MATLAB: Select first n nonzero elements in each row of matrix

\nmatrixnonzero

Hello
How to select first n nonzero elements from each row?
If I have matrix A, and I want result like B (three nonzero elements from each row):
Rusult also can be cell..
A=[1 3 0 2 5
    0 2 0 1 0
    2 0 0 0 0
    3 1 1 0 0];
B=[1 3 2
    2 1 0
    2 0 0
    3 1 1];

Best Answer

Linear indexing does this simply and efficiently. The trick is to work down the columns, which requires transposing:
>> A = [1,3,0,2,5;0,2,0,1,0;2,0,0,0,0;3,1,1,0,0]
A =
   1   3   0   2   5
   0   2   0   1   0
   2   0   0   0   0
   3   1   1   0   0
>> N = 3;
>> Z = A.';
>> S = size(Z);
>> [~,R] = sort(Z==0,1);
>> [~,C] = ndgrid(1:N,1:S(2));
>> X = sub2ind(S,R(1:N,:),C);
>> B = Z(X).'
B =
   1   3   2
   2   1   0
   2   0   0
   3   1   1
Probably the most efficient approach would be to use a simple loop, e.g. (not particularly optimized):
R = size(A,1);
B = zeros(R,N);
for k = 1:R
    tmp = nonzeros(A(k,:));
    idx = 1:min(N,numel(tmp));
    B(k,idx) = tmp(idx);
end
Some timings (1e3 iterations):
Elapsed time is 5.358 seconds. % madhan ravi's with loop and CELLFUN
Elapsed time is 0.606 seconds. % my answer with SUB2IND
Elapsed time is 0.265 seconds. % my answer with loop and indexing
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