[R] Creating 3 vectors that sum to 1

[Petr Savicky]

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.