How to reverse order a vector?

Suppose I have a vector v, how do I get its reverse, i.e. last element first?

The first thing that comes to me is v[length(v):1], but it returns NA when v is numeric(0), while user normally expect sorting nothing returns nothing, not sorting nothing returns the unavailable thing - it does make a big difference in my case.

You are almost there; rev does what you need:

rev(1:3)
# [1] 3 2 1
rev(numeric(0))
# numeric(0)

Here's why:

rev.default
# function (x) 
# if (length(x)) x[length(x):1L] else x
# <bytecode: 0x0b5c6184>
# <environment: namespace:base>

In the case of numeric(0), length(x) returns 0. As if requires a logical condition, it coerces length(x) to TRUE or FALSE. It happens that as.logical(x) is FALSE when x is 0 and TRUE for any other number.

Thus, if (length(x)) tests precisely what you want - whether x is of length zero. If it isn't, length(x):1L has a desirable effect, and otherwise there is no need to reverse anything, as @floder has explained in the comment.