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uot;big","large"),ordered=T)
> x
[1] middle small big large
Levels: small < middle < big < large # R在输出有序因子时会显示因子水平的顺序
> y <- factor(x=c("middle","small","big","large"),levels=c("small","middle","big","large"),ordered=T,labels=c(1,2,3,4))
> y
[1] 2 1 3 4
Levels: 1 < 2 < 3 < 4
> z <- factor(x=c("middle","small","big","large"),levels=c("small","middle","big","large"),exclude=c("small","middle"))
> z
[1] <NA> <NA> big large # 由于去除了水平small和middle，所以原始数据中水平为small和middle的值输出为NA
Levels: big large
> x.integer <- unclass(x) # 通过移除因子x的类属性创建整型向量x.integer 注意此时因子x本身并没有发生变化
> x.integer
[1] 2 1 3 4
attr(,"levels")
[1] "small" "middle" "big" "large"
> class(x.integer)
[1] "integer"
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