How do I reorder data.table columns?

Use setcolorder:

> library(data.table)
> dt <- data.table(a=1:3,b=4:6)
> setcolorder(dt, c("b", "a"))
> dt
   b a
1: 4 1
2: 5 2
3: 6 3

This is how you do it in data.table (without modifying original table):

dt[, list(b, a)]

or

dt[, c("b", "a")]

or

dt[, c(2, 1)]