How to replace the elements in a big sparse matrix?

问题

I have quite a big sparse matrix, 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 as the code below:

H = [];
for i = 1: size(A,2)
    [a,b,c] = find(A(i,:)); % extract the rows
    if size(c,2)==1  % only 2
        add = 0;
    elseif size(c,2) > 1 && any(c<2)== 0 % many 2s
        add = c;
        add(1)   = -2;
        add(end) = 2;
        add(2:end-1) = 0;
    elseif size(c,2) > 1 && any(c<2)~= 0 % 2 and 1
        k = find(diff(c)==-1); % find right 2 position
        add = c;
        add(1)   = -1;
        add(end) = -1;
        add(k) = 2;
        add(2:k-1) = 0;
        add(k+1:end-1) = 0;
    end
    %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

The rule is

Keep the element 2 on the very right in the row
The first and last elements in the row are replaced by -1
All other elements become 0.
In case there is just only one element 2, it will be replaced by 0.
In case there are two or more 2s, the very right one will remain and the very left one will be replaced by -2, the middle elementsare replaced by 0.

and here is the figure that visualises these rules

The left figure is non-zero elements of rows extracted from the sparse matrix and I want the output to be like the right figure. I just selected some examples to explain the rules.

I did ask a similar question here and got a wonderful answer. The rule was differences between 2 adjacent elements in the row so that it could be integrated into the accumarray function. But in this case, the rule is a bit different and I don't know how to perform this task quickly. The sparse matrix is quite big so the for loop is a terrible thing to do here.

Thank you very much.

来源：https://stackoverflow.com/questions/45813822/how-to-replace-the-elements-in-a-big-sparse-matrix

标签

matlab

replace

sparse-matrix