R - Data.table - 使用负位置按引用删除列

Question

I like data table's way of deleting columns:

library(data.table)
data <- mtcars
setDT(data)
data[, 1:3 := NULL]

Deleting columns by position is not always the best solution, but it is often convenient. I expected this to work (delete all columns that are not in position 1:3):

data[, !1:3 := NULL]

But that returns this error:

Error in `[.data.table`(data, , `:=`(!1:3, NULL)) : 
  LHS of := isn't column names ('character') or positions ('integer' or 'numeric')

Inspecting data in the same way does work:

data[, !1:3]
    hp drat    wt  qsec vs am gear carb
 1: 110 3.90 2.620 16.46  0  1    4    4
 2: 110 3.90 2.875 17.02  0  1    4    4
 3:  93 3.85 2.320 18.61  1  1    4    1
 4: 110 3.08 3.215 19.44  1  0    3    1
...

I can do something like:

delCol <- colnames(data)
delCol <- tail(delCol, -3)
data[, c(delCol) := NULL]

But it is less convenient. Is there a better way?

Answer 1

Of course, you can select just the first three columns, as suggested in comments. But if you really want to delete by reference, you might consider working with colnames :

library(data.table)
data <- mtcars
setDT(data)

data[, colnames(data)[-c(1:3)] := NULL]

head(data)
#>     mpg cyl disp
#> 1: 21.0   6  160
#> 2: 21.0   6  160
#> 3: 22.8   4  108
#> 4: 21.4   6  258
#> 5: 18.7   8  360
#> 6: 18.1   6  225

Alternatively, something like this would work:

data[, setdiff(1:ncol(data), 1:3) := NULL]