Try the duplicated() function.  As in

m[!duplicated(id), ]

-tgs


On Wed, Apr 21, 2010 at 10:17 PM, gallon li <gallon.li@gmail.com> wrote:

> Dear r-helpers,
>
> I have a very simple question. Suppose my data is like
>
> id=c(rep(1,2),rep(2,2))
> b=c(2,3,4,5)
> m=cbind(id,b)
>
> > m
>     id b
> [1,]  1 2
> [2,]  1 3
> [3,]  2 4
> [4,]  2 5
> I wish to select the first observation for each id. That is, I want to
> quickly select two rows:
>
> id b
> 1 2
> 2 4
>
> only. how should i do this?
>
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>
> ______________________________________________
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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