r - Group by ID, each element of the new table is a vector

Question

I have a table like this

data.table(ID = c(1,2,3,4,5,6), 
         R = c("s","s","n","n","s","s"), 
         S = c("a","a","a","b","b","b"))

and I'm trying to get this result

      a     b
s   1, 2      5, 6
n     3         4

Is there any option in data.table can do this?

Answer 1

Here's an alternative that uses plain old data.table syntax:

DT[,lapply(split(ID,S),list),by=R]

# or...

DT[,lapply(split(ID,S),toString),by=R]