"Access Elements of a Nonscalar Structure Array" issue

Hello,

I try to access data stored in a structure of this type:

I would love to have access easily for exemple to all the masses of all the subsystems, but i cannot manage to do it , i tried : (I pasted the full code at the end)

systems(:).subsystem.mass

but it returned: Reference to non-existent field 'subsystem'.

The real problem has a lot more system than that, do you have an idea on how i could access to all the masses contained in all the subsystems?

Thank you for your help!

I tried to follow this MatLab tutorial but it doesn't seem to work in my case:

https://fr.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

clear all
clc
%% creation of the structure
systems=struct();

system1.mass=1000;
system1.CG=[0,1,2];
system1.subsystem.mass=[0;10;100];
system1.subsystem.CG=[0,1,2;3,4,5;6,7,8];
systems.system1=system1;

system2.CG=[0,1,2];
system2.mass=1000;
system2.subsystem.mass=[0;10;100];
system2.subsystem.CG=[0,1,2;3,4,5;6,7,8];
systems.system2=system2;



systems.mass=5000;
systems.CG=[10,20,30];

clear system1
clear system2

%% access  to data
systems(:).subsystem.mass

[Stephen23]

Why do you need to brutally CLEAR ALL cached functions from memory?

Is there a reason you cannot just use much more gentle CLEARVARS?

You did not create any non-scalar structures, so the documentation you linked to is not relevant:

isscalar(systems)

ans = logical
   1

isscalar(systems.system1)

ans = logical
   1

isscalar(systems.system2)

ans = logical
   1

Possibly you are confusing the size of a structure with how many fields a structure has (whereas in fact these are totally independent from each other). The design of your scalar structures with multiuple fields does not offer any obvious trivial way to access the nested structures.

[Thomas Fournier]

Thank you for your answer,

Indeed CLEARALL is not necessary, i recreated the problem in a separated file so it works just well this way.

Answer 1

[Jan]

You cannot access fields of deeply nested structs in Matlab. You need a loop to do this.

[Thomas Fournier]

Thank you for your answers, I may have to reconsider how i build my structure then,

But if needed here is a solution which is not pretty but is working:

mass=[];
fn=fieldnames(systems);

for k=1:numel(fn)
    if isstruct(systems.(fn{k}))
        fn2=fieldnames(systems.(fn{k}));
        for l=1:numel(fn2)
           if  string(fn2(l))=='subsystem'
              mass_temp=systems.(fn{k}).subsystem.mass;
              mass=[mass,mass_temp];
           end

        end
    end
end

[Jan]

Hiding an index in the fieldname as in "system1" is a bad idea. Deeply nested structs are inefficient also, if you need to access nested fields. So restructuring the data might be the best approach.

Answer 2

[Stephen23]

sys = fakedata()

sys = struct with fields:
    system1: [1×1 struct]
    system2: [1×1 struct]
       mass: 5000
         CG: [10 20 30]

fnm = regexp(fieldnames(sys),'^system\d+$','match');
fnm = [fnm{:}]

fnm = 1×2 cell array
    {'system1'}    {'system2'}

fun = @(f)sys.(f).subsystem.mass;
out = cellfun(fun,fnm,'uni',0)

out = 1×2 cell array
    {3×1 double}    {3×1 double}

Checking (tip for the future: using different values for fake data is a simple sanity check):

out{1}

ans = 3×1

0 10 100

out{2}

ans = 3×1

0 10 100