> -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > On Behalf Of Dennis Murphy > Sent: Thursday, July 21, 2011 8:55 PM > To: karena > Cc: r-help@r-project.org > Subject: Re: [R] Random number generation > > Hi: > > Here's one way using uniform(0, 1) pseudo-random numbers, but there > are many ways you could go about this. > > # each row comprises a set of three pseudo-random numbers > u <- matrix(runif(30), nrow = 10) > # divide each element in a row by its row sum > v <- t(apply(u, 1, function(x) x/sum(x))) > rowSums(v) > [1] 1 1 1 1 1 1 1 1 1 1 > > # An equivalent way (about equally fast) is > u/outer(rowSums(u), rep(1, 3)) > > Now try > > hist(unlist(v)) > > and notice that the distribution of the constrained sets is not really > uniform. This is a consequence of setting a constraint on the sum of > each sample. Another way to see this is to plot > > plot(sort(unlist(v))) > > A 'truly' uniform random sample would lie approximately on a straight > line in this plot. > > It would seem to me that a better approach would be to sample from a > simplex embedded in the unit cube. I'd suggest looking into the > compositions package (because you are effectively generating > compositional data) and look into its capabilities. At least a couple > of the references in the package's overview page seem to be germane to > the problem. > The pair of runif.* functions appear to be relevant. > > HTH, > Dennis > > > > On Thu, Jul 21, 2011 at 4:18 PM, karena <dr.jz...@gmail.com> wrote: > > Hi, > > > > I want to generate multiple sets of random numbers. > > The requirement is that: > > 1) each set have 3 random numbers; > > 2) the sum of the three number is always 1. > > > > how to do this? > > > > thank you, > > > > karena > >
Karena, if you search the R-help archive you will find a couple of threads discussing this topic. Here is one URL to get you started. http://tolstoy.newcastle.edu.au/R/e2/help/06/10/2520.html Hope this is helpful, Dan Daniel Nordlund Bothell, WA USA ______________________________________________ R-help@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.
