How to access the last value in a vector?

Question

Suppose I have a vector that is nested in a dataframe one or two levels. Is there a quick and dirty way to access the last value, without using the length() fu

Answer 1

Combining lindelof's and Gregg Lind's ideas:

last <- function(x) { tail(x, n = 1) }

Working at the prompt, I usually omit the n=, i.e. tail(x, 1).

Unlike last from the pastecs package, head and tail (from utils) work not only on vectors but also on data frames etc., and also can return data "without first/last n elements", e.g.

but.last <- function(x) { head(x, n = -1) }

(Note that you have to use head for this, instead of tail.)

Answer 2

I use the tail function:

tail(vector, n=1)

The nice thing with tail is that it works on dataframes too, unlike the x[length(x)] idiom.

Answer 3

The dplyr package includes a function last():

last(mtcars$mpg)
# [1] 21.4

Answer 4

I just benchmarked these two approaches on data frame with 663,552 rows using the following code:

system.time(
  resultsByLevel$subject <- sapply(resultsByLevel$variable, function(x) {
    s <- strsplit(x, ".", fixed=TRUE)[[1]]
    s[length(s)]
  })
  )

 user  system elapsed 
  3.722   0.000   3.594 

and

system.time(
  resultsByLevel$subject <- sapply(resultsByLevel$variable, function(x) {
    s <- strsplit(x, ".", fixed=TRUE)[[1]]
    tail(s, n=1)
  })
  )

   user  system elapsed 
 28.174   0.000  27.662 

So, assuming you're working with vectors, accessing the length position is significantly faster.

Answer 5

Package data.table includes last function

library(data.table)
last(c(1:10))
# [1] 10