[R] vector indexing problem in multilevel data: assigning a specific value to all group members

[Bertolt Meyer]

Dear List,

I work with multilevel data from psychological group experiments and  
have frequently encountered a situation for which I haven't found an  
elegant solution: I need to assign the value of a specific group  
member to all members of the group. For example, I have a group leader  
(identified by a binary vector) and some attribute for all group  
members. I want to create a new vector that holds the attribute of the  
group leader for each individual in the data frame (code at the bottom  
of the post):

   personId groupId groupLeader someAttribute leaderAttribute
1        1      17           0   0.145833333              NA
2        2      17           1   0.218750000              NA
3        3      17           0   0.089743590              NA
4        4      22           0   0.003875969              NA
5        5      22           0   0.086486486              NA
6        6      22           0   0.218750000              NA
7        7      22           1   0.089743590              NA
8        8      37           1   0.016129032              NA
9        9      37           0   0.151898734              NA

I need this:

   personId groupId groupLeader someAttribute leaderAttribute
1        1      17           0   0.145833333     0.218750000
2        2      17           1   0.218750000     0.218750000
3        3      17           0   0.089743590     0.218750000
4        4      22           0   0.003875969     0.089743590
5        5      22           0   0.086486486     0.089743590
6        6      22           0   0.218750000     0.089743590
7        7      22           1   0.089743590     0.089743590
8        8      37           1   0.016129032     0.016129032
9        9      37           0   0.151898734     0.016129032

So far, my attemps along the lines of

df$leaderAttribute <- df$someAttribute[df$groupLeader == 1][df$groupId]

have failed if the groups were not numbered with 1, 2, 3... as in the  
example above. I need something simple for transforming the groupId  
vector from 17, 17, 17, 22... to 1,1,1,2... for doing the second  
indexing. It seems like a simple problem, but I am unable to get it  
right. I had to fall back to building a specific function employing  
nested for() loops for achieving this. However, this is error prone  
and very slow, as some of my data sets are very large. What am I  
missing?

Any help would be greatly appreciated.
Regards,
Bertolt

Code:

personId <- c(1,2,3,4,5,6,7,8,9)
groupId <- c(17,17,17,22,22,22,22,37,37)
groupLeader <- c(0,1,0,0,0,0,1,1,0)
someAttribute <- c(0.145833333, 0.218750000, 0.089743590, 0.003875969,  
0.086486486, 0.218750000, 0.089743590, 0.016129032, 0.151898734)
leaderAttribute <- c(NA,NA,NA,NA,NA,NA,NA,NA,NA)

df <- cbind(personId, groupId, groupLeader, someAttribute,  
leaderAttribute)
df <- as.data.frame(df)
df

rm(personId, groupId, groupLeader, someAttribute, leaderAttribute, df)

-- 
Dr. Bertolt Meyer
Senior research and teaching associate
Social Psychology, Institute of Psychology, University of Zurich
Binzmuehlestrasse 14, Box 15
CH-8050 Zurich
Switzerland

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