[R] selection by two unique variables

[Bert Gunter]

?tapply

?with is also useful here

as in (untested)
with(yourdataframe, tapply(lastpk, id, unique))

-- Bert

On Wed, May 2, 2012 at 7:58 AM, Ayyappa Chaturvedula
<ayyappach at gmail.com> wrote:
> Dear Group,
>
> I am working with a large dataset where I need to select for each unique id
> the the unique lastpk row.  Here is a sample subject:
>
>          id             wtdt           wt         lastpk
>
>  64050256 2010-09-18   275  2010-09-16
>
>  64050256 2010-09-19   277  2010-09-18
>
>  64050256 2010-09-20   272  2010-09-18
>
>  64050256 2010-09-21   277  2010-09-18
>
>
>
> I want the result as:
>
>       id               wtdt         wt      lastpk
>
> 64050256 2010-09-18 275 2010-09-16
>
> 64050256 2010-09-19 277 2010-09-18
>
>
>
> I am using !(duplicated(data$id)) to select the first row but now I want to
> select the first row of the unique lastpk in each unique id.
>
>
>
> I appreciate your help on this.
>
>
>
> Regards,
>
> Ayyappa
>
>        [[alternative HTML version deleted]]
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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