[R] lookup in R - possible to avoid loops?

[Phil Spector]

Dimitri -
    While merge is most likely the fastest way to solve
your problem, I just want to point out that you can use
a named vector as a lookup table.  For your example:

categories = my.lookup$category
names(categories) = my.lookup$names

creates the lookup table, and

my.df$category = categories[my.df$names]

creates the category column.
                                            - Phil



On Mon, 8 Nov 2010, Dimitri Liakhovitski wrote:

> Hello!
> Hope there is a nifty way to speed up my code by avoiding loops.
> My task is simple - analogous to the vlookup formula in Excel. Here is
> how I programmed it:
>
> # My example data frame:
> set.seed(1245)
> my.df<-data.frame(names=rep(letters[1:3],3),value=round(rnorm(9,mean=20,sd=5),0))
> my.df<-my.df[order(my.df$names),]
> my.df$names<-as.character(my.df$names)
> (my.df)
>
> # My example lookup table:
> my.lookup<-data.frame(names=letters[1:3],category=c("AAA","BBB","CCC"))
> my.lookup$names<-as.character(my.lookup$names)
> my.lookup$category<-as.character(my.lookup$category)
> (my.lookup)
>
> # Just adding an extra column to my.df that contains the categories of
> the names in the column "names":
> my.df2<-my.df
> my.df2$category<-NA
> for(i in unique(my.df$names)){
> 	my.df2$category[my.df2$names %in%
> i]<-my.lookup$category[my.lookup$names %in% i]
> }
> (my.df2)
>
> It does what I need, but it's way too slow - I need to run it for
> hundreds and hundreds of names in >100 of huge files (tens of
> thousands of rows in each).
> Any way to speed it up?
>
>
> Thanks a lot!
>
> -- 
> Dimitri Liakhovitski
> Ninah Consulting
> www.ninah.com
>
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