Check if the previous value is present in the dataset with a logical operation [R]

I have this dataset

structure(list(N = c("a", "b", "a", "b", "c", "a", "c"), S = c("3", 
"3", "2", "2", "2", "1", "1")), class = "data.frame", row.names = c(NA, 
-7L))

And I would like to verify if all group ‘N’ have a previous observation ‘S’. And that with a logical operation

library(tidyverse)
df %>% group_by(N) %>% arrange(desc(S)) %>% mutate(L = ifelse(****))

The output should looks like this

>Solution :

How about this – it sorts by S within group and then identifies the first (smallest value of S) as FALSE and the others as TRUE.

library(dplyr)
dat <- structure(list(N = c("a", "b", "a", "b", "c", "a", "c"), 
S = c("3", "3", "2", "2", "2", "1", "1")), class = "data.frame", row.names = c(NA,-7L))

dat %>% 
  arrange(S, .by_group = TRUE) %>% 
  group_by(N) %>% 
  mutate(1:n() > 1)
#> # A tibble: 7 × 3
#> # Groups:   N [3]
#>   N     S     `1:n() > 1`
#>   <chr> <chr> <lgl>      
#> 1 a     1     FALSE      
#> 2 c     1     FALSE      
#> 3 a     2     TRUE       
#> 4 b     2     FALSE      
#> 5 c     2     TRUE       
#> 6 a     3     TRUE       
#> 7 b     3     TRUE

^{Created on 2023-03-09 with reprex v2.0.2}