R help - Jul 2010 - Repeated analysis over groups / Splitting by group variable

I am performing some analysis over a large data frame and would like
to conduct repeated analysis over grouped-up subsets. How can I do
that?

Here some example code for clarification:

require("flexmix")	# for Kullback-Leibler divergence
n <- 23
groups <- c(1,2,3)
mydata <- data.frame(
	sequence=c(1:n),
	data1=c(rnorm(n)),
	data2=c(rnorm(n)),
	group=rep(sample(groups, n, replace=TRUE))
)
# Part 1: full stats (works fine)
dataOnly <- cbind(mydata$data1, mydata$data2, mydata$group)
KLdiv(dataOnly)

#
# Part 2: again - but once for each group (error)
#
by(dataOnly, groups, KLdiv(dataOnly))

The error I am getting is: Error in tapply(1L:23L, list(INDICES = c(1,
2, 3)), function (x)  :
  arguments must have same length

Are there better ways than 'by' ? I would like to use different stats
and functions and therefore I am looking for a splitter whose output I
can hand to any statical function I want.

Any ideas?

Ralf

Ralf -
    If you want to use by(), I think it should look like 
this:

by(dataOnly,dataOnly[,3],function(x)KLdiv(as.matrix(x)))

But you might find the following more useful:

lapply(split(as.data.frame(dataOnly),dataOnly[,3]),
              function(x)KLdiv(as.matrix(x)))

since it returns its results in a list.

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu



On Thu, 15 Jul 2010, Ralf B wrote:
> I am performing some analysis over a large data frame and would like
> to conduct repeated analysis over grouped-up subsets. How can I do
> that?
>
> Here some example code for clarification:
>
> require("flexmix")	# for Kullback-Leibler divergence
> n <- 23
> groups <- c(1,2,3)
> mydata <- data.frame(
> 	sequence=c(1:n),
> 	data1=c(rnorm(n)),
> 	data2=c(rnorm(n)),
> 	group=rep(sample(groups, n, replace=TRUE))
> )
> # Part 1: full stats (works fine)
> dataOnly <- cbind(mydata$data1, mydata$data2, mydata$group)
> KLdiv(dataOnly)
>
> #
> # Part 2: again - but once for each group (error)
> #
> by(dataOnly, groups, KLdiv(dataOnly))
>
> The error I am getting is: Error in tapply(1L:23L, list(INDICES = c(1,
> 2, 3)), function (x)  :
>  arguments must have same length
>
> Are there better ways than 'by' ? I would like to use different
stats
> and functions and therefore I am looking for a splitter whose output I
> can hand to any statical function I want.
>
> Any ideas?
>
> Ralf
>
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