[R] selecting maximum values

[Sean Davis]

see ?aggregate.

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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