Assign value to field of nested structure in a structure array
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Konstantinos Tompoulidis
on 16 Oct 2016
Edited: broken_arrow
on 30 Sep 2021
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
2 Comments
Pico Technology
on 17 Oct 2016
Is this the actual code run or should it be
[person(1:4).FamilyInfo.NumberOfsiblings] = 3;
Accepted Answer
Guillaume
on 17 Oct 2016
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));
2 Comments
Guillaume
on 17 Oct 2016
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.
More Answers (2)
Jan
on 17 Oct 2016
Edited: Jan
on 17 Oct 2016
This does not work "at once". Simply use a loop:
for k = 1:4
person(k).FamilyInfo.NumberOfsiblings = 3;
end
4 Comments
Jan
on 19 Oct 2016
@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
on 30 Sep 2021
Edited: broken_arrow
on 30 Sep 2021
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)
Jos (10584)
on 17 Oct 2016
[person(1:4).FamilyInfo] = deal(struct('NumerOfsiblings',3)) ;
1 Comment
Guillaume
on 17 Oct 2016
That will actually remove the FatherName and MotherName subfields for any of the FamilyInfo fields that have them.
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