MATLAB: Parallel computation for a for loop

parallel computingParallel Computing Toolbox

Below is the part of the code I have written, in which I am trying to use parallel computation. But it does give me an error as below:
"Error: The variable k_1 in a parfor cannot be classified."
In each parfor iteration some specific rows of "k_1" matrix will be updated irrelavant to the rest, so I cannot see why I get this message. Any help in this regard will be highly appreciated. Please find the parallel portion of my code below:
k_1 = k;
M_1 = M;
% pi = 0;
parfor ppi=1:NP
    for pii=1:kkk
%         pi= pi + 1;
        pi = (ppi-1)*kkk+pii;
    Tempo1 = zeros(1,1);
    Tempo1_M = zeros(1,1);
    Tempo2 = zeros(1,1);
    Tempo2_M = zeros(1,1);
    for irow = 1:p(pi)
        [xx,inside_angle] = find(irow==[p;q]);
        Tempo1(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [k(p(inside_angle),p(pi))+lambda(pi)*k(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [k(q(inside_angle),p(pi))+lambda(pi)*k(q(inside_angle),q(pi))];
        Tempo1_M(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [M(p(inside_angle),p(pi))+lambda(pi)*M(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [M(q(inside_angle),p(pi))+lambda(pi)*M(q(inside_angle),q(pi))];
    end
    
    for irow = 1:q(pi)
        [xx,inside_angle] = find(irow==[p;q]);
        Tempo2(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [alpha(pi)*k(p(inside_angle),p(pi))+k(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [alpha(pi)*k(q(inside_angle),p(pi))+k(q(inside_angle),q(pi))];
        Tempo2_M(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [alpha(pi)*M(p(inside_angle),p(pi))+M(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [alpha(pi)*M(q(inside_angle),p(pi))+M(q(inside_angle),q(pi))];
    end
    
    %Assign tempos to k
    for irow = 1:p(pi)
        k_1(irow,p(pi)) = Tempo1(irow);
        k_1(p(pi),irow) = Tempo1(irow);
        M_1(irow,p(pi)) = Tempo1_M(irow);
        M_1(p(pi),irow) = Tempo1_M(irow);
    end
    for irow = 1:q(pi)
        k_1(irow,q(pi)) = Tempo2(irow);
        k_1(q(pi),irow) = Tempo2(irow);
        M_1(irow,q(pi)) = Tempo2_M(irow);
        M_1(q(pi),irow) = Tempo2_M(irow);
    end              
    end           
end
k=k_1;
M=M_1;
Please note that k and M matrices are defined by me at the start of the code!

Best Answer

parfor pi=1:NP*kkk
        
    Tempo1 = zeros(  p(pi)  ,1 );
    Tempo1_M = zeros( p(pi) ,1);
    Tempo2 = zeros(q(pi) ,1);
    Tempo2_M = zeros(q(pi) ,1);
    
    pSubs = zeros(  p(pi)  ,2 );  %new



    qSubs = zeros(q(pi) ,2);      
    
    
    for irow = 1:p(pi)
        [xx,inside_angle] = find(irow==[p;q]);
        Tempo1(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [k(p(inside_angle),p(pi))+lambda(pi)*k(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [k(q(inside_angle),p(pi))+lambda(pi)*k(q(inside_angle),q(pi))];
        Tempo1_M(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [M(p(inside_angle),p(pi))+lambda(pi)*M(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [M(q(inside_angle),p(pi))+lambda(pi)*M(q(inside_angle),q(pi))];
        
         pSubs(irow,:)=[irow,p(pi)];  %new
         
    end
    
    for irow = 1:q(pi)
        [xx,inside_angle] = find(irow==[p;q]);
        Tempo2(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [alpha(pi)*k(p(inside_angle),p(pi))+k(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [alpha(pi)*k(q(inside_angle),p(pi))+k(q(inside_angle),q(pi))];
        Tempo2_M(irow) = [(xx-1)*alpha(inside_angle)+(2-xx)]* ...
            [alpha(pi)*M(p(inside_angle),p(pi))+M(p(inside_angle),q(pi))] + ...
            [(2-xx)*lambda(inside_angle)+(xx-1)]* ...
            [alpha(pi)*M(q(inside_angle),p(pi))+M(q(inside_angle),q(pi))];
        
         qSubs(irow,:)=[irow,q(pi)];  %new
    end
    
    subsCell{pi}=[pSubs;qSubs];  %new
    kValCell{pi}=[Tempo1;Tempo2];  
    MValCell{pi}=[Tempo1_M;Tempo2_M];  
end
 subs=cell2mat(subsCell);
 kVal=cell2mat(kValCell);
 MVal=cell2mat(MValCell);
 
 
 k_1=accumarray(subs,kVal,size(k));
 M_1=accumarray(subs,MVal,size(M));
 
 k=k_1 + tril(k_1.',-1); %make symmetric 
 M=M_1 + tril(M_1.',-1);
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