Trouble With Indexing and Parfor

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Jacob Mevorach 2017-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/332777-trouble-with-indexing-and-parfor

评论： Jacob Mevorach 2017-4-5

I've been trying to implement parfor in the following loop for a while but can't seem to find a way to get around the problem that parfor does not allow one to use indexes for variables that are not the loop variable. However given that the index in this case should be able to be sliced I'm wondering if there's a way I could implement parfor or maybe even spmd I'm not seeing.

    % function tracks_Out = updateUnassignedTracks(tracks, unassignedTracks)
        tracks_Out = tracks;
        for i = 1:length(unassignedTracks)
            ind = unassignedTracks(i);
            tracks_Out(ind).age = tracks_Out(ind).age + 1;
            tracks_Out(ind).consecutiveInvisibleCount = ...
            tracks_Out(ind).consecutiveInvisibleCount + 1;
        end
    end

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Answer 1

Matt J 2017-4-4

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https://ww2.mathworks.cn/matlabcentral/answers/332777-trouble-with-indexing-and-parfor#answer_261720

编辑：Matt J 2017-4-4

在 MATLAB Online 中打开

Your loop iterations are not independent - and hence not suitable for parfor - unless all the values in "unassignedTracks" are unique. If you are confident that they are unique, you can do as follows,

function tracks_Out = updateUnassignedTracks(tracks, unassignedTracks)
        u=unique(unassignedTracks);
         N=length(u);
         if N~=length(unassignedTracks)
             error 'unassignedTracks are not unique'
         end
        tmp = tracks(unassignedTracks) ;
        parfor ind = 1:N
            tmp(ind).age = tmp(ind).age + 1;
            tmp(ind).consecutiveInvisibleCount = ...
                       tmp(ind).consecutiveInvisibleCount + 1;
        end
       tracks_Out=tracks;
       tracks_Out(unassignedTracks)=tmp;
    end

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Jacob Mevorach 2017-4-5

This worked beautifully! Thank you so much!

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Answer 2

Faiz Gouri 2017-4-3

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https://ww2.mathworks.cn/matlabcentral/answers/332777-trouble-with-indexing-and-parfor#answer_261593

I understand that you would like to use indexes for variable that is not the loop variable in parfor loop. When MATLAB recognizes a name in a parfor-loop as a variable, the variable is classified in one of several categories(Loop. sliced, broadcast, reduction, temporary). Here "ind" is a temporary variable and you cannot use it as loop variable. Refer this document for more details on parfor variables