Random numbers that add to 100: Matlab

Question

[I\'m splitting a population number into different matrices and want to test my code using random numbers for now.]

Quick question guys and thanks for your help in a

Answer 1

It is not too late to give the right answer

Let's talk about sampling X1...XN in the range [0...1] such that Sum(X1, ..., XN) is equal to 1. Then you could rescale it to 100

This is called Dirichlet distribution, and below is the code to sample from it. Simplest case is when all parameters are equal to 1, then all marginal distributions for X1, ..., XN would be U(0,1). In general case, with parameters different from 1s, marginal distributions might have peaks.

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The Dirichlet is a vector of unit-scale gamma random variables, normalized by their sum. So, with no error checking, this will get you that:

a = [1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0]; // 9 numbers to sample
n = 10000;
r = drchrnd(a,n)

function r = drchrnd(a,n)
  p = length(a);
  r = gamrnd(repmat(a,n,1),1,n,p);
  r = r ./ repmat(sum(r,2),1,p);