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

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

[Stephen23]

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

Do not do this.

Accessing lots of numbered variable names forces you into writing slow, complex, inefficient, obfuscated, insecure, buggy code:

https://www.mathworks.com/matlabcentral/discussions/tips/849901-tutorial-why-variables-should-not-be-named-dynamically-eval

Best avoided.

So far you did not tell us the most important information: how did you get all of those variables into the workspace? That would be the best place to fix that bad data design. For example, rather than LOADing directly into the workspace, always LOAD into an output variable (which is a scalar structure):

S = load(..)

and then access the field/s of that structure. Once you explain exactly how you imported that data, then we can start to show you how to write neat & efficient code to process it.

[Christian]

Dear Stephen,

thank you for your swift reply.

I got the data into the workspace by loading all .txt files from the folder:

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

Thank you also for pointing about about dynamic variables, but as a beginner I am lacking options. I have data from a montitoring system which saves a file every 20 mins (so the amount of lines is always the same). The file contains 20 rows and I want to combine lets say row 4 from each file.

So my idea was top operate with the amount of files per folder.

[Walter Roberson]

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]

Dear Walter,

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

Answer 1

[Matt J]

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]}

[Stephen23]

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]

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.

Answer 2

[Stephen23]

"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!