使用dplyr过滤数据帧后，从因子中删除未使用的级别

install.packages("dplyr")
library(dplyr)

df <- data.frame(
  id = c(1, 2, 3, 4, 5),
  category = factor(c("A", "B", "C", "A", "B"))
)

filtered_df <- df %>% filter(id > 2)

filtered_df$category <- droplevels(filtered_df$category)

levels(filtered_df$category)