I find this function useful for digging out months from Date objects
Month <- function(date, ...)
factor(month.abb[as.POSIXlt(date)$mon + 1], levels = month.abb)
For this little data set below this is what it gives
> with(data, tapply(value, Month(date), median, na.rm = TRUE))
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
NA NA NA NA NA NA NA NA NA NA 0.035 0.030
Here is another useful little one:
Year <- function(date, ...)
as.POSIXlt(date)$year + 1900
So if you wanted the median by year and month you could do
> with(data, tapply(value, list(Year(date), Month(date)), median, na.rm = TRUE))
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2008 NA NA NA NA NA NA NA NA NA NA 0.035 0.03
(The result is a matrix, which in this case has only one row, of course.)
See how you go.
Bill Venables.
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf Of HUXTERE
Sent: Monday, 20 December 2010 8:32 AM
To: r-help@r-project.org
Subject: [R] monthly median in a daily dataset
Hello,
I have a multi-year dataset (see below) with date, a data value and a flag
for the data value. I want to find the monthly median for each month in this
dataset and then plot it. If anyone has suggestions they would be greatly
apperciated. It should be noted that there are some dates with no values and
they should be removed.
Thanks
Emily
> print ( str(data$flow$daily) )
'data.frame': 16071 obs. of 3 variables:
$ date :Class 'Date' num [1:16071] -1826 -1825 -1824 -1823 -1822 ...
$ value: num NA NA NA NA NA NA NA NA NA NA ...
$ flag : chr "" "" "" "" ...
NULL
520 2008-11-01 0.034
1041 2008-11-02 0.034
1562 2008-11-03 0.034
2083 2008-11-04 0.038
2604 2008-11-05 0.036
3125 2008-11-06 0.035
3646 2008-11-07 0.036
4167 2008-11-08 0.039
4688 2008-11-09 0.039
5209 2008-11-10 0.039
5730 2008-11-11 0.038
6251 2008-11-12 0.039
6772 2008-11-13 0.039
7293 2008-11-14 0.038
7814 2008-11-15 0.037
8335 2008-11-16 0.037
8855 2008-11-17 0.037
9375 2008-11-18 0.037
9895 2008-11-19 0.034 B
10415 2008-11-20 0.034 B
10935 2008-11-21 0.033 B
11455 2008-11-22 0.034 B
11975 2008-11-23 0.034 B
12495 2008-11-24 0.034 B
13016 2008-11-25 0.034 B
13537 2008-11-26 0.033 B
14058 2008-11-27 0.033 B
14579 2008-11-28 0.033 B
15068 2008-11-29 0.034 B
15546 2008-11-30 0.035 B
521 2008-12-01 0.035 B
1042 2008-12-02 0.034 B
1563 2008-12-03 0.033 B
2084 2008-12-04 0.031 B
2605 2008-12-05 0.031 B
3126 2008-12-06 0.031 B
3647 2008-12-07 0.032 B
4168 2008-12-08 0.032 B
4689 2008-12-09 0.032 B
5210 2008-12-10 0.033 B
5731 2008-12-11 0.033 B
6252 2008-12-12 0.032 B
6773 2008-12-13 0.031 B
7294 2008-12-14 0.030 B
7815 2008-12-15 0.030 B
8336 2008-12-16 0.029 B
8856 2008-12-17 0.028 B
9376 2008-12-18 0.028 B
9896 2008-12-19 0.028 B
10416 2008-12-20 0.027 B
10936 2008-12-21 0.027 B
11456 2008-12-22 0.028 B
11976 2008-12-23 0.028 B
12496 2008-12-24 0.029 B
13017 2008-12-25 0.029 B
13538 2008-12-26 0.029 B
14059 2008-12-27 0.030 B
14580 2008-12-28 0.030 B
15069 2008-12-29 0.030 B
15547 2008-12-30 0.031 B
15851 2008-12-31 0.031 B
--
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
