Slicing structure variable in parfor loop

Question

Roman Hochuli 2012-6-19

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https://ww2.mathworks.cn/matlabcentral/answers/41547-slicing-structure-variable-in-parfor-loop

Hi,

I'd like to use a parfor loop instead of a for loop in some code I have written. I currently call a function within a for loop to which I input a structure 'photon'. However, I would only like to input certain entries from the fields within this structure: e.g.

    %%loop over all bins
    parfor bin = 1:numel(s_uniq)
        % determine current step length
        s_crnt = s_uniq(bin);
        % create photon wight property with photons currently being simulated
        photon_crnt.W = photon.W(s_int==s_crnt);

MLint tells me I cannot use 'photon' in this way and that I should slice it. I have tried to do this but continue getting this message.

I'd appreciate your help.

Thanks!

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Answer 1

Walter Roberson 2012-6-19

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https://ww2.mathworks.cn/matlabcentral/answers/41547-slicing-structure-variable-in-parfor-loop#answer_51173

Each of your loops is writing over the same fieldname in the same structure array entry, photon_crnt.W . You cannot do that unless photon_crnt is a variable that is local to the parfor loop.

If you were to write to photon_crnt(bin).W then that would be allowed by parfor; whether it makes sense for your code is something you will need to decide.

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Roman Hochuli 2012-6-19

Thank you for your answer. I understand what you mean, but the problem arises due to the use of 'photon', not 'photon_crnt'. Moreover, 'photon_crnt' is, as you say, local to the loop. See here:

%% loop over all bins

parfor bin = 1:numel(s_uniq)

% determine current step length

s_crnt = s_uniq(bin);

% create photon wight property with photons currently being simulated

photon_crnt.W = photon.W(s_int==s_crnt);

% if total photon weight currently being simuluated greater than thresh

if any(photon_crnt.W>threshold)

% create photon position property with photons currently being simulated

photon_crnt.pos = photon.pos(s_int==s_crnt,:);

% create photon direction property with photons currently being simulated

photon_crnt.direc = photon.direc(s_int==s_crnt,:);

% absorb over a single step of path length

[photon_crnt, Q] = absorb(map, Q, s, s_int, s_crnt, photon_crnt, direcCosy, direcCosx, SF, threshold,[],[]);

% replace photon data with new data

photon.W(s_int==s_crnt) = photon_crnt.W;

photon.pos(s_int==s_crnt,:) = photon_crnt.pos;

photon.direc(s_int==s_crnt,:) = photon_crnt.direc;

end

The alternative referencing you mention would not make sense in this case. Using cell array might be a possibility, but I'm not sure I fully understand how to slice variables yet.

What else could I try?

Thanks!

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