How to avoid eval when eval seems unavoidable

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Leo Simon
Leo Simon on 1 Nov 2021
Commented: Walter Roberson on 1 Nov 2021
Everybody on this forum agrees that eval is a terrible terible thing to use. But often it seems to me to be unavoidable. The one below is only the most recent of these that I have encountered. Suppose I want to "unpack" a struct, using the fields of the struct as variable names.
For example
S.a = 1
S.b = 2
etc
Now I want to create variables a=1 and b=2, etc. The only way I know how to do this goes something like this;
Fields = fieldnames(S);
for ii=1:numel(Fields);
field = Fields{ii};
value = getfield(S,field);
eval([ field '=' num2str(value) ';']);
end
a
b
How can I do a loop like this, without using eval?
  2 Comments
Leo Simon
Leo Simon on 1 Nov 2021
definitely wnat (though don't necessarily need) to unpack the struct.

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Answers (2)

Matt J
Matt J on 1 Nov 2021
Edited: Matt J on 1 Nov 2021
One way to avoid eval in this case is to use an automatic code-writing tool instead of unpacking at run time. I created the following precisely for this purpose,
  2 Comments
Matt J
Matt J on 1 Nov 2021
I don't see what advantage you anticipate in having the unpacking done at runtime. The expressed purpose of your post was to avoid the disadvantages of eval(). However, you can make it work with eval() by doing,
eval(structvars(S).')

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Walter Roberson
Walter Roberson on 1 Nov 2021
orderfields, struct2cell, cell expansion in the context of a multiple variable output.
Assuming that the list of fields is fixed but not necessarily their order.
  3 Comments
Walter Roberson
Walter Roberson on 1 Nov 2021
It is not clear to me why my suggestion is not suitable for your purposes? Do you need to have struct with unpredictable field names that have to be moved into variables?

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