Most efficient way to select ending item of an array?

I'm looking for the most efficent way (i.e. the lesser keys pressed) to indexing the last element of an array.

Then something like

a <- c(1,2,3)
n <- length(a)
b <- a[n]

should not be used, I would like to use just a single command.

In the example above I could use

b <- a[length(a)]

but I wonder if something shorter does exist.

Let I want to select a part of an array, like

a <- seq(from = 1, to = 10, by = 1)
b <- a[3:length(a)]

Is there a shorter way to do it?