Question

I have some observed data by hour. I am trying to subset this data by the day or even week intervals. I am not sure how to proceed with this task in R.

The sample of the data is below.

date                                 obs
2011-10-24 01:00:00                  12
2011-10-24 02:00:00                  4
2011-10-24 19:00:00                  18
2011-10-24 20:00:00                  7
2011-10-24 21:00:00                  4
2011-10-24 22:00:00                  2
2011-10-25 00:00:00                  4
2011-10-25 01:00:00                  2
2011-10-25 02:00:00                  2
2011-10-25 15:00:00                  12
2011-10-25 18:00:00                  2
2011-10-25 19:00:00                  3
2011-10-25 21:00:00                  2
2011-10-25 23:00:00                  9
2011-10-26 00:00:00                  13
2011-10-26 01:00:00                  11

Answer 1

First I entered the data with the multiple spaces replaced with tabs.

dat$date <- as.POSIXct(dat$date, format="%Y-%m-%d %H:%M:%S")
split(dat , as.POSIXlt(dat$date)$yday)
# Notice these are not the same functions
#---------------------
$`296`
                 date obs
1 2011-10-24 01:00:00  12
2 2011-10-24 02:00:00   4
3 2011-10-24 19:00:00  18
4 2011-10-24 20:00:00   7
5 2011-10-24 21:00:00   4
6 2011-10-24 22:00:00   2

$`297`
                  date obs
7  2011-10-25 00:00:00   4
8  2011-10-25 01:00:00   2
9  2011-10-25 02:00:00   2
10 2011-10-25 15:00:00  12
11 2011-10-25 18:00:00   2
12 2011-10-25 19:00:00   3
13 2011-10-25 21:00:00   2
14 2011-10-25 23:00:00   9

$`298`
                  date obs
15 2011-10-26 00:00:00  13
16 2011-10-26 01:00:00  11

The POSIXlt class does not work well inside dataframes but it can ve very handy for creating time based groups. It’s a list structure with these indices: ‘yday’, ‘wday’, ‘year’, ‘mon’, ‘mday’, ‘hour’, ‘min’, ‘sec’ and ‘isdt’. The cut.POSIXt function adds divisions at other natural boundaries; E.g.

?cut.POSIXt
  split(dat , cut(dat$date, "week") )

If you wanted to sum within date:

tapply(dat$obs, as.POSIXlt(dat$date)$yday, sum)
#-------
296 297 298 
 47  36  24