利用factor() 或者ordered() 两个函数将普通变量转换成有序因子。
因子用来存储类别变量(categorical variables)和有序变量，用来分类或者计数。可以利用factor() 或者ordered() 两个函数将普通变量转换成有序因子
例如：
> A=c("a","c","e","b","d")
factor
> B=factor(A,levels=c("a","b","c","d","e"),ordered = T)
> B
[1] a c e b d
Levels: a < b < c < d < e
ordered
> C=ordered(A,labels=c("a","b","c","d","e"))
> C
[1] a c e b d
Levels: a < b < c < d < e
