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Select first n nonzero elements in each row of matrix

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giometar
giometar on 23 Jun 2020
Edited: giometar on 25 Jun 2020
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];

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Accepted Answer

Stephen Cobeldick
Stephen Cobeldick on 24 Jun 2020
Edited: Stephen Cobeldick on 24 Jun 2020
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

More Answers (1)

madhan ravi
madhan ravi on 23 Jun 2020
Edited: madhan ravi on 23 Jun 2020
ix = cumprod(A ~= 0, 2);
B = A(:, max(ix) ~= 0)
%OR
ix = cumprod(A ~= 0, 2); % remove cumprod(...) if you don't expect n consecutive nonzero elements
n = 3;
idx = find(cumsum(ix,2) == n, 1);
[~, c] = ind2sub(size(A), idx);
B = A(:, 1:c)
% OR
% if you don't want to specify n by yourself
ix = cumprod(A ~= 0, 2); % remove cumprod(...) if you don't expect n consecutive nonzero elements
ix1 = cumsum(ix,2);
idx = find(ix1 == max(max(ix1)), 1); % use max(..., [], 'all') for later versions
[~, c] = ind2sub(size(A), idx);
B = A(:, 1:c)

  3 Comments

giometar
giometar on 23 Jun 2020
Thanks for answer, but solution is not as expected. With your code I get B as:
B =
1 3 0
0 2 0
2 0 0
3 1 1
But B must be
B=[1 3 2
2 1 0
2 0 0
3 1 1];
madhan ravi
madhan ravi on 23 Jun 2020
n = 3; % n non-zero elements
B = cell(size(A,1),1);
for k = 1:size(A,1)
B{k} = nonzeros(A(k,:)).';
end
B = cellfun(@(x) x(1:numel(x) < n+1), B, 'un', 0);
B = cell2mat(cellfun(@(x) [x, zeros(1, n-numel(x))], B, 'un',0))
giometar
giometar on 25 Jun 2020
Thanks madhan ravi for help. Your solution is also warking but solution from
Stephen Cobeldick is better for me

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