R help - Sep 2008 - splitting time vector into days

Greetings -- I have a dataframe a with one element a vector, time, of  
POSIXct values.  What's a good way to split the data frame into  
periods of a$time, e.g. days, and apply a function, e.g. mean, to some  
other column of the dataframe, e.g. a$value?

Cheers,
Alexy

?aggregate
?window.zoo
?rollapply

anyway have a look at package zoo

On Tue, Sep 9, 2008 at 3:25 PM, Alexy Khrabrov <deliverable at gmail.com>
wrote:
> Greetings -- I have a dataframe a with one element a vector, time, of
> POSIXct values.  What's a good way to split the data frame into periods
of
> a$time, e.g. days, and apply a function, e.g. mean, to some other column of
> the dataframe, e.g. a$value?
>
> Cheers,
> Alexy
>
> ______________________________________________
> 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.
>


-- 
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

Here is one way of doing it:
> x <- data.frame(dates=seq(as.POSIXct('2008-09-08'), by='7
hours', length=10),
+     values=1:10)
> # split into days
> x.s <- split(x, format(x$dates, "%Y%m%d"))
> x.s
$`20080908`
                dates values
1 2008-09-08 00:00:00      1
2 2008-09-08 07:00:00      2
3 2008-09-08 14:00:00      3
4 2008-09-08 21:00:00      4

$`20080909`
                dates values
5 2008-09-09 04:00:00      5
6 2008-09-09 11:00:00      6
7 2008-09-09 18:00:00      7

$`20080910`
                 dates values
8  2008-09-10 01:00:00      8
9  2008-09-10 08:00:00      9
10 2008-09-10 15:00:00     10
> lapply(x.s, function(.df) mean(.df$values))
$`20080908`
[1] 2.5

$`20080909`
[1] 6

$`20080910`
[1] 9
>

On Tue, Sep 9, 2008 at 3:25 PM, Alexy Khrabrov <deliverable at gmail.com>
wrote:
> Greetings -- I have a dataframe a with one element a vector, time, of
> POSIXct values.  What's a good way to split the data frame into periods
of
> a$time, e.g. days, and apply a function, e.g. mean, to some other column of
> the dataframe, e.g. a$value?
>
> Cheers,
> Alexy
>
> ______________________________________________
> 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.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?