[R] splitting time vector into days

[jim holtman]

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

What is the problem that you are trying to solve?