Parfor and symmetric matrix

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Maria
Maria 2021-8-12
编辑： Matt J 2021-8-13
Hi,
I need to fill in a matrix. Since it is symmetrical, the two for loops look like this
for i = 1 : m
for j = i : n
operation on the matrix ....
end
end
The other elements of the matrix are filled in by using "permute" (it is a 3D matrix).
I am now modifying the code to make it parallelizable. I tried with the parfor like this
parfor i = 1 : m
for j = i : n
operation on the matrix ....
end
end
At running time, I get the error because matlab does not know the value of "i". I guess this is because the two "for" are not independent.
My question is:
  • is there a way to fill in a symmetric matrix by exploiting the symmetry, and using the parfor? Or shall I ignore the symmetry, have the second "for" starting from 1, and getting rid of "permute" ?

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回答（1 个）

Matt J
Matt J 2021-8-12
编辑：Matt J 2021-8-12
[I,J]=ndgrid(1:m,1:n);
keep=J>=I;
I=I(keep);
J=J(keep);
parfor k=1:numel(I)
i=I(k);
j=J(k);
operation on the matrix ....
end
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Matt J
Matt J 2021-8-13
编辑：Matt J 2021-8-13
Here's one possible fix.
temp = zeros(m,n);
H=0;
parfor k=1:numel(I)
dH=temp;
i=I(k);
j=J(k);
dH(i,j) = 3;
H=H+dH;
end
Matt J
Matt J 2021-8-13
编辑：Matt J 2021-8-13
Another possible fix (probably better):
temp = zeros(size(I));
H=0;
parfor k=1:numel(I)
temp(k)=3;
end
idx=sub2ind([m,n],I,J);
H(idx)=temp(:);

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