Extract a column from a data.table as a vector, by position
Solution 1:
A data.table inherits from class data.frame. Therefore it is a list (of column vectors) internally and can be treated as such.
is.list(DT)
#[1] TRUE
Fortunately, list subsetting, i.e. [[, is very fast and, in contrast to [, package data.table doesn't define a method for it. Thus, you can simply use [[ to extract by an index:
DT[[2]]
#[1] 3 4
Solution 2:
DT[,get(names(DT)[colNb])]
where colNb can be an integer (the desired column number) or a variable containing the column number.