R/date2ISOweek.R

In UweBlock/ISOweek: Week of the Year and Weekday According to ISO 8601

#' Converts date from standard notation to week notation according to ISO 8601
#'
#' Returns the year, the week of the year, and the day of week of a given date 
#' according to ISO 8601. It is an alternative to \code{strftime(x, "\%G-W\%V-\%u")}.  
#' 
#' The conversion specifications \code{\%g}, \code{\%G}, \code{\%u}, \code{\%V}
#' where not available on Windows and MacOS before \code{R 3.1.0}.
#' According to ISO 8601, the year of the week can differ from the calendar year (see the examples).
#'
#' @param date Vector which can be coerced to class \code{Date}
#' @return A character vector of year, week, and weekday in format "\code{\%G-W\%V-\%u}"
#' @seealso The inverse function \code{\link{ISOweek2date}}. \code{\link{strftime}} for a 
#' description of conversion specifications and references on ISO 8601. 
#' @export
#' @author Uwe Block \email{u.block.mz@@gmail.com}
#' @examples
#' x <- paste(1999:2011, "-12-31", sep = "")
#' y <- as.Date(x)
#' data.frame(date = format(y), weekdate = date2ISOweek(y))
#' data.frame(date = x, weekdate = date2ISOweek(x))
date2ISOweek <- function(date) {
  return(ifelse(is.na(date), NA_character_, paste(ISOweek(date), ISOweekday(date), sep = "-")))
}