create array

Hi considering I want to make a a 3x6624 matrix of random numbers (>0 and <1) but I want the sum of each column of the3 numbers to add up to 1. Any help?

Thanks

Answer 1

[Paulo Silva]

a=zeros(3,6624);
for b=1:size(a,2)
c(1)=rand/3;
c(2)=rand/3;
c(3)=1-c(1)-c(2);
cr=randperm(3);
a(:,b)=[c(cr(1));c(cr(2));c(cr(3))];
end

[Mate 2u]

Is there anyway to do it faster without a loop?

Answer 2

[Mate 2u]

Is there a way to do this without a loop? So it could perform faster?

Answer 3

[Sean de Wolski]

A = rand(3,6624);
A = bsxfun(@rdivide,A,sum(A,1));

Also look at Roger's randfixedsum on the FEX.

Answer 4

[Teja Muppirala]

Paulo's algorithm without a loop:

a=zeros(3,6624);
a(1:2,:)=rand(2,6624)/3;
a(3,:)=1-sum(a);

P = randperm(6624*3);
[~,ord] = sort(ceil(P/3));
a(:) = a(P(ord));
figure
hist(a',.05:.1:.95);

Another possible algorithm:

a=zeros(3,6624);
a(1,:) = rand(1,6624);
a(2,:) = (1-sum(a)).*rand(1,6624);
a(3,:) = (1-sum(a)).*rand(1,6624);

a = bsxfun(@rdivide,a,sum(a));

P = randperm(6624*3);
[~,ord] = sort(ceil(P/3));
a(:) = a(P(ord));

figure
hist(a',.05:.1:.95);