You might take a look at the transformBy function in the doBy package For example, new.df=transformBy(~group,data=my.df, new=y/max(y))
David Freedman baptiste auguie-2 wrote: > > Dear list, > > I have a data.frame with x, y values and a 3-level factor "group", > say. I want to create a new column in this data.frame with the values > of y scaled to 1 by group. Perhaps the example below describes it best: > >> x <- seq(0, 10, len=100) >> my.df <- data.frame(x = rep(x, 3), y=c(3*sin(x), 2*cos(x), >> cos(2*x)), # note how the y values have a different maximum >> depending on the group >> group = factor(rep(c("sin", "cos", "cos2"), each=100))) >> library(reshape) >> df.melt <- melt(my.df, id=c("x","group")) # make a long format >> df.melt <- df.melt[ order(df.melt$group) ,] # order the data.frame >> by the group factor >> df.melt$norm <- do.call(c, tapply(df.melt$value, df.melt$group, >> function(.v) {.v / max(.v)})) # calculate the normalised value per >> group and assign it to a new column >> library(lattice) >> xyplot(norm + value ~ x,groups=group, data=df.melt, auto.key=T) # >> check that it worked > > > This procedure works, but it feels like I'm reinventing the wheel > using hammer and saw. I tried to use aggregate, by, ddply (plyr > package), but I coudn't find anything straight-forward. > > I'll appreciate any input, > > Baptiste > > > > > > _____________________________ > > Baptiste AuguiƩ > > School of Physics > University of Exeter > Stocker Road, > Exeter, Devon, > EX4 4QL, UK > > Phone: +44 1392 264187 > > http://newton.ex.ac.uk/research/emag > > ______________________________________________ > 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. > > ----- David Freedman Atlanta -- View this message in context: http://www.nabble.com/tapply-within-a-data.frame%3A-a-simpler-alternative--tp20939647p20958347.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.