Error using sym>convertChar (line 1448)

Question

Dursman Mchabe 2018-8-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/414641-error-using-sym-convertchar-line-1448

回答： ahmed 2024-5-21

采纳的回答： Tiasa Ghosh

SolveExample.m

Hi everyone,

I get the error message:

"Error using sym>convertChar (line 1448) Character vectors and strings in the first argument can only specify a variable or number. To evaluate character vectors and strings representing symbolic expressions, use 'str2sym'.

Error in sym>tomupad (line 1214) S = convertChar(x);

Error in sym (line 211) S.s = tomupad(x);

Error in SolveExample>TurnEqIntoDEq (line 43) depVarsLong(k) = sym([char(depVars(k)) '(' char(indepVar) ')']);

Error in SolveExample (line 21) [dEq3, initEq3] = TurnEqIntoDEq(Eq1, [a b c], t, 0);"

when I run the attached code. What can I do?

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Answer 1

Tiasa Ghosh 2018-8-13

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https://ww2.mathworks.cn/matlabcentral/answers/414641-error-using-sym-convertchar-line-1448#answer_332578

编辑：Tiasa Ghosh 2018-8-13

answer.m

You have used the function subs() for substitution in your function TurnEqIntoDEq . The input to subs (L47 in your code file) is a string. But the function 'subs' expects a symbolic variable or expression. So in the parent file, convert the Eq1 to symbolic string with str2sym and then pass it to TurnEqIntoDEq function. Also, use str2sym for depVarsLong(k) and depVarsShort(k) in TurnEqIntoDEq. I have done the same to the file attached.

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Dursman Mchabe 2018-8-13

Thanks a lot. The code your amendments worked. I truly appreciate.

Dursman Mchabe 2018-8-13

Thanks a lot. The code you uploaded works. I truly appreciate.

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Answer 2

ahmed 2024-5-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/414641-error-using-sym-convertchar-line-1448#answer_1461231

在 MATLAB Online 中打开

syms s U Y
% Define the Laplace transform of u(t) and y(t)
U = laplace(sym('u(t)', 't'), s);
Y = laplace(sym('y(t)', 't'), s);
% Define the differential equation in Laplace domain
eqn = 6*s^2*U - 3*U + Y/s == -3*s^3*Y - Y;
% Solve for the Laplace transform of the output Y(s)
Y_s = solve(eqn, Y);
% Define the Laplace transform of the input, U(s)
U_s = laplace(sym('u(t)', 't'), s);
% Calculate the transfer function H(s) = Y(s) / U(s)
H_s = simplify(Y_s / U_s);
% Display the transfer function
disp('Transfer Function:');
disp(H_s);