Array of Structures (not structure array)

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AlexMatLab
AlexMatLab am 18 Sep. 2020
Kommentiert: AlexMatLab am 20 Sep. 2020
I have a problem to solve, I think an array of structures would solve my problem, but, it's not possible in matlab as far as I've seen. Please help me with a work around!
I do not want a structure array. I need independent names for the structures.
So I need to be able to do this:
asdf = struct('A', 3, 'B', 0)
zxcv = struct('A', 7, 'B', 0)
arr = [asdf , zxcv]
FUNCTION 1:
for i = 1 : length(arr)
arr(i).A = arr(i).B
end
FUNCTION 2:
for i = 1 : length(arr)
arr(i).B = arr(i).A
end
FUNCTION 3 (structure is called by name):
asdf.A = 65
FUNCTION 4:
var = asdf.A + zxcv.A
Function 3 is important, I can't just have a structure array because there are so many independtly named structures, and I have to add and/or remove many at a time, and often. So keeping track of that exact index number for asdf in a massive structure array, is a moving target, and every index would need to be verified in the code for structure in every location.
I've seen other solutions, have not found one that fits all three functions shown above.
Solutions I looked at:
2) There was another thread, suggesting using a structure of strings of the names of the structures, but it doesn't allow you to write to the the structure, like in functions 1 and 2 above. I can't find the thread though. It was also answered by Stephen Cobeldick.
3) I even tried using eval with an array of strings named after the structures, but you can't write to eval, so that didn't work. https://www.mathworks.com/matlabcentral/answers/289850-not-sure-why-eval-name-of-a-variable-cannot-be-assigned-a-value
4) My only thought was to use this custom pointer library and use an array of pointers to the structures? I'm not super keen on using a custom library if matlab doesn't usually use pointers. https://www.mathworks.com/matlabcentral/answers/5798-array-pointers-in-matl
Any other solutions are welcome? Otherwise, maybe I need to switch to python or something.
Thanks in advance!
  4 Kommentare
Bruno Luong
Bruno Luong am 19 Sep. 2020
Bearbeitet: Bruno Luong am 19 Sep. 2020
I largely recommend structure of array. Put A, B in (2 x D) arrays, column corresponding to variable names
arr = struct('name', ["asdf", "zxcv"], ...
'A', [3, 7], ...
'B', [0, 0]);
% function 1
arr.A = arr.B;
% function 2
arr.B = arr.A;
% function 3
arr.A(:,arr.name=="asdf") = 65
% function 4
var = sum(arr.A(:, ismember(arr.name, ["asdf", "zxcv"])))
Close to that struct of array above is table. But personally I don't care about table excepted when I want to display data nicely on screen or display data with uitable.
AlexMatLab
AlexMatLab am 20 Sep. 2020
Thank you for your response, but I think for my case, Steven's answer better solves my problem.

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Akzeptierte Antwort

Steven Lord
Steven Lord am 18 Sep. 2020
Consider a struct whose fields are themselves struct arrays.
arr.asdf = struct('A', 3, 'B', 2);
arr.zxcv = struct('A', 7, 'B', 5, 'C', 42);
You can access the fields in arr using dynamic field names
F = fieldnames(arr);
y = zeros(size(F));
Q = struct;
for k = 1:numel(F)
disp("Processing field " + F{k} + " of arr.")
y(k) = 2 * arr.(F{k}).A;
Q.(F{k}) = arr.(F{k}).B.^2;
end
disp('y is')
disp(y)
disp('Q is')
disp(Q)
arr.zxcv.B = arr.zxcv.A + 1;

Weitere Antworten (1)

Matt J
Matt J am 18 Sep. 2020
Bearbeitet: Matt J am 18 Sep. 2020
I don't know if this is something you explored in (2), but if so, I don't see why it wouldn't cover what you are trying to do.
asdf = struct('A', 3, 'B', 0,'ID',"asdf")
zxcv = struct('A', 7, 'B', 0, 'ID',"zxcv")
arr = [asdf , zxcv];
arr([arr.ID]=="asdf").A=65;
  8 Kommentare
AlexMatLab
AlexMatLab am 20 Sep. 2020
Steven,
Ok, this makes a lot of sense. I think this is a really good solution.
Thank you Steven!
Good point about var variable name.
AlexMatLab
AlexMatLab am 20 Sep. 2020
Thank you everyone for your responses.

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