splitting time vector into days

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

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?