parallel loop in matlab

Question

Abhishek Saini 2020-7-26

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https://ww2.mathworks.cn/matlabcentral/answers/570640-parallel-loop-in-matlab

回答： Edric Ellis 2020-7-28

Hi ,

I am new to use parfor in MATLAB. I am trying to convert my for loop in to parfor, but getting problem in varaible definition. Here is the a section of the code where I want to implement parfor. Kindly advice how to run this correctly.

%% matrices initialisation
KB=zeros(sdof,sdof);
KS=zeros(sdof,sdof);
KK=zeros(sdof,sdof);
FF=zeros(sdof,1);
KG=zeros(sdof,sdof);
[DB,DS]=material_mat(E,nu,G,shcof);
parfor i=1:nelem
  
    elecon=elemconn(i,:);
    nodes =cor(elecon,1:2);
    index1=nodedof(elecon,:).';
    index=reshape(index1,1,size(index1,1)*size(index1,2));
    
    [KB_final]=elementstiffness_bending(nodes,DB);
    [KS_final,F_final]=elementstiffness_shear(nodes,DS,P);
    [KG_final]=geometricstiff(sigma_i,nodes);
 
    KK1=KB_final+KS_final;
    
    [KK,FF,KG] = paral(KK1,F_final,KG_final,index,KK,FF,KG,i);
    
    KB(index,index) = KB(index,index) + KB_final;
end

I am getting problem in defining Kb variable. How to rectify this?

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Answer 1

Edric Ellis 2020-7-28

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在 MATLAB Online 中打开

Output variables in parfor loops must be either sliced outputs or reduction outputs. More info here in the doc. In this case, you need KB to be a "reduction" variable since each iteration of your loop computes an increment to that matrix. To fit in with the parfor rules, you cannot index a "reduction" variable, so you need to modify things a little to have each iteration compute a full-sized increment. Something a bit like this

parfor i = 1:nelem
    % compute KB_final and index
    KB_increment = zeros(sdof);
    KB_increment(index,index) = KB_final;
    % Use a reduction assignment into KB:
    KB = KB + KB_increment;
end