[R] extracting and summarizing data from data.frame with table-like output

[Kim Milferstedt]

Hello,

I would like to summarize and extract statistics (calculate means, 
stderr etc) from a data set that comes as a large table. This table 
needs to be sorted according to certain categories (in the example 
below "day", "angle", "distance" and "location"). I would like to 
have the output in a table similar to the original data, but now with 
the mean (or stderr etc) for all individual measurement of "res.1" at 
any "location" in one column, in a second colum the means for "res.2" 
at any "location" etc.

1. How can I get the means that I extract from my data set into a 
format similar to the initial data and not as a matrix as I get it 
with tapply()?
2. How can I calculate the means for many variables at once and not 
just for one as in tapply()?
3. How does R know what data to use with the function "order()" or "tapply()"?

Thanks for your help,

Kim

Here is an example:

## The code below creates an artificial data set "jj" that resembles 
my real data.
res.1       <-  c(rbinom(36, size=50, prob=0.6))
res.2       <-  c(rbinom(36, size=20, prob=0.4))
day         <-  rep(rep(1:3,rep(6,3)),2)
angle       <-  rep(1:3, 12)
distance    <-  rep(rep(1:2,rep(3,2)),6)
location    <-  rep(1:2,c(18,18))
jj          <-  cbind(res.1,res.2,day,angle,distance, location)

## I order the data
pp          <-  order(day, angle, distance)
jj[pp,]

## Now I calculate the mean for "res.1" over the variable "location"
ss          <-  tapply(res.1,list(day, angle, distance, location), mean)
ss

## The result "ss" are four matrices but I want a table like output, 
possibly also with the means for res.2.


__________________________________________

Kim Milferstedt
University of Illinois at Urbana-Champaign
Department of Civil and Environmental Engineering
4125 Newmark Civil Engineering Building
205 North Mathews Avenue MC 250
Urbana, IL 61801
USA
phone: (001) 217 333-9663
fax: (001) 217 333-6968
email: milferst at uiuc.edu
http://cee.uiuc.edu/research/morgenroth/index.asp