How can I use the data of a set of matrices labelled in order

Question

Christian 2024-7-10，20:56

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编辑： Matt J 2024-7-11，12:06

Hi, I managed to open all files in a folder which leaves me with the matrices data_1, data_2, data_3 .....data_n.

Now I want to add one of the row from each of the n matrices into one single set of data but I don't know how i can address the matrix in a for loop.

for i=1:1:n
    ag((((i-1)*n)+1):i*ni)=data'i';
end

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Walter Roberson 2024-7-10，21:58

在 MATLAB Online 中打开

files = dir('C:\Work\HumenTestsJune24\2023-Taifun-Sula\8-31\31\*.txt');
filenames = fullfile({files.folder}, {files.name});
for k = 1:length(files)
    Data{k} = load(filenames{k}, '-ascii');
end

Christian 2024-7-11，2:30

编辑：Christian 2024-7-11，2:30

Dear Walter,

thank you, this worked perfectly. I can adress the cells and take it from there!

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Answer 1

Matt J 2024-7-11，0:32

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编辑：Matt J 2024-7-11，0:34

在 MATLAB Online 中打开

data_1=10;
data_2=20;
data_3=30;
data_4=40;
data_5=50;
Data=arrayfun(@(i)evalin('caller', "data_"+i), 1:5,'uni',0)
Data = 1x5 cell array
    {[10]}    {[20]}    {[30]}    {[40]}    {[50]}

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Stephen23 2024-7-11，4:52

Note that the EVALIN documentation states "In most cases, using the evalin function is also less efficient than using other MATLAB functions and language constructs, and the resulting code can be more difficult to read and debug. Consider using an alternative to evalin."

The simpler and much more efficient alternative is to use indexing.

Matt J 2024-7-11，12:06

编辑：Matt J 2024-7-11，12:06

The point of the answer is not to be efficient, however. It is to undo the damage and convert the data to a form where it can be indexed.

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Answer 2

Stephen23 2024-7-11，4:50

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编辑：Stephen23 2024-7-11，6:26

在 MATLAB Online 中打开

"Thank you also for pointing about about dynamic variables, but as a beginner I am lacking options."

You decided to use LOAD, so one option is to read the LOAD documentation. At the bottom of the LOAD page are some links to some related topics**, including to this page:

https://www.mathworks.com/help/matlab/import_export/process-a-sequence-of-files.html

It shows the recommended approach using indexing. You should use indexing. Indexing is simple, reliable, and efficient. Indexing is a MATLAB superpower!

READMATRIX would likely be a better choice than LOAD. You could also use the same structure returned by DIR:

S = dir('C:\Work\HumenTestsJune24\2023-Taifun-Sula\8-31\31\*.txt');
S = natsortfiles(S); % optional, see text below
for k = 1:numel(S)
    F = fullfile(S(k).folder,S(k).name);
    S(k).data = readmatrix(F);
end

All of the imported filedata will be in the structure S. This has the benefit that the imported filedata are stored together with corresponding filenames in one array. You can trivially access it using indexing, for example the 2nd file:

S(2).name
S(2).data

If you expect the files to be processed in alphanumeric order (of their filenames) then you could download my function NATSORTFILES and use it to sort the directory S as shown:

https://www.mathworks.com/matlabcentral/fileexchange/47434-natural-order-filename-sort

** The MATLAB documentation contains a lot of information. The more you practice browsing it, the more you will be able to use it effectively!