[R] Processing dates and generating sequences of dates

[Gavin Simpson]

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On Mon, 2008-02-04 at 10:48 -0500, Gabor Grothendieck wrote:
> Using zoo's yearmon class:
> 
> library(zoo)
> my.dates[!duplicated(as.yearmon(my.dates))]
> 
> or, although you seem to disallow this in your question,
> this would be an option:
> 
> my.dates[!duplicated(format(my.dates, "%Y-%m"))]

Gabor,

Many thanks. Both are exactly what I was looking for. My head was stuck
very firmly in a rut of using tools that work with "Date" objects and
didn't think about using duplicated!

The latter is especially fine because:
     1. I didn't mean to imply that I didn't want to convert to/from
        character - just that my doing so repeatedly in the example
        didn't seem very elegant.
     2. This whole thing may end up being wrapped into a function for
        use by people who have restricted computing environments, so a
        pure R solution is advantageous.

All the best,

G

> 
> 
> On Feb 4, 2008 10:39 AM, Gavin Simpson <gavin.simpson at ucl.ac.uk> wrote:
> > hits=-2.6 tests=BAYES_00
> > X-USF-Spam-Flag: NO
> >
> > Dear List,
> >
> > Say I have the following sequence of dates [*]:
> >
> > start <- as.Date("2005-01-05", format = "%Y-%d-%m")
> > end <- as.Date("2007-10-07", format = "%Y-%d-%m")
> > my.dates <- seq(start, end, by = "days")
> >
> > What I would like to generate is a sequence of dates, by month that goes
> > from the first day of 2005 (the year of the start date) to the last day
> > of 2007 (the year of the end date), so that the output is a vector of 36
> > dates containing all months of the three calendar years that the
> > sampling spanned.
> >
> > I could do it via manipulation of dates as so:
> >
> > new.dates <- seq(as.Date(paste(format(min(my.dates), format = "%Y"),
> >                               "1", "1", sep = "/")),
> >                 as.Date(paste(format(max(my.dates), format = "%Y"),
> >                               "12", "31", sep = "/")),
> >                 by = "months")
> >
> > And then manipulate that to get only the month and year parts of the 36
> > generated dates.
> >
> > This doesn't seem very elegant to me. Is there a better/easier way than
> > converting back and forth between characters and objects of class
> > "Date"?
> >
> > Many thanks,
> >
> > G
> >
> > [*] FWIW, my actual application is similar to my.dates, but not sampled
> > every day - indeed there are months where there are no samples - and I
> > am trying to do a levelplot of the numbers of observations per month,
> > per year. Given the following data
> >
> > dat <- data.frame(temp = rnorm(length(my.dates)),
> >                  my.dates = my.dates)
> > dat$year <- as.numeric(format(dat$my.dates, format = "%Y"))
> > dat$month <- format(dat$my.dates, format = "%b")
> > dat$month <- factor(dat$month, levels = c("Jan","Feb","Mar","Apr",
> >                                          "May","Jun","Jul","Aug",
> >                                          "Sep","Oct","Nov","Dec"))
> >
> > I can get a table of the number of observations per month per year via
> >
> > (obs.yearmon <- with(dat, table(year, month)))
> >
> > Which when converted to a vector provides what I need for levelplot()'s
> > formula method. Now I just need to generate the sequence of 36 months
> > ("Jan", "Feb" etc) and years to go with it. The matrix method of
> > levelplot works, but I am having to hard code a lot of details that I
> > believe the formula method will handle automagically for me.
> > --
> > %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%
> >  Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
> >  ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
> >  Pearson Building,             [e] gavin.simpsonATNOSPAMucl.ac.uk
> >  Gower Street, London          [w] http://www.ucl.ac.uk/~ucfagls/
> >  UK. WC1E 6BT.                 [w] http://www.freshwaters.org.uk
> > %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > 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.
> >
-- 
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 Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
 ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
 Pearson Building,             [e] gavin.simpsonATNOSPAMucl.ac.uk
 Gower Street, London          [w] http://www.ucl.ac.uk/~ucfagls/
 UK. WC1E 6BT.                 [w] http://www.freshwaters.org.uk
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