vector to struct without loop

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Fernando 2017-1-1

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https://ww2.mathworks.cn/matlabcentral/answers/318741-vector-to-struct-without-loop

评论： Fernando 2017-1-2

I have two vectors, one with ids and another with information for each id. For each id, the vector with information has, potentially, a different number of entries. I want to assign this to a struct, ideally without using a loop as the information vector/struct is used within an optimization routine and it changes every time there is a new iteration.

This is the example:

id=[1 1 1 2 2 3]';
info = [0.5 1 10 1 0.3 -200]';

In this case, id 1 has three elements in the information vector, id 2 has two, and id 3 has one. With the loop, I would do

for i = 1:max(id),
m(i).info = info(id==i);
end

Is there anyway to do this without using the loop?

Thanks

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Answer 1

Walter Roberson 2017-1-1

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t1 = [find(diff(id).'), length(id)];
t2 = [t1(1), diff(t1)];
m = struct('info', mat2cell(info, t2, 1));

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Fernando 2017-1-1

This is great, thanks!

Matt J 2017-1-1

Note, there are for-loops inside mat2cell.m.

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Answer 2

Matt J 2017-1-1

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I'm not sure there's a fast way without loops, but here's a way that at least doesn't use any Mcoded for-loops as far as I know

n=max(id);
data=accumarray(id,info,[],@(x){x});
[m(1:n).info]=deal(data{:});

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Fernando 2017-1-2

Thanks for the suggestion. In my application, with id going from 1 to ~30,000, this was slightly slower than the first approach, but both significantly faster than my Mcoded for-loop.

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Answer 3

Fernando 2017-1-2

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https://ww2.mathworks.cn/matlabcentral/answers/318741-vector-to-struct-without-loop#answer_249146

Thanks for both suggestions, I have it working now.

Out of curiosity, how would I go the other way around? If I had the the information in structs of different dimensions for each id and I wanted to go from the structs to the vector info?

Thanks.