[R] Determining which.max() within groups

[Bert Gunter]

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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