Than you so much for your suggestions, everyone! Arun's solution is elegant and it works - but it takes the same amount of time as my original code. Bill's code works too, it's a tiny-bit faster (for 10,000 iterations of the same code - it took about 16.2 secs vs. 16.6 sec for Arun's solution and mine). Bert's solution is the chempion - takes about 75% of time of the first 2 solutions. Very impressive! Thanks again! Dimitri
On Tue, Feb 12, 2013 at 6:15 PM, William Dunlap <wdun...@tibco.com> wrote: > One could try using unlist(mylist, recursive=FALSE) to make a list of all > the columns > of the data.frames in mylist and subscripting from that. E.g., here is > some barely tested > code: > > nCols <- vapply(mylist, ncol, 0L) > > neededcolumns<-c(1,2,0) # I assume 0 means no column wanted from 3rd > df in list > > i <- neededcolumns + c(0, cumsum(nCols[-length(nCols)])) > > i <- i[neededcolumns>0] > > data.frame(unlist(mylist, recursive=FALSE)[i]) > a b > 1 1 5 > 2 2 6 > > > Bill Dunlap > Spotfire, TIBCO Software > wdunlap tibco.com > > > > -----Original Message----- > > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > On Behalf > > Of Bert Gunter > > Sent: Tuesday, February 12, 2013 2:53 PM > > To: Dimitri Liakhovitski > > Cc: r-help > > Subject: Re: [R] grabbing from elements of a list without a loop > > > > The answer is essentially no, a loop is required (as best I can see), > > although it can be in the form of an apply type call instead. Also, > > your code will fail with a 0 index. Something like this should work: > > > > newlist <- lapply(1:3,function(i)if(!neededcolumns[i])NULL else > > mylist[[c(i,neededcolumns[i])]]) > > > > ## note the use of [[c(i,j)]] form for selecting columns as an > > element from a list of lists > > > > ## Note that your cbind call produces a matrix, not a list. > > > > -- Bert > > > > > > You might wish to check the parallel package, as this looks like the > > sort of thing parallellization could be profitably used for; but I > > have no experience to offer beyond that suggestion. > > > > -- Bert > > > > On Tue, Feb 12, 2013 at 1:33 PM, Dimitri Liakhovitski > > <dimitri.liakhovit...@gmail.com> wrote: > > > Hello! > > > > > > # I have a list with several data frames: > > > mylist<-list(data.frame(a=1:2,b=2:3), > > > data.frame(a=3:4,b=5:6),data.frame(a=7:8,b=9:10)) > > > (mylist) > > > > > > # I want to grab only one specific column from each list element > > > neededcolumns<-c(1,2,0) # number of the column I need from each > element of > > > the list > > > > > > # Below, I am doing it using a loop: > > > newlist<-NULL > > > for(i in 1:length(mylist) ) { > > > newlist[[i]]<-mylist[[i]] [neededcolumns[i]] > > > } > > > newlist<-do.call(cbind,newlist) > > > (newlist) > > > > > > I was wondering if there is any way to avoid the loop above and make it > > > faster. > > > In reality, I have a much longer list, each of my data frames is much > > > larger and I have to do it MANY-MANY times. > > > Thanks a lot! > > > > > > Dimitri Liakhovitski > > > gfk.com <http://marketfusionanalytics.com/> > > > > > > [[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<http://www.r-project.org/posting-guide.html> > > > and provide commented, minimal, self-contained, reproducible code. > > > > > > > > -- > > > > Bert Gunter > > Genentech Nonclinical Biostatistics > > > > Internal Contact Info: > > Phone: 467-7374 > > Website: > > http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb- > > biostatistics/pdb-ncb-home.htm > > > > ______________________________________________ > > 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<http://www.r-project.org/posting-guide.html> > > and provide commented, minimal, self-contained, reproducible code. > -- Dimitri Liakhovitski gfk.com <http://marketfusionanalytics.com/> [[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.
