parfor Subscripted assignment dimension mismatch.

Hello!

i have a problem performing a long parfor...

i'm running it with a 4 core i7, and it seems that when a core finish its job it give me the Subscripted assignment dimension mismatch error becouse the error occour after it reach i = 1 (from the output i discovered that apparently it goes downward) i tried it with a simple for and it works perfectly, but since every iteraction need about 20sec and i need 256 iteraction (16 for testing) it take too much...

this is the code, thank you in advance for your help (i'm sorry for my english, i know it's bad)

parfor i = 1:16 %256
    tic
    for j = 1:256
        %disp(j)
        tmp = I1(i,j,:);
        if tmp == 0
            T2(i,j) = 0;
        else
            options = fitoptions('Method','NonlinearLeastSquares','Algorithm','trust-region','MaxFunEvals',5000,'MaxIter',2000);
            options.StartPoint = [tmp(2),20,2]; % S0,T2*,C
            options.Upper = [tmp(2)+100,70,10];
            options.Lower = [0,0,0];
            ffun = fittype('(S0.*exp(-t./T2))+C','coeff','S0','T2','C'},'independent','t','options',options);

              [Sfit, G] = fit(TE1',reshape(tmp,10,1),ffun);
              T2(i,j) = Sfit.T2;
              err(i,j) = G.rmse;
          end
      end
      toc
      disp(i)
  end

the output for this section is:

Elapsed time is 19.673806 seconds.
     6

Elapsed time is 20.312663 seconds.
     3

Elapsed time is 21.246476 seconds.
     9

Elapsed time is 22.603829 seconds.
    11

Elapsed time is 17.124702 seconds.
     2

Elapsed time is 19.613248 seconds.
     5

Elapsed time is 21.034833 seconds.
     8

Elapsed time is 21.052930 seconds.
    10

Elapsed time is 19.624255 seconds.
     1

Subscripted assignment dimension mismatch.

Caused by:
    Subscripted assignment dimension mismatch.

Answer 1

[jacopo]

ok i found the solution... i forgot preallocating variable err so when the first core finish computating the first branch of parfor and goes to the next (higher i value) it couldn't reallocate the variable so it was out of bound

i also changed if tmp == 0 using all() thank you!

Answer 2

[Walter Roberson]

I do not know if this will correct your difficulty, but I see a problem in your code.

tmp = I1(i,j,:);
if tmp == 0

We do not know the dimensions of I1, but it appears likely that I1(i,j,:) would have multiple elements and so that tmp would be a vector. Then in the "if" you compare that vector to 0, producing a vector of logical results, that is then evaluated by "if". When "if" is applied to a vector or matrix, then the condition is only considered to be true if all elements of the vector or matrix are true -- so the "if" would only be true if I1(i,j,:) are all 0. Is that what you want? If it is, then it is best to code that meaning for clarity, using

if all(tmp == 0)