Sort a data.table programmatically using character vector of multiple column names

Question

I need to sort a data.table on multiple columns provided as character vector of variable names.

This is my approach so far:

DT = data.table(x = rep(c("b","a","c"), each = 3), y = c(1,3,6), v = 1:9)

#column names to sort by, stored in a vector
keycol <- c("x", "y")

DT[order(keycol)]
   x y v
1: b 1 1
2: b 3 2

Somehow It displays just 2 rows and removes other records. But if I do this:

DT[order(x, y)]
   x y v
1: a 1 4
2: a 3 5
3: a 6 6
4: b 1 1
5: b 3 2
6: b 6 3
7: c 1 7
8: c 3 8
9: c 6 9

It works like fluid. Can anyone help with sorting using column name vector?

Answer 1

You need ?setorderv and its cols argument:

A character vector of column names of x by which to order

library(data.table)
DT = data.table(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)
#column vector
keycol <-c("x","y")
setorderv(DT, keycol)
DT
   x y v
1: a 1 4
2: a 3 5
3: a 6 6
4: b 1 1
5: b 3 2
6: b 6 3
7: c 1 7
8: c 3 8
9: c 6 9

Note that there is no need to assign the output of setorderv back to DT. The function updates DT by reference.