Get variable content from his name Matlab [closed] - matlab

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I want to get the content of a variable from its name as a string:
%suppose that I have structures variables in the workspace:
struct_1=struct('field1' ,"str",'field2',0:5)
struct_2=struct('field1' ,"str",'field2',0:10)
struct_3=struct('field1' ,"str",'field2',0:20)
%and I have an other variable like:
a=5
var2='hello'
%and I want to concatenate all these structures in the same structure
%So i wan to get their name.
%I don't know if there is an other way to get just the structure variables
structures=who(str*)
array=[]
for i=1:length(structures)
array=[array; structures(i). field2]; % here I get an error because the content of structures are string representing the variable name
end
%create the new struct
str='newstr_value'
struct_4=struct('field1',str, 'field2',array)
How to fix this error and is there any way to do this better ?

Answer 1

While I would still highly recommend going back to the point of origin of these dynamically named structures (shame 🔔🔔🔔 on the toolbox creator if this is actually the output...) and fixing this problem there, there is an approach that does not require the use of eval.
Similar to my approach for a tangentially related question, you can save the desired structures to a temporary *.mat file and take advantage of load's optional structure output to consolidate the data structures for access in a more robust programmatic matter.
For example:
struct_1=struct('field1' ,"str",'field2',0:5);
struct_2=struct('field1' ,"str",'field2',0:10);
struct_3=struct('field1' ,"str",'field2',0:20);
save('tmp.mat', '-regexp', '^struct_');
clear
oldworkspace = load('tmp.mat');
varnames = fieldnames(oldworkspace);
nvars = numel(varnames);
% Get original structure fields
% The subsequent steps assume that all input structs have the same fields
fields = fieldnames(oldworkspace.(varnames{1}));
nfields = numel(fields);
% Initialize output struct
for ii = 1:nfields
newstruct.(fields{ii}) = [];
end
newstruct(nvars) = newstruct;
for ii = 1:nvars
for jj = 1:nfields
newstruct(ii).(fields{jj}) = oldworkspace.(varnames{ii}).(fields{jj});
end
end
Gives us:
Yay.

Answer 2

Not sure if your situation absolutely requires dynamic calls of variables by their string names. eval can solve your problem. However, eval is slow. At times, it is unreliable. You can easily have conflicting code as you build various components of your project. And in my experience, there have always been alternatives to eval. In your case, Adriaan outlined an example in his comment.
Since you are only trying to interface with a third-party toolbox, if the number of variables you read is reasonable, you can do the followings with eval.
array=struct;
for i=1:length(structures)
eval(['array(',num2str(i),').field2=',structures{i},';'])
end
If you know all the structs are named struct_#, you can also do
array=struct;
num=3; % the number of outputs from your toolbox
for i=1:num
eval(['array(',num2str(i),').field2=struct_',num2str(i),'.field2;'])
end
Note that your field2 values are all arrays. You can't just create an array and expect each "cell" to carry another array value. Arrays, or rather matrices, in Matlab are a special data type. They only take numerical values. (Matrices are highly optimized in Matlab in various ways. That is why we use Matlab.)
Also note that when you set equality of fields of structs, the equality is by reference. There is no deep copy of the data. Modifying one will modify another. (Just beware of this but don't necessarily rely on it. Deciding when to deep copy things is one of the things we let Matlab handle.)
As well, be careful with the result from who. You need to be absolutely clear with what is in the local scope. Above is assuming your result from calling structures=who(str*) is correct for your application.