How to convert a layered structure into arrays.

I have a structure like this

A.a.b.first, A.a.b.second, A.a.b.third
A.c.d.first, A.c.d.second, A.c.d.third
A.e.f.first, A.e.f.second, A.e.f.third

I want to extract the fields "first", "second" and "third" into arrays so that one array has all the firsts, another has all the seconds, and a thrid and all the thirds. Is there a simulink method to use or a matlab function? I am currently brute forcing it in matlab code. That is,

B(1) = A.a.b.first
B(2) = A.c.d.first
B(3) = A.e.f.first

and so forth. So it would be much better to have a loop or something even more streamlined.

[Matt J]

Whoever gave you data in this form has a lousy sense of humor.... According to your pattern, there could only be 9 more steps., am I right? If so, that doesn't seem like such an onerous thing to type by hand.

B(4) = A.g.h.first
.
.
B(13) = A.y.z.first

[John Petersen]

I didn't want to take all the time to explain the organization fo the data, and so I see I didn't actually explain it exactly correct. There are many more fields and they are there for orgainization of the data. The main point is that the last structure is only the three fields, first, second, third (and those aren't the actual names either). So there are many more fields than the a,b,c,d, ... and I need to add more fields and so I wanted to simplify the code so that if ever we had to add more again, the code would not have to change, it would just handle it.

[Matt J]

Are the desired fields always at a fixed depth (4 fields deep in your example)? Also, if the field names are not a simple alphabetical progression, what decides the order of the elements in B(i)? We need to know the general pattern before a solution can be recommended.

[John Petersen]

No. Most are four, but some are only three deep. There is a bus in simulink that defines the order. Is it possible to use that? There is also an enum that is used in the code that defines the same order.

Answer 1

[Matt J]

Here's a way you can recursively search down through a tree of nested structs. Since I don't know how you want to handle multiple occurrences of the target field name (e.g. "first") on the same branch of the treee, I propose one possibility:

A.b.c.d.e.first=3;
A.g.h.first=4;

out = findField(A,'first')

out = 1×2 cell array
    {[3]}    {[4]}

function out=findField(S,tfield)

  if ~isstruct(S)
     
     out=[];

  elseif isfield(S, tfield)

      out=S.(tfield); %found a match
      
  else%recurse 
      
    f=fieldnames(S);
      
    if numel(f)==1  
      
        out=findField(  S.(f{1})  , tfield);
        
    else   
      for i=1:numel(f)
       out{i}=findField(  S.(f{i})  , tfield);
      end
    end
    
  end

end

[John Petersen]

Thanks, that looks like it might work.

Answer 2

[Matt J]

A more automated option,

f=string(num2cell('a':'z'));

for i=1:13

    B(i)=A.(  f(2*i-1)  ).(  f(2*i)  ).first;
    C(i)=A.(  f(2*i-1)  ).(  f(2*i)  ).second;
    D(i)=A.(  f(2*i-1)  ).(  f(2*i)  ).third;
    
end

[John Petersen]

I was looking for a more general approach that was able to extract the last fields in the structure. The fields are not actually 'a', 'b', 'c'....