Keeping k largest values in each column of a sparse matrix

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Wolfgang Schwanghart 2011-7-3

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Hi everyone,

I am trying to implement an algorithm that involves a pruning of a large sparse matrix. The pruning scheme should keep the k largest values (all nonzero values are positive) in each column of the sparse matrix M.

My current solution is:

function Mp = pruning(M,k)
[i,j,v] = find(M);
t = sortrows([i j v],[2 -3]);
dzero = cumsum([0;diff(t(:,2))]==0);
dz = [1;diff(t(:,2))]>0;
dd = dzero.*dz;
for r = 2:numel(dd); 
    dd(r) = max(dd([r-1 r])); 
end;
I = (dzero-dd+1) <= k;
Mp = sparse(t(I,1),t(I,2),t(I,3),size(M,1),size(M,2));

Do you have better solutions (that avoid the loop)?

Thanks, W.

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Wolfgang Schwanghart 2011-7-19

since nobody answered, I'll just give it another try...

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采纳的回答

Andrei Bobrov 2011-7-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/10784-keeping-k-largest-values-in-each-column-of-a-sparse-matrix#answer_16286

在 MATLAB Online 中打开

k = triu(bsxfun(@minus,dd,dd'));
pl = sum(k<0)+1==1:numel(dd);
m = dd(pl);
dd = m(cumsum(pl));

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Sean de Wolski 2011-7-19

That is nice Andrei!

Andrei Bobrov 2011-7-19

Thanks, friends!

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MATLAB Mathematics Sparse Matrices

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