Difference between neighbouring elements of a vector

Sounds like diff function

diff(x)

You can also use this code:

x[-1] - x[-length(x)]

x[-1] - vector x without first element

x[-length(x)] - vector x without last element

x <- c(1,3,3,9) 
(z <- x[-1] - head(x, -1))
# [1] 2 0 6