Access `by` variable from within `j` in data.table

data.table, r

Solution

Use `.BY` - which is a list of `by` variables:

d <- data.table(x=1:10, y=c('a', 'b'))
d[, .BY[[1]], by = y]  # [[1]] to access the first by variable, which is y
                       # if you had by = list(x, y) you'd do .BY[[2]] to access y
#   y V1
#1: a  a
#2: b  b

Additionally the list is named, so you can also access it by name:

d[, .BY$y, by = y]

Problem

If I have a `data.table` and I'm doing some function call in the `j`, do I have access to the current value of the `by` variable? ``` library(data.table) d <- data.table(x=1:10, y=c('a', 'b')) myfun <- function(DT) { print (DT$y) } d[, myfun(.SD), by=y] ``` For more context, I am passing a second argument (another `data.table`) to `myfun` and want to subset that based on the current value of `y`. It could be done with a dummy variable, but that seems yucky...

Original source