r - 按因子过滤后如何删除未使用的级别？

Question

这是一个取自 a fellow SO member 的示例.

# define a %not% to be the opposite of %in%
library(dplyr)
# data
f <- c("a","a","a","b","b","c")
s <- c("fall","spring","other", "fall", "other", "other")
v <- c(3,5,1,4,5,2)
(dat0 <- data.frame(f, s, v))
#  f      s v
#1 a   fall 3
#2 a spring 5
#3 a  other 1
#4 b   fall 4
#5 b  other 5
#6 c  other 2
(sp.tmp <- filter(dat0, s == "spring"))
#  f      s v
#1 a spring 5
(str(sp.tmp))
#'data.frame':  1 obs. of  3 variables:
# $ f: Factor w/ 3 levels "a","b","c": 1
# $ s: Factor w/ 3 levels "fall","other",..: 3
# $ v: num 5

由filter()产生的df保留了原始 df 的所有级别。

删除未使用级别的推荐方法是什么，即 "fall"和"others" ，在dplyr内框架？

Answer 1

你可以这样做:

dat1 <- dat0 %>%
  filter(s == "spring") %>% 
  droplevels()

然后

str(df)
#'data.frame':  1 obs. of  3 variables:
# $ f: Factor w/ 1 level "a": 1
# $ s: Factor w/ 1 level "spring": 1
# $ v: num 5