correlation by group (was Re: [R] kunamorph@web.de)

[Sundar Dorai-Raj]

Christfried Kunath wrote on 4/8/2005 10:39 AM:
> Hello,
> 
> how can I use the function "cor()" with x and y in function 
> "aggregate()" or "by()"?
> 
> The data are like this:
> x   y   group
> 1   4   B
> 2   4   B
> 3   5   C
> 
> I would like obtain the correlation between x and y for each subset. I 
> don't want to use the workaround with the function subset(), because I 
> have many groups.
> 
> Thanks in advance.
> Christfried Kunath KC
> 

(Please use an informative subject as the posting guide recommends.)

Your example is not very useful, but perhaps you need something like:

tmp <- data.frame(x = rnorm(100), y = rnorm(100),
                   group = rep(letters[1:5], each = 20))
sapply(split(tmp, tmp["group"]), function(z) cor(z$x, z$y))
# OR
rbind(by(tmp, tmp["group"], function(z) cor(z$x, z$y)))

--sundar