syntax issues when adapting an old code from 2009a

Dear All,

I'm currently struggeling with adapting a short code from the book "Modeling of Curves and Surfaces with MATLAB" from 2010 based on Matlab 7.8 Release 2009a. I would like it to run under Matlab 2020a. The function in the book is presented as:

function f = sgeod(r)

syms u v;

ru = diff(r, u); rv = diff(r, v); % these 2 lines

E = ru*ru’; F = ru*rv’; G = rv*rv’; % can be replaced by

% S=EFG(r); E = S(1); F = S(2); G = S(3); % the following line:

g = E*G - F*F;

Eu = diff(E, u); Ev = diff(E, v);

Fu = diff(F, u); Fv = diff(F, v);

Gu = diff(G, u); Gv = diff(G, v);

f(1)= simplify(-subs((G*Eu - F*(2*Fu - Ev))/(2*g)*’x(3)*x(3)’+ (G*Ev - F*Gu)/

g*’x(3)*x(4)’ +G*(2*Fv - Gu) - F*Gv)/(2*g)*’x(4)*x(4)’, {u v}, {’x(1)’, ’x(2)’}));

f(2) = simplify(-subs((E*(2*Fu -Ev) -F*Eu)/(2*g)*’x(3)*x(3)’+ (E*Gu - F*Ev)/

g*’x(3)*x(4)’+(E*Gv - F*(2*Fv - Gu))/(2*g)*’x(4)*x(4)’, {u v}, {’x(1)’, ’x(2)’}));

end

It would be great if someone could help me with what would be the correct sytax for this code for matlab 2020a

thanks a lot and best greetings

Andreas

[Jan]

Most of all replace the ’ (char(8217)) characters by quotes: ' (char(39)) .

Then try it again. If you get an error message, post a copy of the complete message and explain, what you use as input r.

[Andreas Roschger]

Thanks for the quick reply, i now replaced the characters and added the definition of "r"

the code now reads:

syms u v;

r = [(5 + cos(u))*cos(v), (5 + cos(u))*sin(v), sin(u)];

ru = diff(r, u); rv = diff(r, v); % these 2 lines

E = ru*ru'; F = ru*rv'; G = rv*rv'; % can be replaced by

% S=EFG(r); E = S(1); F = S(2); G = S(3); % the following line:

g = E*G - F*F;

Eu = diff(E, u); Ev = diff(E, v);

Fu = diff(F, u); Fv = diff(F, v);

Gu = diff(G, u); Gv = diff(G, v);

f(1) = simplify(-subs((G*Eu - F*(2*Fu - Ev)) /(2*g)*'x(3)*x(3)'+ (G*Ev - F*Gu)/g*'x(3)*x(4)'+(G*(2*Fv - Gu) - F*Gv)/(2*g)*'x(4)*x(4)', {u v}, {'x(1)', 'x(2)'}));

f(2) = simplify(-subs((E*(2*Fu -Ev) -F*Eu) /(2*g)*'x(3)*x(3)'+ (E*Gu - F*Ev)/g*'x(3)*x(4)'+(E*Gv - F*(2*Fv - Gu))/(2*g)*'x(4)*x(4)', {u v}, {'x(1)', 'x(2)'}));

the syntax for x seems strage to me.

when i try to run that, the error message reads:

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 sym/privResolveArgs (line 1005)

argout{k} = sym(arg);

Error in sym/privBinaryOp (line 1029)

args = privResolveArgs(A, B);

Error in * (line 320)

X = privBinaryOp(A, B, 'symobj::mtimes');

Error in test123 (line 13)

f(1) = simplify(-subs((G*Eu - F*(2*Fu - Ev)) /(2*g)*'x(3)*x(3)'+ (G*Ev - F*Gu)/g*'x(3)*x(4)'+(G*(2*Fv - Gu) - F*Gv)/(2*g)*'x(4)*x(4)', {u v}, {'x(1)', 'x(2)'}));

Answer 1

[Navya Singam]

Hi,

In this line of code,

f(1) = simplify(-subs((G*Eu - F*(2*Fu - Ev)) /(2*g)*'x(3)*x(3)'+ (G*Ev - F*Gu)/g*'x(3)*x(4)'+(G*(2*Fv - Gu) - F*Gv)/(2*g)*'x(4)*x(4)', {u v}, {'x(1)', 'x(2)'}));

"subs" function expects the first argument to be symbolic expression or symbolic function. But 'x(3)*x(3)' in first argument is of the type char, to convert it into symbolic expression you can use the str2sym function.