Extracting data from multiple structures in a .mat file

Hello,

I have some data which is in nested structures names an example structure is below.

Signal_00.y_values.values

I want to extract this data using a foor loop and place it all in m x n array when m is the original structure, and n the number of points. Here's where I got to:

sigName = {'Signal_00';'Signal_01';'Signal_02';'Signal_03'...}
for k = 1:24
    tData(k,:) = sigName(k).y_values.values
end

The error I recieve in command window when I try to open the data is:

>> sNames(1).y_values.values
Dot indexing is not supported for variables of this type.

What is the correct way to generate a structure path which can be used in the for loop?

[KSSV]

First of all: does this code work:

sigName = {'Signal_00';'Signal_01';'Signal_02';'Signal_03'...}
for k = 1:24
    tData(k,:) = sigName(k).y_values.values
end

[Stephen23]

@KSSV: "First of all: does this code work:"

sigName = {'Signal_00';'Signal_01';'Signal_02';'Signal_03'...}
for k = 1:24
    tData(k,:) = sigName(k).y_values.values
end

if sigName is a cell array of character vectors, then sigName(k) will return a scalar cell array... and what do you expect the dot notation to achieve with a scalar cell array?

[John Doe]

Nope!

Answer 1

[Stephen23]

Do NOT load directly in the workspace.

If you do that, then you will force yourself into writing slow, complex, buggy code by trying to access the variable names dynamically. Read this to know why this is a bad approach to writing code:

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

The solution is very simple: always load into an output variable (which is a scalar structure):

S = load(...)

The variables in the .mat file are the fields of that scalar structure. You can trivially and efficiently iterate over the fields of that structure:

F = fieldnames(S);
for k = 1:numel(F)
     S.(F{k}).y_values.values
end

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

Note that if you have multiple .mat files then your code would also be much simpler if each .mat contained variables with exactly the same names. Having lots of numbered variables is a sign that you are doing something wrong (i.e. you should be using indexing).

[John Doe]

I have read that, for the sake of this question I didn't think it was relevant my code, I wrote the code specifically for this question. I still have the same question.

How do I generate the structure path in a for loop to export this multiple times?

For one structure I could do this:

data(1,:) = S.Signal_00.y_values.values

However I would like to access multiple signals from that structure using a for loop.

[Stephen23]

"For one structure I could do this:"

Yes, that works fine... but that is NOT what I told you to do.

"However I would like to access multiple signals from that structure using a for loop. "

Did you read my answer and see the code where I showed you exactly how to do that?

Here is the code again, in case you missed it the first time:

F = fieldnames(S);
for k = 1:numel(F)
     S.(F{k}).y_values.values
end

And here is the very informative link again too:

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

[John Doe]

Edit made a typo. This works - apologies for confusion.

Thank you

[Stephen23]

F is a cell array of fieldnames, you need to use idexing, e.g.

S.(F{k}).y_values.values
%   ^^^ do not forget the index!

[John Doe]

Precisely what I forgot! Thank you