On Oct 13, 2010, at 4:17 PM, Erik Iverson wrote:
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.
Perhaps using sampling to derive an index?
A[ tapply(1:100, A[,3], sample, 1), ]
> A[tapply(1:100, A[,3], sample, 1), ]
[,1] [,2] [,3]
[1,] -1.9512142 0.9823905 1
[2,] 1.4983879 0.4961661 2
[3,] 0.7815468 0.3531835 3
[4,] -0.9210731 0.6508500 4
[5,] 0.2354838 0.8616220 5
--
David.
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.
Bogotá,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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David Winsemius, MD
West Hartford, CT
______________________________________________
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