Assign value to field of nested structure in a structure array

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Konstantinos Tompoulidis 2016-10-16

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https://ww2.mathworks.cn/matlabcentral/answers/307525-assign-value-to-field-of-nested-structure-in-a-structure-array

编辑： broken_arrow 2021-9-30

Hello, I will use a simple example to show what I am after. I have a structure array 'person' with fields ('First_Name' , 'Last_Name' , 'FamilyInfo'), where 'FamilyInfo' is a structure with three fields ('FatherName' , 'MotherName' , 'NumberOfSiblings').

I create 4 elements in person structure array and fill all the 'First_Name' and 'Last_Name' fields but the 'FamilyInfo' field is left empty. Now, for all the 4 elements in the 'person' structure array, I want to assign at once the same value to the field "NumberOfsiblings" in the nested structure 'Family', but MATLAB throws an error. Here is the code;

Create an empty structre array

person = struct('First_Name',[],'Last_Name',[],...
'FamilyInfo',struct('FatherName',[],'MotherName',[],'NumberOfsiblings',[]));

Create elements in the array but the Family field is left empty

FirstNames = {'Joe','Jack','William','Averel'};
[person(1:4).First_Name]=deal(FirstNames{:});
[person(1:4).Last_Name]=deal('Dalton');

For all the elements in person structure array, assign the same value (3) to the NumberOfsiblings field, of the Family field.

[person(1:4).FamilyInfo.NumerOfsiblings] = 3;

Error message pops up;

Expected one output from a curly brace or dot indexing expression, but there were 4 results.

Is there a way to do it without using a for loop? One-line code is preferred. Thanks

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Pico Technology 2016-10-17

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Is this the actual code run or should it be

[person(1:4).FamilyInfo.NumberOfsiblings] = 3;

Konstantinos Tompoulidis 2016-10-17

I don't understand your question. What I wrote is the code I run but it pops an error. I don't know what the correct code is. I am still looking for it.

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Answer 1

Guillaume 2016-10-17

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That's one of the downside of multilevel structures. As Jan says, it's not possible to assign at once to fields of a substructure.

More importantly, note that when you grow the root source structure (by adding the first names of the 4 persons, only the 1st element of the structure array is a FamilyInfo substructure. The other 3 are initialised empty, because matlab does not copy the type of fields when it grows structure arrays. So if you actually want to have the correct FamilyInfo substructure after growing, you actually need to deal a FamilyInfo field to each element. You may as well set the value of NumberOfSiblings to the correct one:

[person.FamilyInfo] = deal(struct('FatherName',[],'MotherName',[],'NumberOfsiblings',3);

Note that if you want to avoid losing the FamilyInfo struct, don't grow the array but initialise it at the correct size to start with:

numpersons = 4;
person = struct('First_Name',[],'Last_Name',[],...
'FamilyInfo',repmat({struct('FatherName',[],'MotherName',[],'NumberOfsiblings',[])}, 1, numpersons));

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Konstantinos Tompoulidis 2016-10-17

编辑：Konstantinos Tompoulidis 2016-10-17

Could you explain a little bit the last line of code that you wrote, please?

Thanks!

Guillaume 2016-10-17

I'm not sure what you want me to explain. If you pass a cell array for one the field values to struct, it creates a structure array by distributing the content of the cell array to each struct array element.

Therefore, here I'm just repmat'ing a scalar cell to the desired array size, so each structure is automatically initialised to the same value.

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Answer 2

Jan 2016-10-17

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https://ww2.mathworks.cn/matlabcentral/answers/307525-assign-value-to-field-of-nested-structure-in-a-structure-array#answer_239344

编辑：Jan 2016-10-17

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This does not work "at once". Simply use a loop:

for k = 1:4
  person(k).FamilyInfo.NumberOfsiblings = 3;
end

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Jan 2016-10-19

@Konstantinos: Discussing your wanting to avoid a loop takes more time than writing it. A loop is clean and efficient here, nice and compact.

broken_arrow 2021-9-30

编辑：broken_arrow 2021-9-30

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I agree with @Konstantinos Tompoulidis that this syntax limitation is a nuisance, especially in OOP. After all, structs are constructed in such a way as to imitate the "physical nature" of the objects and one does not want to warp the structure just to comply with syntax limitations. Of course looping is possible, but according to the principle "vectorize where you can", it is not optimal. Reading and writing should also be possible with multiple outputs on multiple levels, like

[a(:).b.c(:).d] = deal(1)

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