Help fixing old Lagrange equation solver - updates have stopped "sym" being able to work on expressions but I don't know how to fix.

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Dylan 2023-7-25

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https://ww2.mathworks.cn/matlabcentral/answers/2000348-help-fixing-old-lagrange-equation-solver-updates-have-stopped-sym-being-able-to-work-on-expressi

回答： Cris LaPierre 2023-7-25

Hi, I am trying to get a MatLab example of contorl for an inverted pendulum running, but the project is quite old and seems to have been broken by an update. The code used to solve the Lagrangian no longer functions, and throws the error below when I try and run it:

I have done some searching, and found references to this problem, but no solutions that I could implement. I was wandering if someone could help me get the EulerLagrange function working again in R2022b? Sorry for the fuss I am relatively new to MatLab.

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Dylan 2023-7-25

I tried to use str2sym myself, changing the qt = arrayfun(...) line to use that instead. It caused the same issue elsewhere in the code, but not a sym() command in the file but somewhere down in the subs() command. I realize I forgot to attach the files themselves, which I will add here if anyone wants to take a look.

Steven Lord 2023-7-25

Please show us the full and exact error message that occurs after you made that modification in your sym calls in your code.

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Answer 1

Cris LaPierre 2023-7-25

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Not sure if the answer is correct, but the errors are removed by

using str2sym
changing '[' to '('

(ln 79)> qt   = arrayfun(@(ii) symfun(str2sym(['q' int2str(ii) '(t)']),t),1:numcoor,'UniformOutput',false);
(ln 115)> VF = subs(VF,str2sym(['Z(',int2str(indqqd(jj)),')']),str2sym(['Y(',int2str(jj),')']));
(ln 152)> Y  = arrayfun(@(ii) str2sym(['Y(' int2str(ii) ')']),1:2*numcoor,'UniformOutput',false);