[R] Group several variables and apply a function to the group

[John Kane]

?aggregate should do it

aggregate(df$x,list(df$comn, df$mi), sd)

There are other ways of course

Using the reshape2 package 

library(reshape2)
x1 <- melt(df, id=c("comn", "mi"))
dcast(x1, comn + mi ~ variable, sd)




--- On Sun, 12/4/11, Aurélien PHILIPPOT <aurelien.philippot at gmail.com> wrote:

> From: Aurélien PHILIPPOT <aurelien.philippot at gmail.com>
> Subject: [R] Group several variables and apply a function to the group
> To: R-help at r-project.org
> Received: Sunday, December 4, 2011, 3:32 PM
> Dear R-experts,
> I am struggling with the following problem, and I am
> looking for advice
> from more experienced R-users: I have a data frame with 2
> identifying
> variables (comn and mi), and an output variable (x). comn
> is a variable for
> a company and mi is a variable for a month.
> 
> comn<-c("abc", "abc", "abc", "abc", "abc", "abc", "xyz",
> "xyz","xyz", "xyz")
> mi<- c("1", "1","1", "2", "2", "2", "1", "1", "3", "3")
> x<- c("-0.0031", "0.0009", "-0.007", "0.1929","0.0087",
> "0.099","-0.089",
> "0.005", "-0.0078", "0.67" )
> df<- data.frame(comn=comn, mi=mi, x=x)
> 
> 
> For each company, within a particular month, I would like
> to compute the
> standard deviation of x: for example, for abc, I would like
> to compute the
> sd of x for month1 (when mi=1) and for month2 (when mi=2).
> 
> In other languages (Stata for instance), I would create a
> grouping variable
> (group comnn and mi) and then, apply the sd function for
> each group.
> 
> However, I don't find an elegant way to do the same in R:
> 
> I was thinking about the following: I could subset my data
> frame by mi and
> create one file per month, and then make a loop and in each
> file, use a
> "by" operator for each comn. I am sure it would work, but I
> feel that it
> would be like killing an ant with a tank.
> 
> I was wondering if anyone knew a more straightforward way
> to implement that
> kind of operation?
> 
> Thanks a lot,
> 
> Best,
> Aurelien
> 
>     [[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org
> mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained,
> reproducible code.
>