On Tue, 6 Jun 2017, Morway, Eric 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?
Like this:
which.max.by.wyr <- with(Daily, which( ave( Q, wyr, FUN=max) == Q))
cbind( Daily[ which.max.by.wyr, ], index=which.max.by.wyr )
Date wyr Q index
4 1911-04-04 1990 5.097032 4
6 1911-04-06 1991 6.569508 6
9 1911-04-09 1992 4.445745 9
15 1911-04-16 1993 3.001586 15
18 1911-04-28 1994 3.369705 18
If there are ties in Q and you do not want more than one max value listed,
you can add a litle fuzz to randomly pick one. i.e.
fuzz <- runif(nrow(Daily), 0, 1e-10)
which.max.by.wyr <- with(Daily, which(ave(Q+fuzz,wyr,FUN=max)==Q+fuzz))
If you want the first tied value, then sort fuzz before determining
which.max.by.wyr.
HTH,
Chuck
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