Re: [R] Sampling from a Matrix
Once again, thanks for your help.
I did not state the problem correctly, though the code is correct for
what I want to do.
A better description of the problem would be that there is a matrix of
probabilities:
set.seed(1)
probs - array(abs(rnorm(25, sd = 0.33)), dim = c(5,5), dimnames = list(1:5,
letters[1:5]))
probs
a b c de
1 0.21 0.27 0.50 0.0148 0.303
2 0.06 0.16 0.13 0.0053 0.258
3 0.28 0.24 0.21 0.3115 0.025
4 0.53 0.19 0.73 0.2710 0.656
5 0.11 0.10 0.37 0.1960 0.205
The column names, dimnames(probs)[[2]], are the names of units to be
sampled.Each row is a trial. For each row (trial), I want to
sample 3 of the units such that for each row I get a vector like the
following:
[1] a, b, a
The samples are to be done with replacement, and these vectors could
be combined to form a matrix of the samples.
The purpose of the probs matrix is to give each unit a probability
that it will be sampled.
One way to do this follows:
index - 1:ncol(probs)
res - matrix(0,
nrow = dim(probs)[1],
ncol = 3
)
for(i in 1:nrow(probs)){
## gets the indexes of the values chosen
res[, i] - sample(index, size = 3, replace = TRUE, prob = probs[i, ])
}
Using apply as Andy described would accomplish the intended result.
-- Dan
Hmm... If I read Daniel's code (which is different from his description)
correctly, that doesn't seem to be what he wanted. Perhaps something like
this:
apply(probs, 1, function(p) sample(1:ncol(probs), 3, replace=TRUE, prob=p))
Andy
Andy,
You are of course correct. I had focused on the description of the
problem, rather than the code provided, presuming that the code was not
correct, including the use of 'replace' and 'prob' in sample().
I suppose it would be up to Daniel for clarification.
Regards,
Marc
--
Daniel Gerlanc
Williams College '07
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Re: [R] Sampling from a Matrix
On Tue, 2006-08-08 at 14:10 -0400, Liaw, Andy wrote:
From: Marc Schwartz
On Fri, 2006-08-04 at 12:46 -0400, Daniel Gerlanc wrote:
Hello all,
Consider the following problem:
There is a matrix of probabilities:
set.seed(1)
probs - array(abs(rnorm(25, sd = 0.33)), dim = c(5,5),
dimnames =
list(1:5, letters[1:5])) probs
a b c de
1 0.21 0.27 0.50 0.0148 0.303
2 0.06 0.16 0.13 0.0053 0.258
3 0.28 0.24 0.21 0.3115 0.025
4 0.53 0.19 0.73 0.2710 0.656
5 0.11 0.10 0.37 0.1960 0.205
I want to sample 3 values from each row.
One way to do this follows:
index - 1:ncol(probs)
for(i in 1:nrow(probs)){
## gets the indexes of the values chosen
sample(index, size = 3, replace = TRUE, prob = probs[i, ])
}
Is there a another way to do this?
Thanks!
t(apply(probs, 1, function(x) sample(x, 3)))
[,1] [,2] [,3]
1 0.210 0.5000 0.0148
2 0.258 0.0053 0.1300
3 0.025 0.2800 0.3115
4 0.190 0.5300 0.2710
5 0.196 0.1000 0.1100
Hmm... If I read Daniel's code (which is different from his description)
correctly, that doesn't seem to be what he wanted. Perhaps something like
this:
apply(probs, 1, function(p) sample(1:ncol(probs), 3, replace=TRUE, prob=p))
Andy
Andy,
You are of course correct. I had focused on the description of the
problem, rather than the code provided, presuming that the code was not
correct, including the use of 'replace' and 'prob' in sample().
I suppose it would be up to Daniel for clarification.
Regards,
Marc
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
Re: [R] Sampling from a Matrix
From: Marc Schwartz
On Fri, 2006-08-04 at 12:46 -0400, Daniel Gerlanc wrote:
Hello all,
Consider the following problem:
There is a matrix of probabilities:
set.seed(1)
probs - array(abs(rnorm(25, sd = 0.33)), dim = c(5,5),
dimnames =
list(1:5, letters[1:5])) probs
a b c de
1 0.21 0.27 0.50 0.0148 0.303
2 0.06 0.16 0.13 0.0053 0.258
3 0.28 0.24 0.21 0.3115 0.025
4 0.53 0.19 0.73 0.2710 0.656
5 0.11 0.10 0.37 0.1960 0.205
I want to sample 3 values from each row.
One way to do this follows:
index - 1:ncol(probs)
for(i in 1:nrow(probs)){
## gets the indexes of the values chosen
sample(index, size = 3, replace = TRUE, prob = probs[i, ])
}
Is there a another way to do this?
Thanks!
t(apply(probs, 1, function(x) sample(x, 3)))
[,1] [,2] [,3]
1 0.210 0.5000 0.0148
2 0.258 0.0053 0.1300
3 0.025 0.2800 0.3115
4 0.190 0.5300 0.2710
5 0.196 0.1000 0.1100
Hmm... If I read Daniel's code (which is different from his description)
correctly, that doesn't seem to be what he wanted. Perhaps something like
this:
apply(probs, 1, function(p) sample(1:ncol(probs), 3, replace=TRUE, prob=p))
Andy
See ?apply and ?t
HTH,
Marc Schwartz
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
Re: [R] Sampling from a Matrix
On Fri, 2006-08-04 at 12:46 -0400, Daniel Gerlanc wrote:
Hello all,
Consider the following problem:
There is a matrix of probabilities:
set.seed(1)
probs - array(abs(rnorm(25, sd = 0.33)), dim = c(5,5), dimnames =
list(1:5, letters[1:5]))
probs
a b c de
1 0.21 0.27 0.50 0.0148 0.303
2 0.06 0.16 0.13 0.0053 0.258
3 0.28 0.24 0.21 0.3115 0.025
4 0.53 0.19 0.73 0.2710 0.656
5 0.11 0.10 0.37 0.1960 0.205
I want to sample 3 values from each row.
One way to do this follows:
index - 1:ncol(probs)
for(i in 1:nrow(probs)){
## gets the indexes of the values chosen
sample(index, size = 3, replace = TRUE, prob = probs[i, ])
}
Is there a another way to do this?
Thanks!
t(apply(probs, 1, function(x) sample(x, 3)))
[,1] [,2] [,3]
1 0.210 0.5000 0.0148
2 0.258 0.0053 0.1300
3 0.025 0.2800 0.3115
4 0.190 0.5300 0.2710
5 0.196 0.1000 0.1100
See ?apply and ?t
HTH,
Marc Schwartz
__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
