Getting the last n elements of a vector. Is there a better way than using the length() function?

by

see ?tail and ?head for some convenient functions:

> x <- 1:10
> tail(x,5)
[1]  6  7  8  9 10

For the argument’s sake : everything but the last five elements would be :

> head(x,n=-5)
[1] 1 2 3 4 5

As @Martin Morgan says in the comments, there are two other possibilities which are faster than the tail solution, in case you have to carry this out a million times on a vector of 100 million values. For readibility, I’d go with tail. 

test                                        elapsed    relative 
tail(x, 5)                                    38.70     5.724852     
x[length(x) - (4:0)]                           6.76     1.000000     
x[seq.int(to = length(x), length.out = 5)]     7.53     1.113905

benchmarking code :

require(rbenchmark)
x <- 1:1e8
do.call(
  benchmark,
  c(list(
    expression(tail(x,5)),
    expression(x[seq.int(to=length(x), length.out=5)]),
    expression(x[length(x)-(4:0)])
  ),  replications=1e6)
)

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