From a structure with "n" fields which each are a vector, I want to make a vector of length "n" made of the 3rd value of each vector of my structure.

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Ezio Antonio Mosciatti Urzua 2023-5-3

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移动：Rik 2023-5-3

This is a situation I have come upon a few times now since I started using structures little time ago.

Specifically in the last case, I have a structure called "file" with 25 fields. On each field I have a vector called "dist" which is a simple 4 value vector. The thing is that I would like a vector with the 3rd value of each of these vectors, somthing like:

a = file(:).dist(4);

Which does not work at all.

I've discovered that if I write:

a = [file(:).dist];

I get a 1x100 vector with all the .dist vectors concatenated. Also, doing:

a=vertcat(file(:).dist);

makes "a" into a 25x4 matrix in which each row is a .dist vector. However, I cannot index directly into that expression as:

a=vertcat(file(:).dist)(:,3);

I realise that I could get this with a little bit of code such as:

for i=1:length(file)
    a(i)=file(i).dist(3);
end

and even faster, with the vertcat function as:

a=vertcat(file(:).dist);
a=a(:,3);

But none of these solutions allow me to plot this directly, which is my ultimate goal, in this case.

Thank you!

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Rik 2023-5-3

移动：Rik 2023-5-3

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The last solution you mention would be my suggestion, except you don't need the (:).

file = struct('dist',{[1 2 3];[4 5 6]});
a = vertcat(file.dist)
a = 2×3
     1     2     3
     4     5     6

You will need intermediate values anyway, so there is no gain or loss in doing what you already show.

Stephen23 2023-5-3

编辑：Stephen23 2023-5-3

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Your description "I have a structure called "file" with 25 fields" contradicts the code you show, which indicates that you actually have a structure array with 25 elements and only one field:

for i=1:length(file)

a(i)=file(i).dist(3);

end

DIST is 1 field, not 25 fields. And the indexing FILE(i) indicates that FILE has multiple elements.

% "On each field I have a vector called "dist" which is a simple 4 value vector."
%          ^^^^^ element  ^^^^^^ field                             ^^^^^ element

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Answer 1

Stephen23 2023-5-3

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1957199-from-a-structure-with-n-fields-which-each-are-a-vector-i-want-to-make-a-vector-of-length-n-made#answer_1228394

编辑：Stephen23 2023-5-3

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You could use ARRAYFUN to iterate over the elements of a structure (but this will be slower than a well-written loop):

file = struct('dist',{1:3,4:6,7:9})
file = 1×3 struct array with fields:
    dist
out = arrayfun(@(s)s.dist(3),file)
out = 1×3
     3     6     9

This is very similar to the example shown here:

https://www.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

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Ezio Antonio Mosciatti Urzua 2023-5-3

Great, it works like a charm, thanks!

I guess I got confused with the nomenclature. I had a structure array, "file", with 25 structures, each of them with a field called "dist".

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From a structure with "n" fields which each are a vector, I want to make a vector of length "n" made of the 3rd value of each vector of my structure.

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From a structure with "n" fields which each are a vector, I want to make a vector of length "n" made of the 3rd value of each vector of my structure.

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