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 0.3297416setcolorder(x, c("c", "b", "a"))x# c b a# [1,] 0.2880365 3 1# [2,] 0.7785115 2 2# [3,] 0.3297416 1 3From ?setcolorder:
In
data.tableparlance, allset*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.
One may find it easier to use the above solution, but instead sort by column number. For example: 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(3,2,1)) > x c b a [1,] 0.2880365 3 1 [2,] 0.7785115 2 2 [3,] 0.3297416 1 3