valid indices in a parfor loop
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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
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Edric Ellis
am 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
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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