[SOLVED ALREADY] Loop through data struct with variable names

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EDIT:
Hi, So it seems like I was able to solve it already so yeah, Imma post the solution as an answer for people to look into it in case they have a similar problem.
Hi,
I am trying to loop through a structure called 'Erruptions'.
Now in this structure I have 3 more sub structs, thus it's length is 3.
The 3 structures have variable names 'E1', 'E2', etc. These are simply 1x2 doubles or vectors if you wanna call them that.
So by calling one of the structs I write
Erruptions.sub_struct
and I get its content
E1: [6.0014 1.0161]
E2: [4.5936 0.5191]
E3: [3.2693 0.2849]
E4: [2.7640 -0.0073]
E5: [2.3443 0.1345]
But when I want to get the number of elements it says it's only 1, doesn't matter if I use
length(Erruptions.sub_struct) % gives me ans = 1
or
size(Erruptions.sub_struct) % gives me ans = [1 1]
which is very confusing. I want to run a loop through every substruct but since it only recognizes them as one single element, opposed to the above shown 5, itleave the substruct after taking the first into consideration.
How can I solve this problem?
  3 Comments
Steven Lord
Steven Lord on 14 Jul 2022
One analogy or mental model I think helps is that of a bookshelf. One bookshelf (struct) may have many shelves (fields) and each shelf may contain just one item or a series of items (data in the field.)

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Accepted Answer

Voss
Voss on 14 Jul 2022
Edited: Voss on 14 Jul 2022
% creating a structure like yours
Erruptions = struct('sub_struct',struct( ...
'E1',[6.0014 1.0161], ...
'E2',[4.5936 0.5191], ...
'E3',[3.2693 0.2849], ...
'E4',[2.7640 -0.0073], ...
'E5',[2.3443 0.1345]))
Erruptions = struct with fields:
sub_struct: [1×1 struct]
% Erruptions.sub_struct is a scalar structure, so its length is 1
Erruptions.sub_struct
ans = struct with fields:
E1: [6.0014 1.0161] E2: [4.5936 0.5191] E3: [3.2693 0.2849] E4: [2.7640 -0.0073] E5: [2.3443 0.1345]
% you want to get the fields of Erruptions.sub_struct
fn = fieldnames(Erruptions.sub_struct)
fn = 5×1 cell array
{'E1'} {'E2'} {'E3'} {'E4'} {'E5'}
% and loop through those
for ii = 1:numel(fn)
Erruptions.sub_struct.(fn{ii})
end
ans = 1×2
6.0014 1.0161
ans = 1×2
4.5936 0.5191
ans = 1×2
3.2693 0.2849
ans = 1×2
2.7640 -0.0073
ans = 1×2
2.3443 0.1345
  2 Comments

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More Answers (2)

Benjamin Thompson
Benjamin Thompson on 14 Jul 2022
You want to index into the structure array, then pass one of those array elements to the length function:
>> S1.E1 = [1 1];
>> S2.E1 = [1 1];
>> SArray(1) = S1
SArray =
struct with fields:
E1: [1 1]
>> SArray(2) = S2
SArray =
1×2 struct array with fields:
E1
>> length(SArray.E1)
Error using length
Too many input arguments.
>> length(SArray(1).E1)
ans =
2
  1 Comment
Stephen23
Stephen23 on 14 Jul 2022
A much simpler way to create that structure:
S(1).E1 = [1,1];
S(2).E1 = [0,1];

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Sokratis Panagiotidis
Sokratis Panagiotidis on 14 Jul 2022
SOLUTION:
Just like stated above I was able to solve it so here's ONE solution I managed to find.
What I did is I took the names of the struct, meaning
fn = fieldnames(struct);
and in the next step created a for loop
for j = 1:length(fn{j})
and THEN I did the exact same thing and took the fieldnames of each substruct, meaning
newfn = fieldnames(struct.(fn{j}));
which is
E1: [6.0014 1.0161]
E2: [4.5936 0.5191]
E3: [3.2693 0.2849]
E4: [2.7640 -0.0073]
E5: [2.3443 0.1345]
Now I try to get its length and voilà! I can use it in aother for loop to continue:
for i = 1:length(newfn)
So yeah, there you have it!

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