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
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