[R] Creating 3 vectors that sum to 1

[Bert Gunter]

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