R/ISOweekday.R

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

#' Day of week according to ISO 8601
#'
#' Returns the weekday of a given date according to ISO 8601.
#' It is an alternative to \code{strftime(x, "\%u")}.  
#' 
#' The conversion specifications \code{\%g}, \code{\%G}, \code{\%V}, \code{\%u} 
#' where not available on Windows and MacOS before \code{R 3.1.0}.
#' 
#' @param date vector which can be coerced to class \code{Date}
#' @return An integer vector of weekdays (1-7, Monday is 1)
#' @author Uwe Block \email{u.block.mz@@gmail.com}
#' @seealso \code{\link{strftime}}
#' @export
#' @examples
#' x <- paste(1999:2011, "-12-31", sep = "")
#' y <- as.Date(x)
#' data.frame(date = format(y), weekday = ISOweekday(y))
#' data.frame(date = x, weekday = ISOweekday(x))
ISOweekday <- function(date) {
  date <- as.Date(date)
  return(as.integer((as.integer(format(date, "%w"))+6) %% 7 + 1))
}