Window function in R

Question

I have table

DF <- tibble(
  CLIENT = c(1,1,1,2,2,2,3,3,3),
  N_DOG = c('N1','N2','N3','N4','N5','N6','N7','N8','N9'),
  DT = as.Date(c('01.06.2021','12.07.2021','04.05.2021',
                 '03.08.2021','21.07.2021','04.01.2022',
                 '07.04.2021','09.11.2021','08.12.2021'), format = '%d.%m.%Y'),
  RANK = c(1,1,2,3,1,1,2,2,3)
)

CLIENT	N_DOG	DT	RANK
1	N1	2021-06-01	1
1	N2	2021-07-12	1
1	N3	2021-05-04	2
2	N4	2021-08-03	3
2	N5	2021-07-21	1
2	N6	2022-01-04	1
3	N7	2021-04-07	2
3	N8	2021-11-09	2
3	N9	2021-12-08	3

I want to find a contract for each client according to the following criteria:

1. Find a contract with a minimum rank
2. If there are several such contracts, we take the last one.

PS. The output must be a unique client and 1 contract

Realization :

MIN_RANK <- DF %>% 
  select(CLIENT, RANK) %>% 
  group_by(CLIENT) %>% 
  filter(RANK == min(RANK)) %>% 
  ungroup() %>% 
  distinct()

MIN_RANK %>% 
  inner_join(DF, by = c('CLIENT','RANK')) %>% 
  select(!c(N_DOG,RANK)) %>% 
  group_by(CLIENT) %>% 
  filter(DT == max(DT)) %>% 
  inner_join(DF, by = c('CLIENT','DT'))

Output

CLIENT	DT	N_DOG	RANK
1	2021-07-12	N2	1
2	2022-01-04	N6	1
3	2021-11-09	N8	2

How can I do it faster ? [ maybe there's some function ? ] the same question at the ru forums.

English is not my native language, so please be kind to my mistakes.

Answer 1

data.table::setDT(DF)[order(RANK,-DT), .SD[1], by=CLIENT]

Output:

   CLIENT  N_DOG         DT  RANK
    <num> <char>     <Date> <num>
1:      2     N6 2022-01-04     1
2:      1     N2 2021-07-12     1
3:      3     N8 2021-11-09     2