On 4/9/07, Muenchen, Robert A (Bob) <[EMAIL PROTECTED]> wrote: > Hi All, > > I'm trying to understand the difference between do.call and lapply for > applying a function to a list. Below is one of the variations of > programs (by Marc Schwartz) discussed here recently to select the first > and last n observations per group. > > I've looked in several books, the R FAQ and searched the archives, but I > can't find enough to figure out why lapply doesn't do what do.call does > in this case. The help files & newsletter descriptions of do.call sound > like it would do the same thing, but I'm sure that's due to my lack of > understanding about their specific terminology. I would appreciate it if > you could take a moment to enlighten me. > > Thanks, > Bob > > mydata <- data.frame( > id = c('001','001','001','002','003','003'), > math = c(80,75,70,65,65,70), > reading = c(65,70,88,NA,90,NA) > ) > mydata > > mylast <- lapply( split(mydata,mydata$id), tail, n=1) > mylast > class(mylast) #It's a list, so lapply will so *something* with it. > > #This gets the desired result: > do.call("rbind", mylast) This is doing a single 'rbind' with the elements of the list as the parameters so you are effectively creating a single data frame from it. > > #This doesn't do the same thing, which confuses me: > lapply(mylast,rbind) This is applying 'rbind' separately to each element of the list (that is what lapply does - call the function with each element) and will return a list which is exactly the same. > > #...and data.frame won't fix it as I've seen it do in other > circumstances: > data.frame( lapply(mylast,rbind) ) What you are effectively doing is calling data.frame with as many parameters as you have elements of the list. See what happens with:
> data.frame(a=list(a=1,b=2), b=list(a=3,b=4)) a.a a.b b.a b.b 1 1 2 3 4 > > ========================================================= > Bob Muenchen (pronounced Min'-chen), Manager > Statistical Consulting Center > U of TN Office of Information Technology > 200 Stokely Management Center, Knoxville, TN 37996-0520 > Voice: (865) 974-5230 > FAX: (865) 974-4810 > Email: [EMAIL PROTECTED] > Web: http://oit.utk.edu/scc, > News: http://listserv.utk.edu/archives/statnews.html > > ______________________________________________ > R-help@stat.math.ethz.ch 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. > -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? ______________________________________________ R-help@stat.math.ethz.ch 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.