# valid indices in a parfor loop

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Neil on 7 Jun 2022
Commented: Neil on 9 Jun 2022
Can someone please explain why the following code does not work in the parfor loop? The error is about invalid indices on the matrix 'mat'.
inds = 1:ptsPerCore*nCores;
inds = reshape(inds,ptsPerCore,nCores);
mat = zeros(ptsPerCore*nCores,n);
parfor ii=1:nCores
par_inds = inds(:,ii);
for jj=1:ptsPerCore
cur_ind = par_inds(jj);
vec = randn(1,n);
mat(cur_ind,:) = vec;
end
end

Matt J on 7 Jun 2022
Edited: Matt J on 7 Jun 2022
You are trying to use the parfor loop to fill a pre-existing matrix mat. The only type of parfor variable for which this is allowed is a Sliced Variable. Indexing into sliced variables must obey certain rules:
Neil on 9 Jun 2022
Thanks @Matt J. Reordering before the loop along with the 3d output proposed by @Edric Ellis solved my problem as I needed to keep the nested loop.

### More Answers (1)

Edric Ellis on 9 Jun 2022
There are a couple of ways to work around this. You can either unroll the inner loop, or else make the output matrix 3d so that it is sliceable (and then use reshape to get back).
%% Original code
ptsPerCore = 7;
nCores = 5;
n = 3;
inds = 1:ptsPerCore*nCores;
inds = reshape(inds,ptsPerCore,nCores);
mat = zeros(ptsPerCore*nCores,n);
for ii=1:nCores
par_inds = inds(:,ii);
for jj=1:ptsPerCore
cur_ind = par_inds(jj);
vec = cur_ind.*(1:n);
mat(cur_ind,:) = vec;
end
end
%% Alternative 1 - run as a single loop
mat2 = zeros(ptsPerCore*nCores,n);
parfor iijj = 1:(nCores * ptsPerCore)
vec = iijj.*(1:n);
mat2(iijj,:) = vec;
end
Starting parallel pool (parpool) using the 'local' profile ... Connected to the parallel pool (number of workers: 2).
assert(isequal(mat2, mat));
%% Alternative 2 - use a 3d output
mat3 = zeros(ptsPerCore, nCores, n);
parfor ii=1:nCores
par_inds = inds(:,ii);
for jj=1:ptsPerCore
cur_ind = par_inds(jj);
vec = cur_ind.*(1:n);
mat3(jj, ii, :) = vec;
end
end
% Need to reshape back to original size
mat3 = reshape(mat3, nCores*ptsPerCore, n);
assert(isequal(mat3, mat));
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Neil on 9 Jun 2022
Thanks @Edric Ellis. The 3d output worked when I reordered before the loop as proposed by @Matt J.

R2019a

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