Error using sym>convertChar (line 1537) when solving an ODE using laplace transform. How to fix the problem?
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Konard Adams
am 30 Apr. 2021
Bearbeitet: Walter Roberson
am 17 Mai 2021
%%CODE
%% Solving 2nd order ODE using laplace transform
clc, clear , close all
syms x t s X F
F = laplace('diff(x(t),t,t)+7*diff(x(t),t)+10*x(t)= 20',s); % solving using laplace transform
F = subs(F,{'x(0)','D(x)(0)'},{5,3}); % initial values
F = subs(F,{'laplace(x(t),t,s)'},{X}); % substituting the initial values then solve Laplace
X = solve(F,'X');
X = ilaplace(X);
X = simplify(X); pretty(X);
disp(X)
%% ERROR BELOW
Error using sym>convertChar (line 1537)
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 1253)
S = convertChar(x);
Error in sym (line 220)
S.s = tomupad(x);
Error in transform (line 22)
if ~isa(f, 'sym'), f = sym(f); end
Error in sym/laplace (line 28)
L = transform('symobj::laplace', 't', 's', 'z', F, varargin{:});
Error in HW_1_3_2_4070H300 (line 4)
F = laplace('diff(x(t),t,t)+7*diff(x(t),t)+10*x(t)= 20',s); %
solving using laplace transform
2 Kommentare
Walter Roberson
am 15 Mai 2021
Since R2017b (I think it is) you cannot pass character vectors to laplace() . You need to construct the symbolic equation and pass that instead.
Akzeptierte Antwort
Star Strider
am 15 Mai 2021
Eliminate the single quotes, use double equal signs in the symbolic expression, express ‘x’ as ‘x(t)’ in the syms declaration (otherwise, ‘x’ is assumed to be a constant), and it works —
syms x(t) t s X F
Dx = diff(x);
D2x = diff(Dx);
F = laplace(D2x+7*Dx+10*x(t) == 20,s); % solving using laplace transform
F = subs(F,{x(0),Dx(0)},{5,3}); % initial values
F = subs(F,{laplace(x(t),t,s)},{X}); % substituting the initial values then solve Laplace
X = solve(F,X);
X = ilaplace(X);
X = simplify(X); pretty(X);
disp(X)
Character arrays have not been allowed for the last few releases. (The one exception that remains is in the sym funciton.)
4 Kommentare
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Walter Roberson
am 15 Mai 2021
Bearbeitet: Walter Roberson
am 15 Mai 2021
%%CODE
%% Solving 2nd order ODE using laplace transform
syms x(t) s X F
Dx = diff(x,t);
D2x = diff(Dx,t);
eqn = D2x + 7*Dx + 10*x(t) == 20
F = laplace(eqn,s) % solving using laplace transform
F = subs(F,{x(0), Dx(0)},{5,3}) % initial values
F = subs(F, {laplace(x(t),t,s)},{X}) % substituting the initial values then solve Laplace
X = solve(F, X)
X = ilaplace(X)
2 Kommentare
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