Error using sym>convertChar (line 1580) Character vectors and strings in the first argument can only specify a variable or number. To evaluate character vectors and strings re

I had been needed to used Marquardt's Method for Multiple Variable Functions, but the code of I search on online has a bit problems. which the code given as below:

%Marquardt's Method for Multiple Variable Functions 
 
clc; 
close all; 
%Insert starting points 
x01=0; 
x02=0; 
%Function F in Symbolic Format 
a='(x1^2+x2-11)^2+(x1+x2^2-7)^2'; 
%Insert the desired magnitude of final direction vector (0.0001 is fine) 
eserror=0.0001; 
Max_Iterations=20000; 
convergance_criterion=0.0001; 
 
k=0 
gamma=100 
 
%Leave the rest unchanged 
%================================= 
syms x1 x2 
syms alpha real %difining alpha as real prevents matlab to solve function with conj(alpha) 
 
% Hessian Of F 
 
A=[diff(diff(a,x1),x1),diff(diff(a,x1),x2);... 
    diff(diff(a,x2),x1),diff(diff(a,x2),x2)] 
%First Order Derivative of F 
 
B=[diff(a,x1);diff(a,x2)]; 
% pretty(simplify(A)) 
 
 
subs(subs(A,x1,x01),x2,x02) 
 
mag_of_B=sqrt(subs(subs(diff(a,x1),x1,x01),x2,x02)^2+subs(subs(diff(a,x2),x1,x01),x2,x02)^2) 
records=[]; 
while (mag_of_B>convergance_criterion)&(k<Max_Iterations) 
    k=k+1; 
    temp=([x01;x02]-inv(subs(subs(A,x1,x01),x2,x02)+gamma*eye(size(subs(subs(A,x1,x01),x2,x02))))*(subs(subs(B,x1,x01),x2,x02)))' 
    disp('x1 x2'); 
    [x01;x02] 
    disp('Hk') 
    subs(subs(A,x1,x01),x2,x02) 
    disp('gamm es'); 
    gamma*eye(size(subs(subs(A,x1,x01),x2,x02))) 
    disp('diff'); 
    (subs(subs(B,x1,x01),x2,x02)) 
 
    disp('------------'); 
    F_eval_old=subs(subs(a,x1,x01),x2,x02); 
    x01=temp(1,1); 
    x02=temp(1,2); 
    F_eval_new=subs(subs(a,x1,x01),x2,x02); 
    if F_eval_new<F_eval_old 
        gamma=.5*gamma; 
    else 
        gamma=2*gamma; 
    end 
    mag_of_B=sqrt(subs(subs(diff(a,x1),x1,x01),x2,x02)^2+subs(subs(diff(a,x2),x1,x01),x2,x02)^2) 
    records=[records;[k,x01,x02,subs(subs(a,x1,x01),x2,x02)]]; 
 
end 
records

The error might be due to the version I used is 2021b, but i didn't know which part should i changed to clear the error. this is the error of the code:

Error using sym>convertChar (line 1580)
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 1296)
        S = convertChar(x);

Error in sym (line 234)
                S.s = tomupad(x);

Error in sym/privResolveArgs (line 1016)
                    argout{k} = sym(arg);

Error in sym/diff (line 22)
args = privResolveArgs(S,varargin{:});

Error in ww (line 25)
A=[diff(diff(a,x1),x1),diff(diff(a,x1),x2);...

Answer 1

[Shivang]

Hi Tan,

I understand you're running into an error regarding the use of symbolic expressions in your code.

Note that you cannot directly define symbolic expressions as a string or character vector. The following line in your code causes the above error:

a='(x1^2+x2-11)^2+(x1+x2^2-7)^2';

To evaluate a string representing a symbolic expression, use the 'str2sym' function. Replace the above line in your code with:

a= str2sym('(x1^2+x2-11)^2+(x1+x2^2-7)^2'); 

You should no longer see any symbolic expression related errors in your code.

Hope this helps.

-Shivang