[R] Determining which.max() within groups

[David L Carlson]

If you want the Julian date, you could use Bert's index on the original data frame:

Daily[out$Q, ]
         Date  wyr        Q
4  1911-04-04 1990 5.097032
6  1911-04-06 1991 6.569508
9  1911-04-09 1992 4.445745
15 1911-04-16 1993 3.001586
18 1911-04-28 1994 3.369705

Another way to get that index would be to use by():

idx <- as.vector(by(Daily, Daily$wyr, function(x) rownames(x)[which.max(x$Q)]))
Daily[idx, ]

-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Bert Gunter
Sent: Tuesday, June 6, 2017 9:16 PM
To: Morway, Eric <emorway at usgs.gov>
Cc: R mailing list <r-help at r-project.org>
Subject: Re: [R] Determining which.max() within groups

cumsum() seems to be what you need.

This can probably be done more elegantly, but ...

out <- aggregate(Q ~ wyr, data = Daily, which.max)
tbl <- table(Daily$wyr)
out$Q <- out$Q + cumsum(c(0,tbl[-length(tbl)]))
out

## yields

   wyr  Q
1 1990  4
2 1991  6
3 1992  9
4 1993 15
5 1994 18

I leave the matter of Julian dates to you or others.

Cheers,
Bert




Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Tue, Jun 6, 2017 at 6:30 PM, Morway, Eric <emorway at usgs.gov> wrote:
> Using the dataset below, I got close to what I'm after, but not quite all
> the way there.  Any suggestions appreciated:
>
> Daily <- read.table(textConnection("     Date  wyr        Q
> 1911-04-01 1990 4.530695
> 1911-04-02 1990 4.700596
> 1911-04-03 1990 4.898814
> 1911-04-04 1990 5.097032
> 1911-04-05 1991 5.295250
> 1911-04-06 1991 6.569508
> 1911-04-07 1991 5.861587
> 1911-04-08 1991 5.153666
> 1911-04-09 1992 4.445745
> 1911-04-10 1992 3.737824
> 1911-04-11 1992 3.001586
> 1911-04-12 1992 3.001586
> 1911-04-13 1993 2.350298
> 1911-04-14 1993 2.661784
> 1911-04-16 1993 3.001586
> 1911-04-17 1993 2.661784
> 1911-04-19 1994 2.661784
> 1911-04-28 1994 3.369705
> 1911-04-29 1994 3.001586
> 1911-05-20 1994 2.661784"),header=TRUE)
>
> aggregate(Q ~ wyr, data = Daily, which.max)
>
> # gives:
> #    wyr Q
> # 1 1990 4
> # 2 1991 2
> # 3 1992 1
> # 4 1993 3
> # 5 1994 2
>
> I can 'see' that it is returning the which.max() relative to each
> grouping.  Is there a way to instead return the absolute position (row) of
> the max value within each group.  i.e.:
>
> # Would instead like to have
> #     wyr  Q
> # 1  1990  4
> # 2  1991  6
> # 3  1992  9
> # 4  1993  15
> # 5  1994  18
>
> The icing on the cake would be to get the Julien Day corresponding to the
> date on which each year's maximum occurs?
>
>         [[alternative HTML version deleted]]
>
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