On Nov 17, 2007 8:55 AM, Emmanuel Charpentier <[EMAIL PROTECTED]> wrote: > Dear Lucia, > > lucia a écrit : > > Hello, > > I'm very new to R, and so my question is simple. > > > > I have data record with 80 years of daily temperatures in one long > > string. The dates are also recorded, in YYMMDD format. I'd like to > > learn an elegant simple way to pull out the annual averages. > > (Obviously, every 4th year has 366 days.) > > > > I know I can set up a formal loop to create annual records and then > > average. But R seems to have such neat methods, is there some better > > way to do this? > > For sake of simplicity, let's say you managed to store your data in a > two-column dataframe df, with columns date and temperature. > > The first step is to know how to extract the "year" part of the dates. > The obvious solution is of course as.numeric(substr(date,1,2)), but I'd > rather transform your date variable in genuine R's Date class variable, > by as.Date(date,format="%y%m%d") or as.POSIXlt(date,format="%y%m%d"), > the latter allowing easy year extraction by reading the "year" component. > > The second step is of course to apply to each of your relevant subtables > the "mean" function ; that's what tapply() is meant for. > > So, a one-liner for your proble might be : > > Means<- tapply(df$temperature, as.POSIXlt(df$date,format="%y%m%d")$year, > FUN=mean, na.rm=TRUE) > > However, this is only a very crude way to work with time series. I'd > consider converting permanently your date variable in a suitable > datetime representation. There are more than one way to do it : Julian > dates, chron objects and, more recently introduced, DateTime classes, > which seems to be "the standard way" to represent dates and times. > Unfortunately, different packages teeem to expect different > representations... <Sigh>.
That is not true of the zoo package. It does not assume a specific date/time class. It supports any date/time class that satisfies the criteria discussed in ?zoo . For example, numbers, Date, chron and POSIXct all satisfy those criteria. ______________________________________________ R-help@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.
