MATLAB: Sliced scalar assignment in parfor

parallelParallel Computing Toolboxparforpct

The following code

    A=rand(10);
    parfor j=1:10
       A(:,j)=1; 
    end

gives me the following error

Error using test (line 5)
Subscripted assignment dimension mismatch.
Caused by:
    Subscripted assignment dimension mismatch.

It seems that you cannot do scalar assignment with parfor sliced variables? If not, is this documented somewhere? I know sliced variables must maintain a constant shape, but scalar assignment does not violate this.

Best Answer

Unfortunately, this is a limitation of PARFOR. The only workaround for now is not to use scalar expansion like that.

Related Solutions

MATLAB: Sliced variables

To slice the variable 'a', you need to use a subscript list that contains:

One instance of the loop variable
Other subscripts which are either constant literal values or ':'

So, for example, in the following case, 'a' will be a sliced output:

parfor n = 1:35
  tmp = 90 * ones(1, 15);
  a(n, :) = tmp;
end

which creates 'a' as a 35x15 matrix with each element containing the value 90.

MATLAB: Parfor: slicing variable

This is not something you can do with parfor. parfor can only be used where the indices written to are a very simple computation from the indices.

I suggest you consider using sub2ind() to build up lists of linear indices to change, and then do a simple non-parallel b(list_of_indices) = nan .

If necessary (very long list) you could compute the indices in a parfor, returning them as data inside cell locations, like

parfor ii = 1 : length(indices)
    start_index = indices(ii);
    end_index = start_index + step(ii);
    bidx{ii} = start_index : end_index - 1;
end
b(horzcat(bidx{:})) = nan;

Best Answer

Related Solutions

MATLAB: Sliced variables

MATLAB: Parfor: slicing variable

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