Dplyr and Summarising according to presence or not of an element

Question

I am trying to carry out a simple operation (everything is explained in the reprex). I just want to be able to summarise a grouped tibble according to whether a certain element appears in a group (in that case, I want to have its corresponding ranking). The element I talk about appears at most once in each group, so it should be easy, but I am banging my head against the wall.

Any help is appreciated!

Thanks!

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union


df <- tibble(year=rep(seq(2017, 2020),6),
                 ranking=seq(24)) %>%
    arrange(year) %>%
    mutate(x=c(letters[1:12], letters[1:12]))



##I want to summarise the tibble this way: for every year, if there is "d" in
## the group, I want the ranking of d. If not, I want to put e.g. -1.

##this is my attempt, but it does not work


df_summary <- df %>%
    group_by(year) %>%
    summarise(my_summary=if_else("d" %in% x, ranking, -1 ))
#> Error: Problem with `summarise()` input `my_summary`.
#> ✖ `true` must be length 1 (length of `condition`), not 6.
#> ℹ Input `my_summary` is `if_else("d" %in% x, ranking, -1)`.
#> ℹ The error occurred in group 1: year = 2017.

^{Created on 2020-09-18 by the reprex package (v0.3.0)}

Answer 1

With the same methode idea as you, and a verification that your variable do not appear more than one time (else you get -1)

       df %>% 
          mutate(is_d = (x=="d")*1) %>% 
          group_by(year) %>%
          summarise(Val=if_else(sum(is_d)==1,
                                sum(is_d*ranking),
                                -1))

Answer 2

has_d <- df %>% 
    filter(x == "d") %>% 
    select(year, ranking)

df_summary <- df %>% 
    distinct(year) %>% 
    anti_join(has_d, by = "year") %>% 
    mutate(ranking = -1L) %>% 
    bind_rows(has_d) %>% 
    arrange(year)