MATLAB: Select matrices with nonzero rows from a bigger matrix

matrixesnonzero elements

Hello guys!
I am struggling with this problem:
I have a big matrix that is made of few zeros raws and many nonzero raws.
This is a fake simple example to explain, because in reality I am working with big data :(:
B= [0 0 0 0;
    0 0 0 0;
    0 0 0 0;
    0 0 0 0 ;
    1 1 2 3;
    0 0 0 3;
    1 1 1 0;
    4 4 4 4;
    0 0 0 0 ;
    0 0 0 0;
    0 0 0 0 ;
    0 0 0 8;
    0 0 1 0;
    0 1 1 0;
    0 0 0 0;
    0 0 0 0 ];
The zero rows in this matrix, indicate that the signal stopped and I just need to analyse the non zero raws in chunks.
Therefore would like to obtain:
chunk_1=[1 1 2 3;0 0 0 3;1 1 1 0;4 4 4 4]
chunk_2=[0 0 0 8;0 0 1 0;0 1 1 0;]
As you can see the chunks don't have to have the same amount of raws, since I analyse them individually, and the zero chunks also don't have the same number of rows.
I hope you guys have more knowledge than ve and could help me!
Thanks in advance !

Best Answer

This nice little function from the image processing toolbox (bwconncomp) will be very useful here.
B = [0 0 0 0;0 0 0 0;0 0 0 0;0 0 0 0 ;1 1 2 3;0 0 0 3;1 1 1 0;4 4 4 4; 0 0 0 0 ;0 0 0 0;0 0 0 0 ;0 0 0 8;0 0 1 0;0 1 1 0;0 0 0 0;0 0 0 0 ];
mask = sum(B, 2)~=0;
comps = bwconncomp(mask);
compMaskList = comps.PixelIdxList;
B_components = cell(numel(compMaskList), 1);
for i=1:numel(compMaskList)
    B_components{i} = B(compMaskList{i}, :);
end
This shorter version is equivalent to the above code
comps = bwconncomp(sum(B, 2)~=0);
B_comps = cellfun(@(x) {B(x, :)}, comps.PixelIdxList);
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