[R] grabbing from elements of a list without a loop

[William Dunlap]

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-bounces at r-project.org [mailto:r-help-bounces at 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.liakhovitski at 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 at 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.
> 
> 
> 
> --
> 
> Bert Gunter
> Genentech Nonclinical Biostatistics
> 
> Internal Contact Info:
> Phone: 467-7374
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> 
> ______________________________________________
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