R help - May 2003 - array question on indexing

Dear friends,
I have struggled to do what is likely rather simple but cannot get it working.
I have a dataframe with 428 obs. of 5 variables on 26 patients. Two 
variables are responses, one an index of patients (1:26), one a 
period-indicator :1 :max 3, and one total time from min 1 to max 36 weeks 
(max 21 entries). A complete set would be 26*21=546 long but here are only 
428 entries.
Now, within each period nested within patients a max of one of the response 
variables is found, with length 78, 6 are NA from MAX <- 
by(response,list(period,id),max).
Then how do I get the individual times corresponding to these within 
period, within patient MAX values ?
(Windows R 1.7)

Best wishes
Troels


Troels Ring, MD
Department of Nephrology
Aalborg Hospital, Denmark
tring@gvdnet.dk
fax 00 45 99 32 22 34
tlf 00 45 99 32 22 42
  00 45 98 14 05 82 (home)
Heliosvej 12
9210 Aalborg SE
Denmark 
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Troels Ring <tring at gvdnet.dk> writes:
> Dear friends,
> I have struggled to do what is likely rather simple but cannot get it
working.
> I have a dataframe with 428 obs. of 5 variables on 26 patients. Two 
> variables are responses, one an index of patients (1:26), one a 
> period-indicator :1 :max 3, and one total time from min 1 to max 36 weeks 
> (max 21 entries). A complete set would be 26*21=546 long but here are only 
> 428 entries.
> Now, within each period nested within patients a max of one of the response
> variables is found, with length 78, 6 are NA from MAX <- 
> by(response,list(period,id),max).
> Then how do I get the individual times corresponding to these within 
> period, within patient MAX values ?
> (Windows R 1.7)
Let's see...

if you had a data frame of one person in one period, you'd do 

time[which.max(response)]

so use by to split the data frame  and write a little function to
handle each subframe:

timeAtMax <- function(d) with(d, time[which.max(response)])
# or: function(d) d$time[which.max(d$response)])
by(mydata, list(period,id), timeAtMax)

[untested of course, and you might want to do something about the fact
that which.max returns a value even in the precense of NA's]

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
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