R语言基础包应用ifelse函数对变量分类并进行因子化（fact

用法如下
ifelse.(conditions, true, false, na = NA)

简单展示如下

a<-c(1:10)

head(a)
## [1] 1 2 3 4 5 6
b<-ifelse(a>5,"大于5","小于5")

b
##  [1] "小于5" "小于5" "小于5" "小于5" "小于5" "大于5" "大于5" "大于5" "大于5"，"大于5"

b<-as.factor(b)
levels(b)
#"大于5" "小于5"
str(b)
#Factor w/ 2 levels "大于5","小于5": 2 2 2 2 2 1 1 1 1 1