How to resolve indexing the sliced variable error in the case of for nested with parfor?

I have following use case:

Nth = 102;
p = 0.0000001:0.1:0.99999;
NthArray = (Nth - 100 > 0)*Nth + (Nth - 100 <=0)*1:1:Nth+100;
III = zeros( length(p), length(NthArray));
parfor ip = 1:numel(p)
    for inth = 1:numel(NthArray)
        III(ip, inth) = g( p(ip), ntharray(inth));
    end
end

However, upon running it I get following error:

Error: When indexing the sliced variable 'III', the range of the for-loop variable 'inth' must be a row vector of positive constant numbers or variables. For more information, see Parallel
for Loops in MATLAB, "Nested for-Loops with Sliced Variables".

How should I resolve this error?

Thanks.

Answer 1

[Edric Ellis]

The problem here is that the inner loop bounds must be a constant - basically this means that you must compute it outside the parfor loop, like so:

Nth = 102;
p = 0.0000001:0.1:0.99999;
NthArray = (Nth - 100 > 0)*Nth + (Nth - 100 <=0)*1:1:Nth+100;
III = zeros( length(p), length(NthArray));
% Must calculate inner loop bounds outside the PARFOR loop:
n_Nth = numel(NthArray);
g = @plus; % Dummy definition of "g"
parfor ip = 1:numel(p)
    for inth = 1:n_Nth
        III(ip, inth) = g( p(ip), NthArray(inth));
    end
end
disp('Success!')

Success!

[Qitr]

I have the same problem in the following code. Even though, both indexes are constant I obtaine error:

When indexing the sliced variable 'RintE_mat', the range of the for-loop variable 'itm2' must

be a row vector of positive constant numbers or variables. For more information, see Parallel

for Loops in MATLAB, "Nested for-Loops with Sliced Variables".

What am I doing wrong?

parfor itm=1:2*NmaxAMM
        gz_real_coeff = squeeze(gzvec_real(itm,:));
        gz_real_interp(itm,:) = sqrt(interp1(grid_vec,gz_real_coeff.^2,fvec,'spline','extrap'));
        for itm2=1:2*NmaxAMM
            RintE_coeff = squeeze(RintE_mat0(itm,itm2,:));
            RintH_coeff = squeeze(RintH_mat0(itm,itm2,:));
            RintE_interp_real = interp1(grid_vec,real(RintE_coeff),fvec,'spline','extrap');
            RintE_interp_imag = interp1(grid_vec,imag(RintE_coeff),fvec,'spline','extrap');
            RintE_mat(itm,itm2,:) = RintE_interp_real + 1i*RintE_interp_imag;
            RintH_interp_real = interp1(grid_vec,real(RintH_coeff),fvec,'spline','extrap');
            RintH_interp_imag = interp1(grid_vec,imag(RintH_coeff),fvec,'spline','extrap');
            RintH_mat(itm,itm2,:) = RintH_interp_real+1i*RintH_interp_imag;
        end
end

[Edric Ellis]

Unfortunately this is a limitation of the parfor implementation. It cannot handle the expression you have there for the nested for-loop bounds (and the error is rather confusing I admit). You simply need to compute that value outside the parfor loop, like so:

limit = 2*NmaxAMM;
parfor itm=1:limit
    % then
    for itm2 = 1:limit
    end
end

[Qitr]

Have you tested that? because I tried it and it does not improved the situation. Finally, I've inserted the inner loop inside the function and it works, but the error is really confusing :(

[Edric Ellis]

I didn't try it with your code because your code is not executable in that form, so I tried it with a simpler case. Let me look again to see if there are further problems.

[Edric Ellis]

Here's an executable version of my previous comment. Hopefully this includes the salient points from your code, and this does work.

limit = 2*3;
out = zeros(limit, limit, 2);
parfor i = 1:limit
    for j = 1:limit
        out(i,j,:) = [i j];
    end
end

Starting parallel pool (parpool) using the 'Processes' profile ...
Parallel pool using the 'Processes' profile is shutting down.

disp(out)

(:,:,1) =

     1     1     1     1     1     1
     2     2     2     2     2     2
     3     3     3     3     3     3
     4     4     4     4     4     4
     5     5     5     5     5     5
     6     6     6     6     6     6


(:,:,2) =

     1     2     3     4     5     6
     1     2     3     4     5     6
     1     2     3     4     5     6
     1     2     3     4     5     6
     1     2     3     4     5     6
     1     2     3     4     5     6

[Qitr]

I cannot say anything more than thank you very much!