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\",         


        

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]