How to reorder data.table columns (without copying)

Use setcolorder():

library(data.table)
x <- data.table(a = 1:3, b = 3:1, c = runif(3))
x
#      a b         c
# [1,] 1 3 0.2880365
# [2,] 2 2 0.7785115
# [3,] 3 1 0.3297416
setcolorder(x, c("c", "b", "a"))
x
#              c b a
# [1,] 0.2880365 3 1
# [2,] 0.7785115 2 2
# [3,] 0.3297416 1 3

From ?setcolorder:

In data.table parlance, all set* functions change their input by reference. That is, no copy is made at all, other than temporary working memory, which is as large as one column.

so should be pretty efficient. See ?setcolorder for details.