[R] Using by() and stacking back sub-data frames to one data frame

[Stephan Lindner]

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