[R] Subsetting data frames without a loop

[Michael_Nielsen/Syd/Synergy.SYNERGY@synergy.com.au]

I KNOW this should be easy, but I'm stuck.

My data frame consists of multiple observations from each of a number of
stations, and what I would like to do is create another data frame that
contains all the variables of the first, but only rows where a certain
variable is at its maximum for the station.

So, for example:

> my.df
   stn obs           v
1    1   1  0.26400396
2    2   1 -0.79194397
3    3   1  0.11924528
4    4   1  0.42596859
5    5   1 -0.50528235
6    1   2 -1.57524853
7    2   2  0.17762482
8    3   2 -0.83013770
9    4   2 -0.53203400
10   5   2 -2.71397275
11   1   3  0.26902053
12   2   3  2.01147908
13   3   3  0.73301643
14   4   3 -0.67333384
15   5   3 -1.36219773
16   1   4 -2.20342109
17   2   4  0.18941702
18   3   4  0.51492032
19   4   4  0.03597370
20   5   4 -1.43502366
21   1   5 -1.34589392
22   2   5  1.00389195
23   3   5 -0.21233041
24   4   5 -1.35141044
25   5   5 -0.02052348

> tapply(v,factor(stn),max)
          1           2           3           4           5
 0.26902053  2.01147908  0.73301643  0.42596859 -0.02052348

so my new data frame should contain (possibly multiple rows per station)

   stn obs           v
1    1   3  0.26902053
2    2   3  2.01147908
3    3   3  0.73301643
4    4   1  0.42596859
5    5   5 -0.02052348

Thanks in advance.

Regards,

Mike

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