[R] selecting maximum values

[Roger Bivand]

On Wed, 4 May 2005, Sean Davis wrote:

> see ?aggregate.

Or maybe tapply, or its close relative, by:

> by(df, list(df$station, df$date), function(x) 
+   x$row[which.max(x$chlorophyll)]) 
: Ancona
: 21/06/01
[1] NA
------------------------------------------------------------ 
: Castagneto
: 21/06/01
[1] 3
------------------------------------------------------------ 
: Ancona
: 23/06/01
[1] 6
------------------------------------------------------------ 
: Castagneto
: 23/06/01
[1] NA

since happily a row ID column was included in the data frame. Note that 
which.max only reports the row of the first maximum if there are ties.

> 
> Sean
> 
> On May 4, 2005, at 11:43 AM, alessandro carletti wrote:
> 
> > Sorry for disturbing you with another newbie question!
> > I have a data frame about coastal waters quality
> > parameters: for some parameters (e.g. NH3) I have only
> > 1 observation for each sampling station and each
> > sampling date, while in other cases (chlorophyll) I
> > have 1 obs for each meter-depth for each station and
> > date. How can I select only the max chlorophyll value
> > for each station/date?
> >
> > example
> >
> > row  station         date        depth     chlorophyll
> > 1     Castagneto      21/06/01     -0.5         2.0
> > 2     Castagneto      21/06/01     -1.5         2.2
> > 3     Castagneto      21/06/01     -2.5         2.4
> > 4     Castagneto      21/06/01     -3.5         2.1
> > 5     Ancona          23/06/01     -0.5         2.4
> > 6     Ancona          23/06/01     -1.5         2.5
> > 7     Ancona          23/06/01     -2.5         2.2
> > 8     Ancona          23/06/01     -3.5         2.1
> > 9     Ancona          23/06/01     -4.5         1.9
> > ...
> >
> > I'd like to select only row 3 and 6, the ones with max
> > chlorophyll values, or have the mean for the rows 1:4
> > and 5:9
> >
> > Thanks
> >
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> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no