Resources for optimizing code

I want to eliminate certain observations in a large dataframe (21000x100).
I have written code which does this using a binary vector (0=delete obs,
1=keep), but it uses for loops, and so it's slow and in the extreme it
causes R to hang for indefinite time periods.

I'm looking for one of two things:
1.  A document which discusses how to avoid for loops and situations in
which it's impossible to avoid for loops.

or

2.  A function which can do the above better than mine.

My code is pasted below.

Thanks so much,

Janet

# asst is a binary vector of length= nrow(DATAFRAME).
# 1= observations you want to keep.  0= observation to get rid of.

remove.xtra.f <-function(asst, DATAFRAME) {
        n<-sum(asst, na.rm=T)
        newdata<-matrix(nrow=n, ncol=ncol(DATAFRAME))
        j<-1
        for(i in 1:length(data)) {
                if (asst[i]==1) {
                        newdata[j,]<-DATAFRAME[i,]
                        j<-j+1
                }
        }
        newdata.f<-as.data.frame(newdata)
        names(newdata.f)<-names(DATAFRAME)
        return(newdata.f)
}
--
Janet Rosenbaum                                 jerosenb at fas.harvard.edu
PhD Candidate in Health Policy, Harvard GSAS
Harvard Injury Control Research Center, Harvard School of Public Health