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

Bert Gunter gunter.berton at gene.com
Tue Feb 12 23:52:31 CET 2013

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

## 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]]
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Bert Gunter
Genentech Nonclinical Biostatistics

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