From: Bert Gunter <gunter.berton_at_gene.com>

Date: Tue, 29 Mar 2011 10:07:25 -0700

Date: Tue, 29 Mar 2011 10:07:25 -0700

So you want to generate random multinomials ? ... Just generate random uniforms and divide by their sum!

z <- runif(3); z/sum(z)

## This can easily be modified to generate lots of random multinomial 3 vectors, e.g.

z <- matrix( runif(3*100), ncol=3)

z/rowSums(z) ## each row is a random multinomial

- Bert

On Tue, Mar 29, 2011 at 9:34 AM, Petr Savicky <savicky_at_praha1.ff.cuni.cz> wrote:

> On Tue, Mar 29, 2011 at 11:20:13AM -0500, Christopher Desjardins wrote:

*>> I have 3 vectors: p1, p2, and p3. I would like each vector to be any
**>> possible value between 0 and 1 and p1 + p2 + p3 = 1. I want to graph these
**>> and I've thought about using scatterplot3d(). Here's what I have so far.
**>>
**>> library(scatterplot3d)
**>> p1 <- c(1,0,0,.5,.5,0,.5,.25,.25,.34,.33,.33,.8,.1,.1,.9,.05,.05)
**>> p2 <- c(0,1,0,.5,0,.5,.25,.5,.25,.33,.34,.33,.1,.8,.1,.05,.9,.05)
**>> p3 <- c(0,0,1,0,.5,.5,.25,.25,.5,.33,.33,.34,.1,.1,.8,.05,.05,.9)
**>> scatterplot3d(p1,p2,p3)
**>>
**>>
**>> However, I wonder if there is an easy way to create vectors p1, p2, and p3.
**>
**> Hi.
**>
**> The vectors p1, p2 and p3 are not uniquely determined. Try, for example,
**> the following
**>
**> n <- 16
**> pp <- expand.grid(p1=0:n, p2=0:n, p3=0:n)
**> pp <- subset(pp, p1 + p2 + p3 == n)
**> p1 <- pp$p1/n
**> p2 <- pp$p2/n
**> p3 <- pp$p3/n
**>
**> If n is a power of 2, then p1 + p2 + p3 will be exactly all ones vector.
**> Otherwise, there may be differences within machine rounding error.
**>
**> Hope this helps.
**>
**> Petr Savicky.
**>
**> ______________________________________________
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**>
*

-- Bert Gunter Genentech Nonclinical Biostatistics 467-7374 http://devo.gene.com/groups/devo/depts/ncb/home.shtml ______________________________________________ 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.Received on Tue 29 Mar 2011 - 17:09:18 GMT

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