r - Extract columns from data table by numeric indices stored in a vector

Question

I want to extract the 4th, 5th, and 6th column from a data table named dt

the following method works:

    dt[, c(4,5,6)]

but the following doesn't:

    a = c(4,5,6)
    dt[, a]

In fact, the second method gives me a reult of:

    4 5 6

Can someone tell me why this is happening? The two method looks equivalent to me.

Answer 1

We can use double dots (..) before the object 'a' to extract the columns

dt[, ..a]
#   col4 col5 col6
#1:    4    5    6
#2:    5    6    7
#3:    6    7    8
#4:    7    8    9

Or another option is with = FALSE

dt[, a, with = FALSE]

data

dt <- data.table(col1 = 1:4, col2 = 2:5, col3 = 3:6, col4 = 4:7, col5 = 5:8, col6 = 6:9)