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

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Rahul Bhadani on 29 Apr 2021
Commented: Qitr on 11 Nov 2023
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.
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### Answers (1)

Edric Ellis on 30 Apr 2021
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!
##### 6 CommentsShow 4 older commentsHide 4 older comments
Edric Ellis on 10 Nov 2023
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 on 11 Nov 2023
I cannot say anything more than thank you very much!

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