[R] merging single column from different dataframe

[Rui Barradas]

Hello,

One way, with limited applicability, would be

df4 <- cbind(df2, var1=NA)
df4$var1[ df2$names2 %in% df1$names1 ] <- df1$var1
df4


Limited because it doesn't resist to duplicate key values (the columns 
names1/2).
But with unique keys it should be much more efficient, speed- and 
memory-wise. Much, much more.
There is also package data.table, designed for this type of problem.

As for the extraction operator, there is An Introduction to R, 
R-intro.pdf, in the doc directory of your R installation.
And several others on-line, on CRAN, for instance.
(Your example wouldn't work, df2$names2 are the values of that variable, 
not row indexes. See df2[[ "names2" ]].)

Hope this helps,

Rui Barradas

Em 03-06-2012 20:22, Kai Mx escreveu:
> Hi all,
> probably really simple to solve, but having no background in programming I
> haven't been able to figure this out: I have two dataframes like
>
> df1<- data.frame(names1=c('aa','ab', 'ac', 'ad'), var1=c(1,5,7,12))
> df2<- data.frame(names2=c('aa', 'ab', 'ac', 'ad', 'ae'),
> var2=c(3,6,9,12,15))
>
> Now I want merge var1 to df2 by matching the dataframes by the 'names'
> columns, i.e. something like
>
> df3<- merge (df2, df1, by.x='names2', by.y='names1', all.x=T)
>
> However, the original dataframes have quite a lot of columns and I thought
> that I should be able to address the var1 column by something like
> df1$var[[df2$name2]]. Could somebody please enlighten me and/or maybe
> suggest a short tutorial for the extraction operator?
> Thanks!
>
>
> Best,
>
> Kai
>
> 	[[alternative HTML version deleted]]
>
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