如何获得一个分组变量的计数，并在R中添加另一个计数的变量？

install.packages("dplyr")

library(dplyr)

df <- data.frame(group_var = c("A", "A", "B", "B", "B", "C"))

df <- df %>%
  group_by(group_var) %>%
  mutate(count_var = n())

print(df)

  group_var count_var
  <chr>         <int>
1 A                 2
2 A                 2
3 B                 3
4 B                 3
5 B                 3
6 C                 1