根据id和r中的日期对第一次观察进行分组

dplyr 通过分组然后切片:

library(tidyverse)
df <- read_table("id date Observation
1  3    A
1  2    B
1  8    B
2  5    B
2  3    A
2  9    A")

df %>%
    group_by(id, Observation) %>%
    slice(1)
#> # A tibble: 4 x 3
#> # Groups:   id, Observation [4]
#>      id  date Observation
#>   <dbl> <dbl> <chr>      
#> 1     1     3 A          
#> 2     1     2 B          
#> 3     2     3 A          
#> 4     2     5 B

^{于2021-04-12由 reprex package}

library(dplyr)

df %>% 
  group_by(id, Observation) %>% 
  slice(1) %>% 
  ungroup()

# OR

df %>% 
  group_by(id, Observation) %>% 
  filter(row_number() == 1) %>%
  ungroup()