vectorizing: selecting one record per group

Hello,

There are probably many ways to do this, but I think
it's easier if you use a data.frame as your object.

The easy solution for the matrix you provide is escaping
me at the moment.

One solution, using the plyr package:


library(plyr)
A <- data.frame(a = rnorm(100),b = runif(100), c = rep(c(1,2,3,4,5), 
20))
ddply(A, .(c), function(x) x[sample(1:nrow(x), 1), ])

           a         b c
1  0.02995847 0.4763819 1
2  0.72035194 0.2948611 2
3  1.34963917 0.2057488 3
4 -1.99427160 0.1147923 4
5 -0.73612703 0.5889539 5


Mauricio Romero wrote:

Hi,
I want to select a subsample from my data, choosing one record from  
each
group. I know how to do this with a for.
For example: lets say I have the data:
A=cbind(rnorm(100),runif(100),(rep(c(1,2,3,4,5),20)))
Where the third column is the group variable. Then what I want is  
to select
5 observations. Each one taken randomly from each group.
 INDEX =NULL
i=1
for(index_g in  unique(A[,3])){
INDEX [i]=sample(which(A[,3]==index_g),1)
i=i+1
}
SEL=A[INDEX,]
 Is there a way to do this without a ?for?? in other words is there  
a way to
?vectorize? this?
Thank you,
 Mauricio Romero Quantil S.A.S.
Bogot?,Colombia
www.quantil.com.co
"It is from the earth that we must find our substance; it is on the  
earth
that we must find solutions to the problems that promise to destroy  
all life
here"
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