[R] 15-min mean values

[bogdan romocea]

Here's another approach which can be easily implemented in SQL.
1. Start with the dates as character vectors,
   dt <- as.character(Sys.time())
2. Extract the minutes and round them to 0,15,30,45:
   minutes <- floor(as.numeric(substr(dt,15,16))/15)*15
   final.mins <- as.character(minutes)
   final.mins[final.mins == "0"] <- "00"
3. Get the dates you need for aggregating:
   final.dt <- paste(substr(dt,1,14),final.mins,":00",sep="")
(If you had wanted to use 10 minutes, it would have been enough to
transform MM:SS to M0:00.)
4. Use aggregate(), SQL GROUP BY etc
5. Finally, convert final.dt from character to datettime.


> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Gabor
> Grothendieck
> Sent: Thursday, February 02, 2006 1:44 AM
> To: Augusto.Sanabria at ga.gov.au
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] 15-min mean values
>
> Assume VDATE is a character vector.  If its a
> factor first convert it using
>
> VDATE <- as.character(VDATE)
>
> Lets assume we only need the times portion
> and later we handle the full case which may or
> may be needed.
>
> We create a times object from the times portion of
> vdate and then in the aggregate statement we use
> trunc.times -- note that trunc.times is a recent
> addition to the chron package so make sure you have
> the latest chron and R 2.2.1.    See ?trunc.times
>
> # test data
> library(chron)
> library(zoo)
> VDATE <- c("1998-10-22:02:11", "1998-10-22:02:12",
> "1998-10-22:02:13", "1998-10-22:02:14", "1998-10-22:02:15")
> WS <- c(12.5, 10.1, 11.2, 10.5, 11.5)
>
> # convert VDATES to times class and aggregate
> vtimes <- times(sub(".*:(..:..)", "\\1:00", VDATE))
> aggregate(zoo(WS), trunc(vtimes, "00:15:00"), mean)
>
> If we need the day part too then its only a little
> harder.
>
> Represent VDATE as a chron object, vdate.  We do this
> by extracting out the date and time portions
> and converting each separately.  We use regular
> expressions to do that conversion but show in a
> comment how to do it without regular expressions.
> See R News 4/1 Help Desk for more info on this and
> the table at the end of the article in particular.
>
> # alternative way to convert to vdate would be:
> # vdate <- chron(dates = as.numeric(as.Date(substring(VDATE, 1, 10))),
> #        times = paste(substring(VDATE, 12), 0, sep =":"))
>
>
> vdate <- chron(dates = sub("(....)-(..)-(..).*",
> "\\2/\\3/\\1", VDATE),
> times = sub(".*:(..:..)", "\\1:00", VDATE))
> aggregate(zoo(WS), chron(trunc(times(vdate), "00:15:00")), mean)
>
> On 2/2/06, Augusto.Sanabria at ga.gov.au
> <Augusto.Sanabria at ga.gov.au> wrote:
> >
> > Good day everyone,
> >
> > I want to use zoo(aggregate) to calculate
> > 15-min mean values from a wind dataset which
> > has 1-min values. The data I have looks like this:
> >
> >     vector VDATE           vector WS
> > 1   1998-10-22:02:11          12.5
> > 2   1998-10-22:02:12          10.1
> > 3   1998-10-22:02:13          11.2
> > 4   1998-10-22:02:14          10.5
> > 5   1998-10-22:02:15          11.5
> >      .
> >      .
> >      .
> > n   2005-06-30:23:59           9.1
> >
> >
> > I want to use:
> >
> > aggregate(zoo(WS),'in 15-min intervals',mean)
> >
> > How do you specify 'in 15-min intervals' using
> > vector VDATE? The length of VDATE cannot be
> > changed, otherwise it would be a trivial problem
> > because I can generate a 15-min spaced vector
> > using 'seq'.
> >
> > Am I missing something?
> >
> > Thanks a lot,
> >
> > Augusto
> >
> >
> > --------------------------------------------
> > Augusto Sanabria. MSc, PhD.
> > Mathematical Modeller
> > Risk Research Group
> > Geospatial & Earth Monitoring Division
> > Geoscience Australia (www.ga.gov.au)
> > Cnr. Jerrabomberra Av. & Hindmarsh Dr.
> > Symonston ACT 2609
> > Ph. (02) 6249-9155
> >
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