From data frame to list object

Thanks David for this reply. However if my data frame has only 2  
columns
then it is working fine. It is not working for a general setting:

dfrm <- data.frame(x=rnorm(18), y=rep(c("a", "b", "c"), each=6),
z=rep(c("x", "y", "z"), each=2))
tapply(dfrm[,1], dfrm$y, c) # this is working fine

tapply(dfrm[,c(1,3)], dfrm$y, c)  # this is giving error!

Error in tapply(dfrm[, c(1, 3)], dfrm$y, c) :
 arguments must have same length

Can you please help me how to modify that?

Thanks,

-----Original Message-----
From: David Winsemius [mailto:dwinsemius at comcast.net]
Sent: 31 January 2011 23:26
To: Bogaso Christofer
Cc: r-help at r-project.org
Subject: Re: [R] From data frame to list object


On Jan 31, 2011, at 1:03 PM, Bogaso Christofer wrote:

Dear all, let say I have following data frame:

dfrm <- data.frame(x=rnorm(18), y=rep(c("a", "b", "c"), each=6))  >

tapply(dfrm$x, dfrm$y, c) $a [1]  0.9711995  1.4018345 -1.4355713  
-0.5106138
-0.8470171 [6]  1.1634586

$b
[1] -0.8058164  0.4977112  1.1556391  0.8158588  0.2549273 [6]   
3.0758260

$c
[1]  0.437345128 -0.415874363  0.003230285 -0.737117910 [5]   
1.247972964
0.903001077

data.frame(x=rnorm(18), y=rep(c("a", "b", "c"), each=6))

            x y

1  -1.072152537 a

2   0.382985265 a

3   0.058877377 a

4  -0.006911939 a

5  -2.355269051 a

6  -0.303095553 a

7   0.484038422 b

8   0.733928931 b

9  -1.136014346 b

10  0.503552090 b

11  1.708609658 b

12 -0.294599403 b

13  1.239308497 c

14  0.754081946 c

15 -0.237346858 c

16 -0.051011439 c

17 -0.618675146 c

18  0.537612359 c

From this data frame I want to create a "list" of length 3, where
each

element of this list will be a vector corresponding to the value of  
y.
For example, 1st element will be all "x" values corresponding to the
"y=a", and similarly the other elements of this list. Can somebody
point me how to do this without having some "for" loop?



Thanks and regards,


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David Winsemius, MD
West Hartford, CT