[R] group by and merge two dataframes

[Massimo Bressan]

yes, thank you for all your replies, they worked out correctly indeed...

...but because of my fault, by then working on my real data I fully 
realised that I should have mentioned something that is changing (quite 
a lot, in fact) the terms of the problem...

please would you consider the following (consistent) variation ?

df1 <- data.frame(id=rep(1:3,each=2), item=c(rep("A",2), rep("B",2), 
rep("C",2)), v=rnorm(6))
df2 <- data.frame(id=c(1,2,3), who=c("tizio","caio","sempronio"))

and again I need to group the first dataframe "df1" both by "id" and by 
the first record of "v", and then merge with the second dataframe "df2" 
(again by "id")

now, how to do that?
(that's why probably I was pointing in my first post to the use of sqldf)

thanks

ps: I'm in doubt wheter I must open another thread or keep going with 
this one (really sorry for the eventual violation of the R-help netiquette)

Il 08/05/2014 17:14, arun ha scritto:
> Hi,
> May be this helps:
>   merge(unique(df1),df2)
> A.K.
>
>
>
>
>
> On Thursday, May 8, 2014 5:46 AM, Massimo Bressan <mbressan at arpa.veneto.it> wrote:
> given this "bare bone" example:
>
> df1 <- data.frame(id=rep(1:3,each=2), item=c(rep("A",2), rep("B",2),
> rep("C",2)))
> df2 <- data.frame(id=c(1,2,3), who=c("tizio","caio","sempronio"))
>
> I need to group the first dataframe "df1" by "id" and then merge with
> the second dataframe "df2" (again by "id")
> so far I've manged to accomplish the task by something like the following...
>
> # start
>
> require(sqldf)
> tmp<-sqldf("select * from df1 group by id")
> merge(tmp, df2)
>
> #end
>
> now I'm wonderng if there is a more efficient and/or elegant way to
> perform it (also because in fact I'm dealing with much more "heavy"
> dataframes);
>
> may be possible through a single sql statement?  or by using a different
> package functions (e.g. dplyr)?
> my attempts towards these alternative approaches miserably failed ...
>
> thanks
>
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