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
采纳的回答
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:
7 个评论
Neil
2022-6-8
Which rule is being violated?
"Form of Indexing. Within the first-level of indexing for a sliced variable, exactly one indexing expression is of the form i, i+k, i-k, or k+i. The index i is the loop variable and k is a scalar integer constant or a simple (non-indexed) broadcast variable. Every other indexing expression is a positive integer constant, a simple (non-indexed) broadcast variable, a nested for-loop index variable, colon, or end"
Is there a workaround?
The whole thing looks equivalent to
mat=randn(ptsPerCore*nCores,n);
Not sure why you don't just do that.
Neil
2022-6-8
Matt J
2022-6-8
It might be possible, but it would be important that the par_inds are unique and don't overlap from worker to worker. Otherwise, the tasks are not truly parallel.
Neil
2022-6-8
If so, use inds to re-order the mat matrix before the loop:
matReordered=reshape( mat(inds,:) ,[ptsPerCore,nCores,n]);
parfor i=1:nCores
matReordered(:,i,:)=rand(ptsPerCore,1,n); %assign something
end
mat(inds,:)=matReordered(:,:);
Neil
2022-6-9
更多回答（1 个）
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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