Use a ready p-file to operate on matrices

Question

Omar Morsy 2021-12-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1603460-use-a-ready-p-file-to-operate-on-matrices

编辑： Stephen23 2021-12-5

I have 345 matrices(S). So, I have from S1 to S345. I have a ready pfile function that operates on S. I want to make a loop to make that function work on each matrix seperately and give me 345 answers.

Lets say the pfile function is called Areas. So, I want to make something like this:

i = 1:345

Area = Areas(S(i))

end

to get the Area of each matrix from S1 to S345.

And to create the 345 matrices I used the following code(which is not mine).

S = ones(1,345);

tfile = tempdir + "assign_S.m";

fid = fopen(tfile, 'w');

for K = 1 : 345

fprintf(fid, 'S%d = S; S%d(1,%d) = 2;\n', K, K, K);

end

fclose(fid);

run(tfile)

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Stephen23 2021-12-5

"So, I have from S1 to S345."

And that is the start of your difficulties.

Using numbered variables is a sign that you are doing something wrong. Read this to know why:

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

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Answer 1

Stephen23 2021-12-5

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1603460-use-a-ready-p-file-to-operate-on-matrices#answer_847970

编辑：Stephen23 2021-12-5

在 MATLAB Online 中打开

Your approach is complex, obfuscated, and inefficient.

Using numbered variables is a sign that you are doing something wrong. Read this to know why:

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

The MATLAB approach is to use simple and efficient indexing, for example using a cell array:

N = 345;
C = cell(1,N)
for k = 1:N
    S = ones(1,N);
    S(k) = k;
    C{k} = Areas(S);
end

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

Because your data are all row vectors you could also do something similar with a simple numeric matrix:

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

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Omar Morsy 2021-12-5

编辑：Omar Morsy 2021-12-5

Yes I saw the cell array C but i am really new to matlab and thats why I do not quite understand it. But when I write your new code. I get Unrecognized function or variable 'A'.

I guess you wanted to write something else there?

I just want to add something. To use the ready function that is given to me. V should be a 1x345 matix(using the notation you used in the last comment).

As I said before. I want to make 345 matrices of S. Matrix S=ones(1x345) and I want to change the first value only of S, which is S(1,1) = 2 then save that matrix seperately lets say as S1(which has all values as 1 except S(1,1)). And after that, change the value only of S(1,2) = 2 and also save that matrix as lets say S2. And so on. Till I have S345 which has all values as 1 except S(1,345) as 2.

And after doing that. I want to use the ready pfile (function) to calculate the area of the 345 matices(S1:S345) that are modified from S.

hope I managed to explain what I want.

Thank you a lot in advance

Stephen23 2021-12-5

编辑：Stephen23 2021-12-5

在 MATLAB Online 中打开

"I get Unrecognized function or variable 'A'. I guess you wanted to write something else there?"

Yes, that was a spelling error: the line should be:

V = S;

"As I said before. I want to make 345 matrices of S...."

And that is what my code does: did you look inside the cell array yet?

Using indexing is how MATLAB works. If you want to use MATLAB, then you need to learn how to use indexing. You should do the Introductory Tutorials here, which teach very basic MATLAB concepts like how to access data in a loop using indexing:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

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