Undefined function 'int' for input arguments of type 'char'. keep getting error using matlab2015b

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Brenda Galabe
Brenda Galabe on 8 Dec 2018
Edited: Brian Hart on 8 Dec 2018
eqa= 'diff(y(t),t)+y(t)-cos(t)';
neweq1a = laplace(eqa,t,s)
neweq1i = laplace(int('y(x)','x','0','t'),t,s)
neweq1 = neweq1a + neweq1i;
neweq2 = subs(neweq1,'y(0)',0)
lneweq2 = subs(neweq2,'laplace(y(t),t,s)',Ys)
soln = solve(lneweq2,Ys)
ysol = ilaplace(soln,s,t)

Answers (2)

Walter Roberson
Walter Roberson on 8 Dec 2018
You probably do not have the Symbolic Toolbox installed.
Note that that code would not work in current versions of MATLAB (R2018a and later.) It should be modernized even for your version.
syms s x y(t) Ys
dy = diff(y(t), t);
eqa= dy + y(t) - cos(t);
neweq1a = laplace(eqa, t, s)
inty = int(y(x), x, 0, t);
neweq1i = laplace(inty, t, s)
neweq1 = neweq1a + neweq1i;
neweq2 = subs(neweq1, y(0), 0)
lapy = laplace(y(t), t, s)
lneweq2 = subs(neweq2, lapy, Ys)
soln = solve(lneweq2, Ys)
ysol = ilaplace(soln, s, t)
This still requires the Symbolic Toolbox, but has been rewritten to avoid needing to process character strings.

Brian Hart
Brian Hart on 8 Dec 2018
Edited: Brian Hart on 8 Dec 2018
In the first line you define an eqution as a text string (in quotes). Then in the second line you pass that text string into the laplace function, which expects a symbolic function. See the help pages for laplace and syms.
I think your specified error comes from the third line. The function int also wants a symbolic function, not text (which is of type 'char').

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