Question

I have a data.table my.data.table and a character vector i (length one) corresponding a to a colname of my.data.table. Using i, I would like to extract the corresponding column of my.data.table as a vector that is not of class data.table or data.frame.

How would I do this?

> my.data.table <- data.table(a=1:2,b=2:3)
> i <- "a"
> class(my.data.table[,i,with=FALSE])
[1] "data.table" "data.frame"
> as.vector(my.data.table[,i,with=FALSE])  ##does not work
   a
1: 1
2: 2
> is.vector(as.vector(my.data.table[,i,with=FALSE])) ##strange behavior
[1] FALSE
> 

I assume there’s a way to do this using with=FALSE and eval(i, <env>) in j but I can’t figure it out.

Answer 1

Either one of these will work in your example, but the second of the two is more generally useful.

my.data.table[[i]]
# [1] 1 2
my.data.table[,..i][[1L]]
# [1] 1 2

.. in data.table means to “look up one level” (in this case for the variable i)