I doubt this to be true. Try this in R: > dmy<-rep(1,5) > dmy[2:5]<-dmy[1:4]+1 This is equivalent to what you propose (even simpler), but it does not, as OP seems to have wanted, fill dmy with 1,2,3,4,5, but, as I had expected, with 1,2,2,2,2.
I would be interested in knowing what exactly the difference beween my example above, and the one you suggest, is. As others have suggested: another way is to use actual recursive calls, but I seriously doubt these to be more efficient. You should probably only use it if you really hate to type the word 'for' (-: Though I would also like to see an example where they prove to be the better way to go (by any criteria, but preferably speed or perhaps other resource usage) Nick Sabbe -- ping: nick.sa...@ugent.be link: http://biomath.ugent.be wink: A1.056, Coupure Links 653, 9000 Gent ring: 09/264.59.36 -- Do Not Disapprove -----Original Message----- From: David Winsemius [mailto:dwinsem...@comcast.net] Sent: maandag 8 november 2010 15:04 To: Nick Sabbe Cc: 'PLucas'; r-help@r-project.org Subject: Re: [R] How to eliminate this for loop ? On Nov 8, 2010, at 4:30 AM, Nick Sabbe wrote: > Whenever you use a recursion (that cannot be expressed otherwise), you > always need a (for) loop. Not necessarily true ... assuming "a" is of length "n": a[2:n] <- a[1:(n-1))]*b + cc[1:(n-1)] # might work if b and n were numeric vectors of length 1 and cc had length >= n. (Never use "c" as a vector name.) # it won't work if there are no values for the nth element at the beginning and you are building up a element by element. And you always need to use operations that appropriate to the object type. So if "a" really is a list, this will always fail since arithmetic does not work on list elements. If on the other hand, the OP were incorrect in calling this a list and "a" were a numeric vector, there might be a chance of success if the rules of indexing were adhered to. The devil is in the details and the OP has not supplied enough code to tell what might happen. -- David. > Apply and the like do not allow to use the intermediary results > (i.e. a[i-1] > to calculate a[i]). > > So: no, it cannot be avoided in your case, I guess. > > > Nick Sabbe > -- > ping: nick.sa...@ugent.be > link: http://biomath.ugent.be > wink: A1.056, Coupure Links 653, 9000 Gent > ring: 09/264.59.36 > > -- Do Not Disapprove > > > > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org > ] On > Behalf Of PLucas > Sent: maandag 8 november 2010 10:26 > To: r-help@r-project.org > Subject: [R] How to eliminate this for loop ? > > > Hi, I would like to create a list recursively and eliminate my for > loop : > > a<-c() > a[1] <- 1; # initial value > for(i in 2:N) { > a[i]<-a[i-1]*b - c[i-1] # b is a value, c is another vector > } > > > Is it possible ? > > Thanks > -- > View this message in context: > http://r.789695.n4.nabble.com/How-to-eliminate-this-for-loop-tp3031667p30316 > 67.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. > > ______________________________________________ > 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 Winsemius, MD West Hartford, CT ______________________________________________ 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.
