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create structure variable within loop using strings??

Asked by Michael on 17 Jun 2015
Latest activity Edited by Stephen Cobeldick on 29 Jun 2015
within a loop I would like to do something along the lines of the following:
for dive = 1:10
for n = 1:3
(['data_split(',num2str(dive),').n',num2str(n),'.var']) = var;
end
end
so that I end up with a structure like:
data_split(1).n1.var for n= 1:3 and dive = 1:10

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I have tried eval and genvarname but to no luck so far..

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2 Answers

Answer by Alberto
on 17 Jun 2015
Edited by Alberto
on 17 Jun 2015
 Accepted Answer

for dive = 1:10
for n = 1:3
data_split(dive).( ['n',num2str(n)]).('var')=var
end
end

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Answer by Stephen Cobeldick on 17 Jun 2015
Edited by Stephen Cobeldick on 18 Jun 2015

This is easy with a bit of help from the documentation, but first we have to understand that these are two separate operations that we are trying to do. Lets look at them:
1) Indexing of a Non-Scalar Structure. Just like any array in MATLAB a structure can be non-scalar, and is indexed using logical or subscript indexing:
>> A(3).data = 9;
>> A(2).data = 3;
>> A(1).data = 1
A =
1x3 struct array with fields:
data
>> [A.data]
ans =
1 3 9
You can use all the usual indexing methods here, plus some very handy shortcuts: check out the link I gave above.
2) Using Dynamic Fieldnames to define or access structure fields. These are also written using parentheses:
>> B.('data') = 2;
>> B.('another_field') = 4
B =
data: 2
another_field: 4
Combining These...
So we can easily combine these two operations like this:
for dive = 1:10
for n = 1:3
tmp = sprintf('n%d',n);
data_split(dive).(tmp).var = XXX;
end
end
Note that using sprintf is usually a more robust way to construct strings, compared to string concatenation. It also makes the intent clearer, as the sequence is clearly defined in the format string.
PS: using eval to generate variable names or perform basic operations in MATLAB really should avoided. Although beginners seem to love variables magically appearing an disappearing from workspace it is not a robust way to program and only makes their own lives more difficult. Read this to know more:

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