naming a column in a list manner with string in data.table

Question

I have a problem that I don\'t manage to solve properly in data.table. I have the following data:

plouf <- data.table(  ID = rep(LETTERS[1:10],each = 10)          


        

Answer 1

We can use setNames

plouf[get(col) > 5, setNames(list(get(col)[1]), col) ,by = ID]

or another option is setnames after getting the result

setnames(plouf[get(col) > 5, .(get(col)[1]) ,by = ID], 'V1', col)[]
#   ID X1
#1:  A  8
#2:  B  7
#3:  C  6
#4:  D 10
#5:  F  9
#6:  G  8
#7:  H 10
#8:  I  6
#9:  J  8

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If we are using dplyr, then the option would be

library(dplyr)
plouf %>%
   filter_at(col, any_vars(.>5)) %>%
   group_by(ID) %>% 
   summarise_at(col, first)
# A tibble: 9 x 2
#  ID       X1
#  <chr> <int>
#1 A         8
#2 B         7
#3 C         6
#4 D        10
#5 F         9
#6 G         8
#7 H        10
#8 I         6
#9 J         8

Or with := and sym from rlang

plouf %>% 
    filter(!! rlang::sym(col) > 5) %>%
    group_by(ID) %>% 
    summarise(!! col := first(!!rlang::sym(col)))