Error in using eval statement

Question

Ramesh Bala 2022-1-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1626230-error-in-using-eval-statement

编辑： Stephen23 2022-1-10

采纳的回答： Paul

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Why it shows an incomplete statement ?

I'm trying to write this statement using eval as follows : Energy_H_A1S1 = rms (H_A1S1); but in eval it shows error ?

Thanks in advance.

 eval(['Energy_H_A',num2str(1),'S',num2str(1),'  = rms(H_A',num2str(1),'S',num2str(1);]);

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Stephen23 2022-1-10

编辑：Stephen23 2022-1-10

"I'm trying to write this statement using eval as follows : Energy_H_A1S1 = ..."

And that is the start of your problems.

Forcing meta-data into variable names makes acessing your data complex, slow, inefficient, and buggy... as your question demonstrates. Meta-data is data. Data should be stored in variables, not in variable names.

Your poor data design forces you into writing bad code, which could be easily avoided by storing the meta-data correctly.

Ramesh Bala 2022-1-10

Thank you Stephen for your valuable reply.Yes that's right using names was wrong as it was indeed slow.

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Answer 1

Paul 2022-1-10

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https://ww2.mathworks.cn/matlabcentral/answers/1626230-error-in-using-eval-statement#answer_872065

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There's almost certainly a better way to organize the data and store the results, but if you must use eval:

['Energy_H_A',num2str(1),'S',num2str(1),'  = rms(H_A',num2str(1),'S',num2str(1);]
ans = 'Energy_H_A1S1  = rms(H_A1S1'

should be:

['Energy_H_A',num2str(1),'S',num2str(1),'  = rms(H_A',num2str(1),'S',num2str(1),');']
ans = 'Energy_H_A1S1  = rms(H_A1S1);'

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Paul 2022-1-10

Alternatives are dicussed in the link posted by @Walter Roberson in his comment.

Stephen23 2022-1-10

编辑：Stephen23 2022-1-10

"Could you please also post an example of the mentioned better way instead of eval as it takes high running time for this varying A1..4..S1..4 data"

The MATLAB introductory tutorials also show how to use MATLAB effectively by using arrays and indexing:

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

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Answer 2

Walter Roberson 2022-1-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1626230-error-in-using-eval-statement#answer_872060

Asking how to use eval without problems is like asking how to use a chainsaw without its chain guard. It would be irresponsible for us to answer in any way other than "Don't do that!"

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