On Sep 3, 2009, at 12:17 PM, Ottorino-Luca Pantani wrote:
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 )
)
)
)
Perhaps this:
by(df.mydata, df.mydata$SAMPLE, function(x) x$CONC/x$CONC[1] )
...or if you wanted to used max(x$CONC) as the standardizing procedure
hat ought to work as well. With your data is gives identical results.
The equivalent tapply construction would be:
tapply(df.mydata$CONC, df.mydata$SAMPLE, function(x) x/x[1] )
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 olpant...@unifi.it
http://www4.unifi.it/dssnp/
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
