Switching the first element with the last one in a loop

Question

Is there a function in R which switches the first element with the last one in a vector? I have a for loop which need that reordering. From:

months = seq(1:12)
[1]  1  2  3  4  5  6  7  8  9 10 11 12

I would like to have:

[1]  12 1  2  3  4  5  6  7  8  9 10 11

and then again:

[1]  11 12  1  2  3  4  5  6  7  8  9 10

... until the 12th position.

Answer 1

I'm afraid that you have to come up with a home-made function, something like this one:

rotate <- function(v,i=1) {
  i <- i %% length(v)
  if (i==0) return(v)
  v[c(seq(i+1,length(v)),seq(i))]
}

Couple of examples:

v <- seq(12)
rotate(v,1)
# [1]  2  3  4  5  6  7  8  9 10 11 12  1

rotate(v,-1)
# [1] 12  1  2  3  4  5  6  7  8  9 10 11