Dear Daniela, Try this:
set.seed(123) myD<-data.frame(code=sample(letters[1:5],200,replace=T),value=rnorm(200)) tapply(myD$value,myD$code,mean) a b c d e 0.04401465 0.07813648 0.07018791 -0.14508544 -0.02369875 See ?tapply for more information. HTH, Jorge On Tue, Jul 8, 2008 at 9:18 AM, Daniela Ottaviani <[EMAIL PROTECTED]> wrote: > Dear All, > > I have a database of 200 observations named myD. > In the dataframe there are a column named code (with codes varying from 1 > to 77), a column named "prevalence" with some quantitative measurements are > given and an column named Pr_mean, with no values. > > I would like to set a cycle to compute the average of prevalence values for > each different code and store the averages under the empty field Pr_mean. > > This is what I wrote: > > # Set a cycle > for (i in 1:nrow(myD)) { > mycode = myD$code[i] > mymean[i] = mean(prevalence) > myD$Pr_mean[i] = mymean[i] > } > > With the above cycle I am able to compute the average of all 200 > observations which is then written in every cell. > I understand that a condition is missing, that indicates that the average > has to be computed amongst the observations showing the same codes values. > > Could you please help me ? > > > D. > > > > Posta, news, sport, oroscopo: tutto in una sola pagina. > Crea l'home page che piace a te! > www.yahoo.it/latuapagina > [[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. > [[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.