[R] Newbie question: How to use tapply() on several vectors simultaneously

[Bertolt Meyer]

Dear R users,

I have a newbie-question that I couldn't resolve after reading through 
several pieces of documentation and searching the archive.

I have a data.frame containing experimental data from a group experiment 
in psychology. Each line represents a single participant, but 
participants were assigned to groups of three or four persons. One 
variable indicates each participants' group number (groupID). For a 
large number of variables, I would like to obtain the mean group value. 
I figured I use tapply() in the fashion of tapply(variable, groupID, 
mean), but that would be a tiresome task for my 150 variables. I am thus 
looking for a way to obtain a data.frame that contains one row for each 
group with the group-mean variables as columns.

Example:

 > test <- as.data.frame(cbind(c(rep(1,5),rep(2,5)), rnorm(10), rnorm(10)))
 > names(test)[1] <- "groupID"
 > test

    groupID          V2          V3
1        1 -0.82990860 -0.61778919
2        1 -0.01379452  0.64609053
3        1 -2.64990839 -1.00570627
4        1 -0.07903878 -0.70864441
5        1  0.61483071 -1.32039565
6        2 -0.18913937  1.38490710
7        2 -0.60017953  0.15893421
8        2 -0.99901931  0.05963436
9        2 -1.46759515  0.35040283
10       2 -0.44650422 -0.08713162

 > tapply(test$V2, test$groupID, mean)
          1          2
-0.5915639 -0.7404875

 > tapply(test$V3, test$groupID, mean)
          1          2
-0.6012890  0.3733494

I am now looking for something that gives me

   groupID	  V2		V3
1	1 -0.5915639	-0.6012890
2	2 -0.7404875 	0.3733494

Any ideas?

Thank you very much,
Bertolt

-- 
Bertolt Meyer
Oberassistent
Sozialpsychologie, Psychologisches Institut der Universität Zürich
Binzmühlestr. 14, Box 15
CH-8050 Zürich

bmeyer at sozpsy.uzh.ch
tel:   +41446357282
fax:   +41446357279
mob:   +41788966111