#####回归树####
library(MASS)
data("Boston")
set.seed(1)
train=sample(1:nrow(Boston),nrow(Boston)/2)
tree.boston=tree(medv~.,Boston,subset=train) #房价中位数
plot(tree.boston)
text(tree.boston,pretty=0)

cv.boston=cv.tree(tree.boston)
plot(cv.bostonsize,cv.bostondev,type="b")

prune.boston=prune.tree(tree.boston,best=5)
plot(prune.boston)
text(prune.boston,pretty=0)

yhat=predict(tree.boston,newdata = Boston[-train,])
boston.test=Boston[-train,"medv"]
plot(yhat,boston.test)
abline(0,1)
mean((yhat-boston.test)^2)  #25