[R] How to apply a function to subsets of a data frame *and* obtain a data frame again?

[Nick Sabbe]

You might want to look at package plyr and use ddply.

HTH,


Nick Sabbe
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> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Marius Hofert
> Sent: woensdag 17 augustus 2011 12:42
> To: Help R
> Subject: [R] How to apply a function to subsets of a data frame *and*
> obtain a data frame again?
> 
> Dear all,
> 
> First, let's create some data to play around:
> 
> set.seed(1)
> (df <- data.frame(Group=rep(c("Group1","Group2","Group3"), each=10),
>                  Value=c(rexp(10, 1), rexp(10, 4), rexp(10,
> 10)))[sample(1:30,30),])
> 
> ## Now we need the empirical distribution function:
> edf <- function(x) ecdf(x)(x) # empirical distribution function
> evaluated at x
> 
> ## The big question is how one can apply the empirical distribution
> function to
> ## each subset of df determined by "Group", so how to apply it to
> Group1, then
> ## to Group2, and finally to Group3. You might suggest (?) to use
> tapply:
> 
> (edf. <- tapply(df$Value, df$Group, FUN=edf))
> 
> ## That's correct. But typically, one would like to obtain not only the
> values,
> ## but a data.frame containing the original information and the new
> (edf-)values.
> ## What's a simple way to get this? (one would be required to first
> sort df
> ## according to Group, then paste the values computed by edf to the
> sorted df;
> ## seems a bit tedious).
> ## A solution I have is the following (but I would like to know if
> there is a
> ## simpler one):
> 
> (edf.. <- do.call("rbind", lapply(unique(df$Group), function(strg){
>     subdata <- subset(df, Group==strg) # sub-data
>     subdata <- cbind(subdata, edf=edf(subdata$Value))
> })) )
> 
> 
> Cheers,
> 
> Marius
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