[R] About populating a dataframe in a loop

[jeremiah rounds]

As a rule never rbind in a loop. It has O(n^2) run time because the rbind
itself can be O(n) (where n is the number of data.frames).  Instead either
put them all into a list with lapply or vector("list", length=) and then
datatable::rbindlist, do.call(rbind, thelist) or use the equivalent from
dplyr.  All of which will be much more efficient.



On Fri, Jan 6, 2017 at 8:46 PM, lily li <chocold12 at gmail.com> wrote:

> Hi Rui,
>
> Thanks for your reply. Yes, when I tried to rbind two dataframes, it works.
> However, if there are more than 50, it got stuck for hours. When I tried to
> terminate the process and open the csv file separately, it has only one
> data frame. What is the problem? Thanks.
>
>
> On Fri, Jan 6, 2017 at 11:12 AM, Rui Barradas <ruipbarradas at sapo.pt>
> wrote:
>
> > Hello,
> >
> > Works with me:
> >
> > set.seed(6574)
> >
> > pre.mat = data.frame()
> > for(i in 1:10){
> >     mat.temp = data.frame(x = rnorm(5), A = sample(LETTERS, 5, TRUE))
> >     pre.mat = rbind(pre.mat, mat.temp)
> > }
> >
> > nrow(pre.mat)  # should be 50
> >
> >
> > Can you give us an example that doesn't work?
> >
> > Rui Barradas
> >
> >
> > Em 06-01-2017 18:00, lily li escreveu:
> >
> >> Hi R users,
> >>
> >> I have a question about filling a dataframe in R using a for loop.
> >>
> >> I created an empty dataframe first and then filled it, using the code:
> >> pre.mat = data.frame()
> >> for(i in 1:10){
> >>      mat.temp = data.frame(some values filled in)
> >>      pre.mat = rbind(pre.mat, mat.temp)
> >> }
> >> However, the resulted dataframe has not all the rows that I desired for.
> >> What is the problem and how to solve it? Thanks.
> >>
> >>         [[alternative HTML version deleted]]
> >>
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> >> ng-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>

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