[SOLVED ALREADY] Loop through data struct with variable names

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Sokratis Panagiotidis 2022-7-14

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1760135-solved-already-loop-through-data-struct-with-variable-names

评论： Voss 2022-7-14

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?

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Sokratis Panagiotidis 2022-7-14

Hey, Now I uderstand my confusion. You were right about the mix up and the solution with 'fieldnames' is the correct one I was also able to solve.

But how would it confuse more people if I post the solution I found, since also someone else (besides you) was able to answer it this way?

Steven Lord 2022-7-14

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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Answer 1

Voss 2022-7-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1760135-solved-already-loop-through-data-struct-with-variable-names#answer_1007510

编辑：Voss 2022-7-14

在 MATLAB Online 中打开

% 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

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Sokratis Panagiotidis 2022-7-14

Yeah this was pretty much the solution I was able to find only two minutes after posting that question except for using 'length' instead of 'numel'.

Thanks !

Voss 2022-7-14

You're welcome!

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Answer 2

Benjamin Thompson 2022-7-14

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https://ww2.mathworks.cn/matlabcentral/answers/1760135-solved-already-loop-through-data-struct-with-variable-names#answer_1007495

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

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Stephen23 2022-7-14

在 MATLAB Online 中打开

A much simpler way to create that structure:

S(1).E1 = [1,1];
S(2).E1 = [0,1];

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Answer 3

Sokratis Panagiotidis 2022-7-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1760135-solved-already-loop-through-data-struct-with-variable-names#answer_1007505

在 MATLAB Online 中打开

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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[SOLVED ALREADY] Loop through data struct with variable names

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