Hi:

There is a months() function associated with Date objects, so you should be
able to do something like

aggregate(value ~ months(date), data = data$flow$daily, FUN = median)

Here's a toy example because your data are not in a ready form:

df <- data.frame(date = seq(as.Date('2010-01-01'), by = 'days', length =
250),
                    val =  rnorm(250))
> aggregate(val ~ months(date), data = df, FUN = median)
  months(date)         val
1        April -0.18864817
2       August -0.16203705
3     February  0.03671700
4      January  0.04500988
5         July -0.12753151
6         June  0.09864811
7        March  0.23652105
8          May  0.25879994
9    September  0.53570764

HTH,
Dennis

On Sun, Dec 19, 2010 at 2:31 PM, HUXTERE <emilyhuxter@gmail.com> wrote:

>
> 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
> --
> View this message in context:
> http://r.789695.n4.nabble.com/monthly-median-in-a-daily-dataset-tp3094917p3094917.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@r-project.org mailing list
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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.
>

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