Cannot give the value to part of the matrix in parallel computing

Question

Archer Ao 2020-7-21

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https://ww2.mathworks.cn/matlabcentral/answers/568158-cannot-give-the-value-to-part-of-the-matrix-in-parallel-computing

编辑： Raymond Norris 2020-7-24

As I'm trying to wrtie a code for matrix multiplying with the 'parfor', I've got an error that 'cannot verify the type of variable C'. Could anyone tell me how to manipulate part of C'value ?

parfor r = 1:d
        i = mod(r-1,b)+1;
        j = floor(r-1/a)+1;
        s = min(i*subsize,m);   t = min(j*subsize,n);
        tmp = zeros(s-(i-1)*subsize,t-(j-1)*subsize);
        for k = 1:c
           q = min(k*subsize,l);
           tmp = tmp+A((i-1)*subsize+1:s,(k-1)*subsize+1:q)*B((k-1)*subsize+1:q,(j-1)*subsize+1:t);
        end
        C((i-1)*subsize+1:s,(j-1)*subsize+1:t) = tmp;
    end

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Answer 1

Raymond Norris 2020-7-24

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https://ww2.mathworks.cn/matlabcentral/answers/568158-cannot-give-the-value-to-part-of-the-matrix-in-parallel-computing#answer_470212

编辑：Raymond Norris 2020-7-24

在 MATLAB Online 中打开

Hi Archer,

The problem is that MATLAB doesn't know how to properly index into C. I would suggest a slight rewrite, using parfeval instead of parfor.

% Assumed a, b, c, d, l, m, n, subsize, A, B are all define above
% Using a nested function so that input arguements don't need to be passed to unitOfWork
p = gcp;
for r = 1:d
    f(r) = p.parfeval(@unitOfWork,5);
end
for r = 1:d
    [~,i,j,s,t,tmp] = f.fetchNext();
    C((i-1)*subsize+1:s,(j-1)*subsize+1:t) = tmp;
end
    function [i,j,s,t,tmp] = unitOfWork()
        i = mod(r-1,b)+1;
        j = floor(r-1/a)+1;
        s = min(i*subsize,m);   t = min(j*subsize,n);
        tmp = zeros(s-(i-1)*subsize,t-(j-1)*subsize);
        for k = 1:c
            q = min(k*subsize,l);
            tmp = tmp+A((i-1)*subsize+1:s,(k-1)*subsize+1:q)*B((k-1)*subsize+1:q,(j-1)*subsize+1:t);
        end
    end
end