How to load variable in a nested structure from file

I have .mat files with data stored as nested structures and I would like to read some of the variables. Here is a MWE:

% Save a nested structure for a MWE
s(1).n(1).a = 1:10;
s(1).n(1).b = 11:20;
s(1).n(2).a = 21:30;
s(1).n(2).b = 31:40;
s(2).n(1).a = 41:50;
s(2).n(1).b = 51:60;
s(2).n(2).a = 61:70;
s(2).n(2).b = 71:80;
save('s.mat', 's');

In reality, the filename is a variable that the user defines. Here, let's say I have it as:

filename = 's.mat';

How do I load the data 1:10 stored in s(1).n(1).a?

Of course this works:

load(filename)
data = s(1).n(1).a;

But the point is avoiding to manually hardcode the s.

The following obviously does not work:

data2 = filename(1).n(1).a;
% Nor something like this:
data3 = load(filename,'????');

But it points in the direction of what I would like to do.

I have read the documentation on nested structures, the load function, etc.

Note: my files actually come from ControlDesk and the name of the file (s) always corresponds to the outer structure (s).

Answer 1

[Stephen23]

Accessing the data without knowing the name of the variable is easy, as long as there is only one variable saved in the .mat file: just use struct2cell like this:

>> filename = 's.mat';
>> T = load(filename);
>> C = struct2cell(T);
>> C{1}
ans =
  1x2 struct array containing the fields:
    n
>> C{1}(1).n(1).a
ans =
  1  2  3  4  5  6  7  8  9  10

You could avoid the intermediate variable T, I just used it for clarity:

>> C = struct2cell(load(filename));

You should read this as well:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

[Joan Vazquez]

Thanks! Indeed, the problem is that in a structure you need to access by name. But in a cell you can access by position, so struct2cell does the job, nice!

[Stephen23]

@Joan Vazquez: both struct2cell and cell2struct are quite handy when saving/loading structures, in situations like you have. Another option is to obtain the fieldname and use it as a dynamic fieldname:

>> filename = 's.mat';
>> T = load(filename);
>> F = fieldnames(T);
>> T.(F{1})(1).n(1).a
ans =
    1    2    3    4    5    6    7    8    9   10

[Joan Vazquez]

While your previous solution worked in my case, dynamic fieldnames are what is needed in the general case, I was not aware of this. Now, I can use a variable as a structure name! Thanks again.

Answer 2

[Christian Heigele]

We use for data like this either xml-files or yaml-files. A good xml-parser is that one here: https://ch.mathworks.com/matlabcentral/fileexchange/12907-xml_io_tools?focused=5171820&tab=function

With this you can serialize your data into a file and reconstruct it later into another item: xml_write('out.xml', s); f = xml_read('out.xml');

If this is what you asked.