R help - Aug 2008 - max and min with the indexes in a zoo object (or anything else that could solve the problem)

library(zoo)
library(chron)
t1 <- chron("1/1/2006", "00:00:00")
t2 <- chron("1/31/2006", "23:45:00")
deltat <- times("00:15:00")
tt <- seq(t1, t2, by = times("00:15:00"))
d <- sample(33:700, 2976, replace=TRUE)
sin.zoo <- zoo(d,tt)

#there are ninety six reading in a day
d.max <- rollapply(sin.zoo, width=96, FUN=max)
d.min <- rollapply(sin.zoo, width=96, FUN=max)

this is about what I would like to do.  I want to be able to get the
max for one day and then also get the index that coincides with this
max time period.  Also, the same thing with the min.  With the real
data I am trying to find out at what time the maximum dissolved oxygen
value is for each day.

Thanks

-- 
Stephen Sefick
Research Scientist
Southeastern Natural Sciences Academy

Let's not spend our time and resources thinking about things that are
so little or so large that all they really do for us is puff us up and
make us feel like gods. We are mammals, and have not exhausted the
annoying little problems of being mammals.

	-K. Mullis

Perform rollapply over the index rather than the series itself:  The
result, sin.mx is a zoo series with three columns: the original
series, the series of maxima and their index into the original series.


library(zoo)
library(chron)
t1 <- chron("1/1/2006", "00:00:00")
t2 <- chron("1/31/2006", "23:45:00")
deltat <- times("00:15:00")
tt <- seq(t1, t2, by = times("00:15:00"))
d <- sample(33:700, 2976, replace=TRUE)
sin.zoo <- zoo(d,tt)

f <- function(idx) idx[which.max(sin.zoo[idx])]
mx.idx <- rollapply(zoo(seq_along(sin.zoo)), 96, f, na.pad = TRUE)
sin.mx <- zoo(coredata(sin.zoo)[mx.idx], time(sin.zoo))

Another possibility is to encode the indexes into the original series.
This one is faster as it uses the optimized rollmax:

sin.zoo.2 <- sin.zoo + seq_along(sin.zoo) / 10000
mx <- rollmax(sin.zoo.2, 96)
sin.mx.2 <- cbind(sin.zoo, floor(mx), 10000 * (mx - floor(mx)))

On Thu, Aug 21, 2008 at 5:48 PM, stephen sefick <ssefick at gmail.com>
wrote:
> library(zoo)
> library(chron)
> t1 <- chron("1/1/2006", "00:00:00")
> t2 <- chron("1/31/2006", "23:45:00")
> deltat <- times("00:15:00")
> tt <- seq(t1, t2, by = times("00:15:00"))
> d <- sample(33:700, 2976, replace=TRUE)
> sin.zoo <- zoo(d,tt)
>
> #there are ninety six reading in a day
> d.max <- rollapply(sin.zoo, width=96, FUN=max)
> d.min <- rollapply(sin.zoo, width=96, FUN=max)
>
> this is about what I would like to do.  I want to be able to get the
> max for one day and then also get the index that coincides with this
> max time period.  Also, the same thing with the min.  With the real
> data I am trying to find out at what time the maximum dissolved oxygen
> value is for each day.
>
> Thanks
>
> --
> Stephen Sefick
> Research Scientist
> Southeastern Natural Sciences Academy
>
> Let's not spend our time and resources thinking about things that are
> so little or so large that all they really do for us is puff us up and
> make us feel like gods. We are mammals, and have not exhausted the
> annoying little problems of being mammals.
>
>        -K. Mullis
>
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