yes, thanks
you pointed me in the right direction: split/unplist was the trick
I completely left behind that possibility!
here the final version
############
mynorm <- function(x) {(x - min(x, na.rm=TRUE))/(max(x, na.rm=TRUE) - min(x,
na.rm=TRUE))}
mydf<-data.frame(blocks=rep(c("a","b","c"),each=5),
v1=round(runif(15,10,25),0), v2=round(rnorm(15,30,5),0))
g <- mydf$blocks
l <- split(mydf, g)
l <- lapply(l, transform, v1.mod = mynorm(v1))
mydf_new <- unsplit(l, g)
############
thanks again
massimo
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