MATLAB: How to find first nonzero element/first ‘1’ per row and set other elements to zero without loops in 3D Matrix

Question

I've a Matrix f.ex. like

A=[0,1,1,0; 1,0,0,1;0,0,0,1;0,1,1,1]; A =

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

only in 3D. So for example a Matrix like this only 100times (size(A)=4 x 4 x 100). Of course different shape/form of the '1s' and '0s'.

I want to as a result only the first '1' of each row and all other elements or further "1" to zero. The result should be:

B=[0,1,0,0;1,0,0,0;0,0,0,1;0,1,0,0];

B =

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

again, a hundred times. So the Result would be again size(B)= 4 x 4x 100.

If it was a 2D-matrix I'd use this: [Y,I] = max(A, [], 2); B = zeros(size(A)); B(sub2ind(size(A), 1:length(I), I')) = Y;

But that doesn't work for a 3Dim-Matrix…And I don't want to use for loop. Thanks!

Answer 1

s = size(A);
out = zeros(s);
i2 = s(2) - sum(cumsum(A,2)>0,2) + 1;
[ii,kk] = ndgrid(1:s(1),1:s(3))
idx = [ii(:),i2(:), kk(:)];
idx = num2cell(idx(idx(:,2) <= s(2),:),1);
out(sub2ind(s,idx{:})) = 1;

or

A2 = reshape(permute(A,[2 1 3]),s(2),[]);
s2 = size(A2);
[ii,jj] = find(A2);
[~,k] = unique(jj,'first');
ind = [ii,jj];
o1 = zeros(s2);
o1(sub2ind(s2,ind(k,1),ind(k,2))) = 1;
out = permute(reshape(o1,s(2),[],s(3)),[2 1 3]);