Select a sequence of columns: `:` works but not `seq`

Question

I\'m trying to subset a dataset by selecting some columns from a data.table. However, my code does not work with some variations.

Here is a sample data.table

Answer 1

The lesson I learned is to use list instead of c:

 DT[ ,list(ID,Capacity)]
 #---------------------------
     ID Capacity
  1:  1      483
  2:  2      703
  3:  3      924
  4:  4      267
  5:  5      588
 ---            
196: 46      761
197: 47      584
198: 48      402
199: 49      416
200: 50      130

It lets you ignore those pesky quotations, and it also moves you in the direction of seeing the j argument as an evaluated expression with an environment of the datatable itself.

To 'get' the named columns by number use the mget function and the names function. R 'names' are language elements, i.e., data objects in the search path from the current environment. Column names of dataframes are not actually R names. So you need a function that will take a character value and cause the interpreter to consider it a fully qualified name. Datatable-[-function syntax for the j item does handle column names as language objects rather than character values as would the [.data.frame-function:

DT[ ,mget(names(DT)[c(1,2)])]
     ID Capacity
  1:  1      483
  2:  2      703
  3:  3      924
  4:  4      267
  5:  5      588
 ---            
196: 46      761
197: 47      584
198: 48      402
199: 49      416
200: 50      130