return a character vector of the last 12 months, ending with current month

order_months.R

order_months <- function(x = NULL, label = "abb") {
  
  # This function takes a given month number or the current month
  # number and returns a character vector of the last 12 months,
  # including current month. For example, if it is now February: 
  # "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec" "Jan" "Feb"
  # is returned with the current month at the end.
  # This makes a nice x axis if you need to plot something for
  # the "last 12 months".
  # if x is NULL the current month is taken from Sys.Date
  # if x is given, x must be 1-12
  # if label is "abb", month names are abbreviated
  # if label is "names", month names are full
  
  # stop if x is out of bounds
  if(!is.null(x))
    if(x > 12 | x < 1) stop("x must be 1-12")
  
  # sets of named integers using abbreviated and full month names
  months_abb <- setNames(1:12, month.abb)
  months_names <- setNames(1:12, month.name)
  
  # current month number
  m <- as.POSIXlt(Sys.Date())$mon + 1
  
  # get a value for x
  if(is.null(x)) x <- m + 1 else x <- x + 1
  
  # a and b components for main if
  # if x is 1 or 12
  a <- 1:12
  # if x is 2 through 12
  b <- c(
    x:12,
    1:(x-1)
    )
  
  # main if
  if(x == 1 | x == 13) new_order <- a else new_order <- b

  # use new_order to set desired order of months
  if(label == "abb") my_month_order <- names(months_abb[new_order])
  if(label == "names") my_month_order <- names(months_names[new_order])
  
  my_month_order
}