Error in using eval with num2str

Question

Farid Khazaeli Moghadam 2022-8-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1784810-error-in-using-eval-with-num2str

编辑： Stephen23 2022-8-24

采纳的回答： Mathieu NOE

在 MATLAB Online 中打开

I want to rename a variable within the loop by using matlab standard eval and num2str functions.

Here is the line of the code I've written to do so:

eval(['DTU_10MW_RWT_s' num2str(seed) '_' num2str(wind) '_' num2str(I) '_' num2str(derate) ' = DTU10MWRWT;'])

When seed,wind, I, derate take integer values, the function gives what I want, but when these variables take rational noninteger values, it gives me this error:

Invalid expression. Check for missing multiplication operator, missing or unbalanced delimiters, or other syntax error. To construct matrices, use brackets instead of parentheses.

Any feedback is very much appreciated.

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Stephen23 2022-8-24

编辑：Stephen23 2022-8-24

"Any feedback is very much appreciated."

In general, will converting "rational noninteger values" to text produce valid variable names? (hint: no).

For example, is the text "1.23e-4" part of a valid variable name? (hint: no, the dot and minus are not permitted).

Your data design forces you into writing complex, buggy, slow, fragile, inefficient code.

Solution: do not force meta-data into variable names.

Meta-data is data. Data should be stored in variables, not in variable names.

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

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Answer 1

Mathieu NOE 2022-8-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1784810-error-in-using-eval-with-num2str#answer_1032050

在 MATLAB Online 中打开

hello

your method works only for integers

otherwise you have to round your numbers with a given number of decimals befoe it hets converted into char but the dot character is not supported in names so I changed it to a "d" character

see example below for "seed" (other parameters are integers here for sake of simplicity) :

DTU10MWRWT = rand(1);
seed = 3*rand(1);
seed_str = num2str(1+0.01*round(seed*100)); % value rounded with 2 decimals
seed_str = strrep(seed_str,'.','d');
out_name = ['DTU_10MW_RWT_s' seed_str '_' num2str(2) '_' num2str(3) '_' num2str(4)];
eval([out_name ' = DTU10MWRWT;'])
% %% alternative to eval 
% 
% variableCreator ( out_name, DTU10MWRWT )
% 
% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%  function variableCreator ( newVar, variable )
%    assignin ( 'caller', newVar, variable );
%  end
%  

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Mathieu NOE 2022-8-24

hi @Stephen23

ok - so this something I have to remember , tx !

a simple reaction when I saw the "eval" not-so-appreciated usage.

Stephen23 2022-8-24

编辑：Stephen23 2022-8-24

"a simple reaction when I saw the "eval" not-so-appreciated usage."

I wrote here about forcing meta-data into variable names, aka dynamic variable names. Variable names can be created dynamically using various methods and functions (e.g. EVAL, ASSIGNIN, LOAD, etc), and they all suffer exactly the same problems:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval#answer_293935

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