Randomly choose one nonzero element in each row of matrix

Hello,

I have a 3D-matrix filled with ones and zeros. For example:

ex(:,:,1) =

     0     1     0     1
     0     0     0     1
     1     0     0     1


ex(:,:,2) =

     0     1     0     0
     1     1     1     1
     1     0     0     1


ex(:,:,3) =

     0     1     1     0
     0     1     1     0
     1     0     1     0

In each row I need to randomly select one nonzero element and get its index. Best would be to get a 3 x 1 x 3 matrix with the (column) indexes of the randomly chosen element for each row.

I think I know how to do it with loops, but I want to avoid them since the matrix will be quite big.

Any ideas on how to solve this with no loops?

Thank you in advance for your answers!

Answer 1

[David Hill]

Need a single loop in third dimension.

ex=randi(2,10,17,12)-1;
E=permute(ex,[2,1,3]);
[r,c]=find(E);
N=zeros(size(ex,1),1,size(ex,3));
for k=1:size(ex,3)*size(ex,1)
    f=find(c==k);
    try%in case some rows don't have any 1's
        N(k)=r(f(randperm(length(f),1)));
    end
end

[Peter Uwsen]

Thanks a lot!! Works great :)