One thing you might consider when working with large dataframes is that
instead of partitioning the dataframe into smaller ones, create a list of
indices and use that to access the subset.  Works especially well when using
'lapply' to cromp through many segments of a data frame:

> y
      suid month esr
1  1074034    12   6
2  1074034     1   2
3  1074034     2   2
4  1074034     3   2
5  1074034    12   1
6  1074034     1   1
7  1074034     2   1
8  1074034     3   1
9  1074034    12   2
10 1074034     1   2
11 1074034     2   2
12 1074034     3   2
13 1074034    12   9
14 1074034     1   9
15 1074034     2   9
16 1074034     3   9
17 1123003    12   2
18 1123003     1   2
19 1123003     2   2
20 1123003     3   2
> y.ind <- split(seq(nrow(y)), y$month)
> y.ind
$`1`
[1]  2  6 10 14 18
$`2`
[1]  3  7 11 15 19
$`3`
[1]  4  8 12 16 20
$`12`
[1]  1  5  9 13 17
> # a subset
> y[y.ind[['12']],]
      suid month esr
1  1074034    12   6
5  1074034    12   1
9  1074034    12   2
13 1074034    12   9
17 1123003    12   2
>


On Wed, Jun 24, 2009 at 11:34 PM, Stephan Lindner <lindners@umich.edu>wrote:

> Dear all,
>
>
> I have a code where I subset a data frame to match entries within
> levels of an factor (actually, the full script uses three difference
> factors do do that). I'm very happy with the precision with which I can
> work with R, but since I loop over factor levels, and the data frame is
> big, the process is slow. So I've been trying to speed up the process
> using by(), but I got stuck at the point where I want to stack back
> the sub- data frames, and I was wondering whether someone could help me
> out.
>
> Here is an example:
>
> <--
>
> > y <- data.frame(suid  = c(rep(1074034,16),rep(1123003,4)),
>                 month = rep(c(12,1,2,3),5),
>                 esr   = c(6,2,2,2,1,1,1,1,2,2,2,2,9,9,9,9,2,2,2,2))
>
>
> > by(y,y$month,function(x)return(x))
>
> y$month: 1
>      suid month esr
> 2  1074034     1   2
> 6  1074034     1   1
> 10 1074034     1   2
> 14 1074034     1   9
> 18 1123003     1   2
> ------------------------------------------------------------
> y$month: 2
>      suid month esr
> 3  1074034     2   2
> 7  1074034     2   1
> 11 1074034     2   2
> 15 1074034     2   9
> 19 1123003     2   2
> ------------------------------------------------------------
> y$month: 3
>      suid month esr
> 4  1074034     3   2
> 8  1074034     3   1
> 12 1074034     3   2
> 16 1074034     3   9
> 20 1123003     3   2
> ------------------------------------------------------------
> y$month: 12
>      suid month esr
> 1  1074034    12   6
> 5  1074034    12   1
> 9  1074034    12   2
> 13 1074034    12   9
> 17 1123003    12   2
>
> -->
>
> What I would like to do is stacking these four data frames back to one
> data frame, which in this simple example would just be y. I tried
> unlist(), unclass() and rbind(), but none of them would work.
>
>
> Thanks a lot,
>
>
>
>        Stephan
>
>
>
>
>
>
>
>
>
>
> --
> -----------------------
> Stephan Lindner
> University of Michigan
>
> ______________________________________________
> 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.
>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

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