[R] dividing a dataframe column by different constants

[Ottorino-Luca Pantani]

Dear R users, today I've got the following problem.
Here you are a dataframe as example.
There are some SAMPLES  for which a CONCentration was recorded through TIME.
The time during which the concentration was recorded is not always the same,
10 points for Sample A, 7 points for Sample B and 11 for sample C

Also the initial concentration was not the same for the three samples.

I would like express the concentrations as % of the concentration at 
time = 1, therefore I wrote the following code that do the job, but is 
impractical when the samples are, as in my real case, more than on hundred.
It is known that at the minimum time is present the maximum 
concentration, by which divide all the other concentrations in the sample.

I'm quite sure that there's a more elegant solution, but I really do not 
even imagine how to write it.

Thanks in advance for your time


(df.mydata <- data.frame(
                         CONC =
                         c(seq( from = 1, to = 0.1, by = -0.1 ),
                           seq( from = 0.8, to = 0.2, by = -0.1 ),
                           seq( from = 0.6, to = 0.1, by = -0.05 )),
                         TIME =
                         c(1:10,
                           2:8,
                           4:14 ),
                         SAMPLE = c( rep( "A", 10 ),
                           rep( "B", 7 ),
                           rep( "C", 11 )
                           )
                         )
 )
MAX <- tapply( df.mydata$CONC, df.mydata$SAMPLE, max )
(df.mydata$PERCENTAGE <-
 ifelse(df.mydata$SAMPLE == "A",  df.mydata$CONC / MAX[1],
        ifelse(df.mydata$SAMPLE == "B",  df.mydata$CONC / MAX[2],
               df.mydata$CONC / MAX[3])))

-- 
Ottorino-Luca Pantani, Università di Firenze
Dip. Scienza del Suolo e Nutrizione della Pianta
P.zle Cascine 28 50144 Firenze Italia
Tel 39 055 3288 202 (348 lab) Fax 39 055 333 273 
OLPantani at unifi.it  http://www4.unifi.it/dssnp/