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.

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!

[Rik]

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]

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

Answer 1

[Stephen23]

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

[Ezio Antonio Mosciatti Urzua]

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".