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Slicing structure variable in parfor loop

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Roman Hochuli
Roman Hochuli on 19 Jun 2012
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!

Answers (1)

Walter Roberson
Walter Roberson on 19 Jun 2012
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.
  1 Comment
Roman Hochuli
Roman Hochuli on 19 Jun 2012
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
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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