[R] 15-min mean values

[Gabor Grothendieck]

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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