Storing a vector in a loop each time with a different name

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Siddhant Chandra 2018-8-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/414424-storing-a-vector-in-a-loop-each-time-with-a-different-name

评论： Siddhant Chandra 2018-8-10

for k = 1:10
    FileName1 = ['EqvStr_',num2str(k),'.csv'];
    filename1 =fullfile('E:\ANSYS_FILES\BigKB', FileName1);
    A = xlsread(filename1,'A2:E1000');
    vectorname = ['a',num2str(k)];
    vectorname = A(:,1);           
end
    disp(a1)
    disp(a2)
    disp(a3)
    disp(a4)
    disp(a5)
    disp(a6)
    disp(a7)
    disp(a8)
    disp(a9)
    disp(a10)

So I am running the above code and MATLAB gives me an error message saying "Undefined function or variable 'a8'". It is weird because it IS showing me the results if I comment out the a8, a9, and a10 lines from the code but gives me the error message if not. There are a 1090 files in the folder in that name format and I am just testing the code with the first 10 but it is showing me this error. Can anyone please help me understand what the problem here is?

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Stephen23 2018-8-10

"Can anyone please help me understand what the problem here is?"

Bad code design. Much better code design would simply use simpler, more efficient indexing with a cell array, exactly as the MATLAB documentation shows:

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

What you are doing will force you into writing slow, buggy, complex code which is harder to debug:

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

Siddhant Chandra 2018-8-10

Thanks Stephen! I will improve it.

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Answer 1

James Tursa 2018-8-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/414424-storing-a-vector-in-a-loop-each-time-with-a-different-name#answer_332362

编辑：James Tursa 2018-8-10

在 MATLAB Online 中打开

This is not the best way to do this in MATLAB. There are many, many posts on this site explaining why. Instead, you should use cell arrays. E.g.,

A{k} = xlsread(filename1,'A2:E1000');

Then downstream in your code simply use A{k} instead of the a1, a2, etc that you are currently using. See this link:

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

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Stephen23 2018-8-10

编辑：Stephen23 2018-8-10

在 MATLAB Online 中打开

@Siddhant Chandra: try it and find out: assuming that C does not exist, try this:

C{4} = 1

What does it create?

But your question is actually a good one, because to make the code clear, efficient, and to prevent interference from existing variables you should preallocate the cell array before the loop, like this:

N = 10;
C = cell(1,N); % preallocate!
for k = 1:N
    ...
    C{k} = xlsread(...);
end

Then it is unambiguous what size C has, and what the indexing refers to. Note that the MATLAB documentation clearly shows the cell array being preallocated before the loop:

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

You can read about array preallocation here:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

https://www.mathworks.com/help/matlab/matlab_prog/preallocate-memory-for-a-cell-array.html

Using clear is not recommended for this, it is better to preallocate.

Siddhant Chandra 2018-8-10