R - 添加在组内按顺序计数但重复重复的列

我正在寻找一种解决方案来添加“desired_result”列，最好使用 dplyr 和/或 ave()。请参阅此处的数据框，其中组是“部分”，我希望“desired_results”列按顺序计数的唯一实例位于“exhibit”中：

structure(list(section = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L), exhibit = structure(c(1L, 
2L, 3L, 3L, 1L, 2L, 2L, 3L), .Label = c("a", "b", "c"), class = "factor"), 
desired_result = c(1L, 2L, 3L, 3L, 1L, 2L, 2L, 3L)), .Names = c("section", 
"exhibit", "desired_result"), class = "data.frame", row.names = c(NA, 
-8L))

dense_rank it is

library(dplyr)
df %>% 
  group_by(section) %>% 
  mutate(desire=dense_rank(exhibit))
#  section exhibit desired_result desire
#1       1       a              1      1
#2       1       b              2      2
#3       1       c              3      3
#4       1       c              3      3
#5       2       a              1      1
#6       2       b              2      2
#7       2       b              2      2
#8       2       c              3      3