Hi Steve, and thanks so much for taking the time to draft your solution! After running through it looks like it's EXACTLY what I was looking/hoping for!
For those interested: I also tried to get this in as a feature request for the lubridate package >> https://github.com/hadley/lubridate/issues/506 Here's to open source, social coding and our active user community! :-) Janko On Wed, Jan 18, 2017 at 1:35 PM, S Ellison <s.elli...@lgcgroup.com> wrote: > > >> -----Original Message----- >> (yw <- format(posix, "%Y-%V")) >> > # [1] "2015-52" "2015-53" "2016-53" "2016-01" >> >> Which, after checking back with a calendar, would give me reason to believe >> that it using %V does in fact seem to work: it's an input to `format()` and R >> doesn't seem to ignore it as the correct week numbers (following ISO 8601) >> are returned. > > Unfortunately it does _not_ work as an input to strptime() or as.Date(). Try > strptime(sprintf("2016-%02d", 1:52), "%Y-%V") > > which all return the same (rather arbitrary-looking) date. (R 3.3.1 on > Windows 7) > > So you can easily extract the week number from a date, but you can't extract > the month number from a week number using base R. > > What you _could_ do is assign an appropriate weekday in each week and use > that day to assign a month. Using the 8601 rules, Thursday seems sensible as > that is the weekday used to define the week number. The following crude > implementation (which allows some variation in the input string and a custom > weekday defaulting to Thursday) seems to work in my UK locale, though I've > not spent a lot of time debugging. You should be able to get it to work with > minimal tweaking in another locale: > > yw.to.month <- function(yw, yearformat="%Y", separator="-", monthformat="%m", > weekday=4) { > # yw is a character vector assumed to be in %Y-%W" or "%y-%W" > # form (or with another separator specified by separator) > # yearformat is a character string used by as.Date to convert the > year. > # separator is the year-week separator string in yw > # monthformat is the output format, passed to format.Date > # weekday is the (numerica) day of the week to be used to place the > week > # in a specific month. The default, 4, takes the Thursday. > > > #Get the year as a character vector > Y <- format(as.Date(yw, yearformat), "%Y") > > #Get the date of the first thursday in each year > Jan01 <- as.Date(sprintf("%s-01-01", Y), "%Y-%m-%d") #constructs 1st > Jan for each year > J1.w <- as.numeric( format(Jan01, "%w") ) #numerical day of week for > jan 1st > date.increment <- ifelse(J1.w > weekday, 7 + weekday - J1.w, weekday > - J1.w) #How far to first thursday? > Thursday1 <- as.Date( Jan01 + date.increment ) #date for first > thursday > > #Add the week number in yw to get the corresponding Thursday's date > wknum <- as.numeric( gsub(sprintf(".+%s(.+)", separator), "\\1", yw) ) > Thursdays <- Thursday1 + 7 * (wknum - 1) > > #... and convert each Thursday date to month number > # using supplied monthformat > > format( Thursdays, monthformat ) > } > > #Examples > #Construct a collection of "YYYY-WW" strings: > yw <- sprintf("%4d-%02d", rep(2015:2017, each=53), rep(1:53, 3)) > #NB: Week 53 does not exist in all years. e.g 'week 53' of 2016 is > week 1 of 2017. > #But the week number is just used as an offset from the first > specified weekday, > #so the function does returns a valid year and month if the week > number is > #after the year end. > > yw.to.month(yw) > > yw.to.month(yw, monthformat="%Y-%m") > > #Use the month in which the Monday falls: > yw.to.month(yw, monthformat="%Y-%m", weekday=1) > > > > Steve E > > > > > > > > > ******************************************************************* > This email and any attachments are confidential. Any u...{{dropped:8}} ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
