Reorder rows in data.table in a specific order

Question

I have a data.table dumdt:

set.seed(123)
dumdt <- data.table(v1=sample(1:10, 5), v2=1:5)

whose rows I\'d like to r

Answer 1

If I understand correctly, you can just add a col and then order by it:

setorder(dumdt[, .r := order(to_ord)], .r)[, .r := NULL]

   v1 v2
1:  4  3
2:  6  5
3:  8  2
4:  3  1
5:  7  4

Answer 2

This capability is not (yet) exported. After looking at the source of setorderv I was able to extract required call to C function which does what you need and supply it with custom order.

library(data.table)
set.seed(123)
dumdt <- data.table(v1=sample(1:10, 5), v2=1:5)
print(dumdt)
#   v1 v2
#1:  3  1
#2:  8  2
#3:  4  3
#4:  7  4
#5:  6  5
setroworder <- function(x, neworder) {
    .Call(data.table:::Creorder, x, as.integer(neworder), PACKAGE = "data.table")
    invisible(x)
}
to_ord <- c(3, 5, 2, 1, 4)
setroworder(x=dumdt, neworder=to_ord)
print(dumdt)
#   v1 v2
#1:  4  3
#2:  6  5
#3:  8  2
#4:  3  1
#5:  7  4

Yet the solution proposed by Frank looks a little bit nicer.