build a decision tree def build_tree(train, max_depth, min_size): root = get_split(train)#获得最好的分割点,下标值，groups split(root, max_depth, min_size, 1) return root # tree=build_tree(train_set, max_depth, min_size) # print(tree) # Make a prediction with a decision tree def predict(node, row): print(row[node['index']]) print(node['value']) if row[node['index']] < node['value']:#用测试集来代入训练的最好分割点，分割点有偏差时，通过搜索左右叉树来进一步比较。 if isinstance(node['left'], dict):#如果是字典类型，执行操作 return predict(node['left'], row) else: return node['left'] else: if isinstance(node['right'], dict): return predict(node['right'], row) else: return node['right'] tree = build_tree(train_set, max_depth, min_size) predictions = list() for row in test_set: prediction = predict(tree, row) predictions.append(prediction)
11.评价决策树
# Evaluate an algorithm using a cross validation split def evaluate_algorithm(dataset, algorithm, n_folds, *args): folds = cross_validation_split(dataset, n_folds) scores = list() for fold in folds: train_set = list(folds) train_set.remove(fold) train_set = sum(train_set, []) test_set = list() for row in fold: row_copy = list(row) test_set.append(row_copy) row_copy[-1] = None predicted = algorithm(train_set, test_set, *args) actual = [row[-1] for row in fold] accuracy = accuracy_metric(actual, predicted) scores.append(accuracy) return scores
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