R/get_week_start.r

In mmparker/mpmisc: A collection of miscellaneous functions for report-making.

#' Identify the start of the week for a vector of dates.
#'
#' This function returns the date of the start of the week for any given date;
#' "start of the week" is defined as any arbitrary day of the week. Can also
#' calculate the start of the week any number of weeks in the past; I use this
#' while aggregating data for several weeks to ensure that the first week
#' included is always a full week (for example, if I aggregated data for the
#' last eight weeks on a Friday, the week furthest back would only have its
#' Friday data included - if I get_week_start for that date, I ensure the
#' full week's data is aggregated).
#'
#' @param dates a vector of dates
#' @param start.day the arbitrary first day of the week - defaults to Monday
#' @param weeks.back how many weeks in the past to evaluate
#' 
#' @export
#' 
#' @examples
#' # The start of the current week
#' get_week_start(Sys.Date())
#' 
#' # Sunday instead of Monday
#' get_week_start(Sys.Date(), start.day = "Sunday")
#' 
#' # Start of the week eight weeks ago
#' get_week_start(Sys.Date(), weeks.back = 8)


get_week_start <- function(dates, start.day = "Monday", weeks.back = 0) {


    week.starts <- sapply(dates, simplify = TRUE, FUN = function(x) {

        if(is.na(x) | is.infinite(x)) { return(as.Date(NA)) }

        last.seven <- seq(from = x, length.out = 7, by = -1)

       week.start <- last.seven[format(last.seven, "%A") %in% start.day] - 
           (weeks.back * 7)

    })

    as.Date(week.starts, origin = as.Date("1970-01-01"))

}