[R] data frames; matching/merging

[hadley wickham]

On Mon, Feb 8, 2010 at 10:39 AM, Jonathan <jonsleepy at gmail.com> wrote:
> Hi all,
>    I'm feeling a little guilty to ask this question, since I've
> written a solution using a rather clunky for loop that gets the job
> done.  But I'm convinced there must be a faster (and probably more
> elegant) way to accomplish what I'm looking to do (perhaps using the
> "merge" function?).  I figured somebody out there might've already
> figured this out:
>
> I have a dataframe with two columns (let's call them V1 and V2).  All
> rows are unique, although column V1 has several redundant entries.
>
> Ex:
>
>     V1     V2
> 1    a        3
> 2    a        2
> 3    b        9
> 4    c        4
> 5    a        7
> 6    b        11
>
>
> What I'd like is to return a dataframe cut down to have only unique
> entires in V1.  V2 should contain a vector, for each V1, that is the
> minimum of all the possible choices from the set of redundant V1's.
>
> Example output:
>
>      V1     V2
> 1     a        2
> 2     b        9
> 3     c        4

With the plyr package:

library(plyr)
ddply(mydf, "V1", summarise, V2 = min(V2))

Hadley


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