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Hi, All,

I have a big sparse matrix A. I want to find out the first non-zero element in all columns from the top. Here is my code:

reorderCol = [];

for jCol = 1 : length(A(1,:))

eee = find(A(:,jCol),1,'first');

reorderCol = [reorderCol eee];

end

For example, I have matrix A = [0 0 0;0 5 0;0 0 1;0 0 0;-1 0 -4]. My code gives the following result:

reorderCol = [5 2 3];

I am wondering if it is possible to obtain reorderCol without iterations. Thanks a lot.

Benson

Les Beckham
on 18 Mar 2020

Edited: Les Beckham
on 18 Mar 2020

Try this. In my test with a 1000x1000 random sparse 0 or 1 matrix (A = sparse(randi([0 1], 1000, 1000));) it is about 4 times faster.

i = find(A(:) ~= 0, 1, 'first');

ij = ind2sub(size(A), i);

For a 5000x5000 A it is actually slower, however (about 83% as fast).

Note that I am using Octave as I don't currently have access to Matlab.

Iteration is not always necessary, or desirable, to avoid. In fact, many of the Matlab 'tricks' are just iteration in disguise.

I hope this helps.

Perhaps your results will be different using Matlab vs. Octave. Let me know.

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