in r dplyr select the first observation by group and create a list

Question

I would like to select the first observation be group and create a list column.

For example:

df <- structure(list(Grp = c("A", "A", "A", "B", "B"), col1 = c(1, 
2, 3, 70, 80), col2 = c(4, 5, 6, 100, 110)), class = c("tbl_df", 
"tbl", "data.frame"), row.names = c(NA, -5L))

I would like to create a column whose first element is c(1,4) and second element is c(70, 100).

I tried:

df %>% group_by(Grp) %>% mutate(L = list(slice(1)))

But got an error.

Answer 1

A possible solution:

library(dplyr)

df %>% 
  group_by(Grp) %>% mutate(L = list(c(first(col1), first(col2)))) %>% 
  ungroup

#> # A tibble: 5 × 4
#>   Grp    col1  col2 L        
#>   <chr> <dbl> <dbl> <list>   
#> 1 A         1     4 <dbl [2]>
#> 2 A         2     5 <dbl [2]>
#> 3 A         3     6 <dbl [2]>
#> 4 B        70   100 <dbl [2]>
#> 5 B        80   110 <dbl [2]>