How to deal with nonzero elements from rows of sparse matrix in Matlab?

Question

I am dealing with quite a big sparse matrix, its size is about 150,000*150,000. I need to access into its rows, extract the non-zero elements and replace these values following the rule as the code below:

tic
H = [];
for i = 1: size(A,2)
    [a,b,c] = find(A(i,:)); % extract the rows
    add = diff([0 c(2:end) 0]); % the replacing rule
    aa = i*ones(1,size(a,2)); % return back the old position of rows
    G0 = [aa' b' add']; % put it back the old position with replaced values 
    H = [H; G0];    
end
H1 = H(:,1);
H2 = H(:,2);
H3 = H(:,3);
ADD = sparse(H1,H2,H3,nm,nm,nzmax);
toc

I found that the find function is really time consuming (0.1s/rows) in this code and with this current size of my sparse matrix, it takes me up to about 33 hours for this job. I do believe there is some ways out but I am such a newborn to coding and dealing with sparse matrix is really scary.

Would you drop me some ideas?

Answer 1

You can use the find function once applying it on the whole array then use accumarray to apply the function on each row:

[a b c]=find(A.');
add=accumarray(b,c,[],@(x){diff([0 ;x(2:end) ;0])});
H = [b a vertcat(add{:})];