R中缺少因子水平

Question

我有一个因子列，我得到了一个缺失的级别，为什么R创建了这个缺失级别？

d0s$y
 [1] E E E E E E G G G G G G G P P P P P P P
Levels:  E G P

levels(d0s$y)
[1] ""  "E" "G" "P"

Answer 1

可能是在子集化之前数据集中存在空白""。一种方法是调用droplevels删除未使用的级别

d0s$y <- droplevels(d0s$y)

或再次致电factor

d0s$y <- factor(d0s$y)

但是，也可能是""元素已经存在，但因为它是factor打印选项不显示

y1 <- factor(rep(c("E", "G", "P", ""), each = 3))
y1
#[1] E E E G G G P P P      
#Levels:  E G P
levels(y1)
#[1] ""  "E" "G" "P"

假设我们将'y1'

分组

 y2 <- y1[y1 %in% c("E", "G", "P")]
 levels(y2) #the unused levels are still there
 #[1] ""  "E" "G" "P"

除非我们放弃这些级别

 levels(droplevels(y2))
 #[1] "E" "G" "P"