R help - Mar 2008 - Generating a new matrix using rbinom and a matrix of probabilities.

I am having a little trouble getting R to do something without writing
a couple of very awkward loops.

I have a matrix of probabilities and I want to generate a new matrix
of ones and zeros where each element in the new matrix is the result
of a draw from a binomial distribution where the probability of
getting a 1 is the corresponding element in the matrix of
probabilities.

Example Code:
--------------------------------------------

## First I generate the matrix of probabilities for example purposes.

probMatrix <- matrix(NA,5,5){

for (i in 1:5)

probVectorI <- runif(5,0,1)

probMatrix[i,] <- probVectorI

}

# Now I want to take each element in probMatrix and use it as the
probability parameter in rbinom draw and generate a new matrix.
Something like this:

binomialMatrix <- rbinom(1,1,probMatrix)

# But that does not work. I know I can run a loop across each vector
of the matrix, but this seems like an bad way to do this.

---------------------------End Code--------------------------------

So any help would be appreciated.

Thanks,

EG

I think the following does what you want:

probMatrix <- matrix(runif(5 * 5), 5, 5)
binomialMatrix <- matrix(rbinom(5 * 5, 1, probMatrix), 5, 5)


Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Economics Guy wrote:
>I am having a little trouble getting R to do something without writing
>a couple of very awkward loops.
>
>I have a matrix of probabilities and I want to generate a new matrix
>of ones and zeros where each element in the new matrix is the result
>of a draw from a binomial distribution where the probability of
>getting a 1 is the corresponding element in the matrix of
>probabilities.
>
>Example Code:
>--------------------------------------------
>
>## First I generate the matrix of probabilities for example purposes.
>
>probMatrix <- matrix(NA,5,5){
>
>for (i in 1:5)
>
>probVectorI <- runif(5,0,1)
>
>probMatrix[i,] <- probVectorI
>
>}
>
># Now I want to take each element in probMatrix and use it as the
>probability parameter in rbinom draw and generate a new matrix.
>Something like this:
>
>binomialMatrix <- rbinom(1,1,probMatrix)
>
># But that does not work. I know I can run a loop across each vector
>of the matrix, but this seems like an bad way to do this.
>
>---------------------------End Code--------------------------------
>
>So any help would be appreciated.
>
>Thanks,
>
>EG
>
>______________________________________________
>R-help at r-project.org 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.
>
>
>  
>

I tried your code and could not get it to run on my installation of R,
so I may be missing something.  But if you have a matrix of
probabilities (call it probs) and want to simulate random binomial
draws, can you not simply create a matrix of the same size of uniform
random numbers (runif()) (call it rands), then do comparisons, thus,

draws <- 0 + (rands < probs)

Ben Fairbank


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Economics Guy
Sent: Tuesday, March 11, 2008 10:16 AM
To: r-help at stat.math.ethz.ch
Subject: [PS] [R] Generating a new matrix using rbinom and a matrix of
probabilities.

I am having a little trouble getting R to do something without writing
a couple of very awkward loops.

I have a matrix of probabilities and I want to generate a new matrix
of ones and zeros where each element in the new matrix is the result
of a draw from a binomial distribution where the probability of
getting a 1 is the corresponding element in the matrix of
probabilities.

Example Code:
--------------------------------------------

## First I generate the matrix of probabilities for example purposes.

probMatrix <- matrix(NA,5,5){

for (i in 1:5)

probVectorI <- runif(5,0,1)

probMatrix[i,] <- probVectorI

}

# Now I want to take each element in probMatrix and use it as the
probability parameter in rbinom draw and generate a new matrix.
Something like this:

binomialMatrix <- rbinom(1,1,probMatrix)

# But that does not work. I know I can run a loop across each vector
of the matrix, but this seems like an bad way to do this.

---------------------------End Code--------------------------------

So any help would be appreciated.

Thanks,

EG

______________________________________________
R-help at r-project.org 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.