assign the content of a structure to another

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etudiant_is 2016-2-1

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回答： etudiant_is 2016-2-2

I have two structures A and B having the same fields but not the same sizes. The size of A can be bigger or smaller than B depending on the code. A is originally defined and I want to change its content at the end of the round to use what is in B. So what I want to do is to assign the content of B to A and if there are more fields in A , remove them. If there is less add the missing ones.

I tried

[A]= B;

It does not give me any error but the result I get is not correct. Then I tried something like

[A(1:numel(B)).field1]=B.field1; 
...
[A(1:numel(B)).fieldn]=B.fieldn;

but I get an error because both structures do not have the same number of elements.

I tried also

[A(1:length(B)).field1]=B.field1;

No errors but the result is not correct either. What is the right way to it? And why the first one does not work?

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Answer 1

etudiant_is 2016-2-2

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Turns out that there was a letter in capital in one of the fields which I mistakenly wrote as lower case and it was the source of all the problems. The assignment a=b is correct and was not the source of the problem. Still I am going to leave the question since it might come handy to someone else.

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Answer 2

Guillaume 2016-2-1

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在 MATLAB Online 中打开

There's something that you're not explaining correctly. "I want to change its content at the end of the round to use what is in B" to me reads as if you simply want to assign B to A, in which case:

A = B; %no need for the [] which doesn't do anything

So, when you say "the result is not correct", in what way is it not correct?

Note that with the above, it does not matter what is originally in A. A could be a structure with whichever size and whatever fields, it could be a matrix, or even not exist. By saying A = B you're essentially creating a new A which has nothing to do with the old one (if it existed).

Now, when you say that

[A(1:numel(B)).field1]=B.field1

gives you "an error because both structures do not have the same number of elements", that shouldn't be the case. Is that the exact command that you entered? It does not give an error for me. If A is smaller than B, it'll be automatically resized to fit the size of B. If A is larger than B, only the first b elements of A will be replaced, where b = numel(B).

Finally, your last instruction (with length) is the same as

[A(1:numel(B)).field1]=B.field1

if B is a vector. If B is a matrix, then it will only copy the first x elements of B where x is the length of the largest dimension of B. Surely not what you want in any case. As a rule avoid length or learn what it means (as per doc length(m) = max(size(m)))

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etudiant_is 2016-2-1

exactly what i want is to erase A and put B inside. But with the first alternative I wrote about, when i display the content of A after the assignment, I found it is the same as it was before. I had been first using numel(A) that's why i got an error, after changing it with numel (B), it became as you said: there are entries who are still there if size(A) bigger than size (B).

etudiant_is 2016-2-1

To be more precise, with the first alternative, the size of A changes to size of B but the field1 is the old one not B.field1 and so on. So I thought something must be wrong with it.

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