Dear Ottorino-Luca, Here is a suggestion using ave(): df.mydata$PERCENTAGE <- with(df.mydata, ave(CONC, list(SAMPLE), FUN = function(x) x / max(x) )) df.mydata[1:5,] # CONC TIME SAMPLE PERCENTAGE # 1 1.0 1 A 1.0 # 2 0.9 2 A 0.9 # 3 0.8 3 A 0.8 # 4 0.7 4 A 0.7 # 5 0.6 5 A 0.6
See ?ave and ?tapply for more information. HTH, Jorge On Thu, Sep 3, 2009 at 12:17 PM, Ottorino-Luca Pantani < ottorino-luca.pant...@unifi.it> wrote: > Dear R users, today I've got the following problem. > Here you are a dataframe as example. > There are some SAMPLES for which a CONCentration was recorded through > TIME. > The time during which the concentration was recorded is not always the > same, > 10 points for Sample A, 7 points for Sample B and 11 for sample C > > Also the initial concentration was not the same for the three samples. > > I would like express the concentrations as % of the concentration at time = > 1, therefore I wrote the following code that do the job, but is impractical > when the samples are, as in my real case, more than on hundred. > It is known that at the minimum time is present the maximum concentration, > by which divide all the other concentrations in the sample. > > I'm quite sure that there's a more elegant solution, but I really do not > even imagine how to write it. > > Thanks in advance for your time > > > (df.mydata <- data.frame( > CONC = > c(seq( from = 1, to = 0.1, by = -0.1 ), > seq( from = 0.8, to = 0.2, by = -0.1 ), > seq( from = 0.6, to = 0.1, by = -0.05 )), > TIME = > c(1:10, > 2:8, > 4:14 ), > SAMPLE = c( rep( "A", 10 ), > rep( "B", 7 ), > rep( "C", 11 ) > ) > ) > ) > MAX <- tapply( df.mydata$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/ > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > [[alternative HTML version deleted]]
______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
