How to convert strings to symbolic expressions?

Here is a function I defined:

function [X]=Taylor(a,b,h,p,m)
n=(b-a)/h;
X=zeros(1,n+1);
X(1)=p;
A=sym(zeros(1,m));
syms t;
f=str2sym('t+x(t)^2');
A(1)=f;
df=f;
for i=2:m
    df0=diff(df,t);
    df=subs(df0,'diff(x(t),t)','t+x(t)^2');
    A(i)=df;
end
A=subs(A,'x(t)','y');
for i=1:n
    B=A;
    B=subs(B,'y',X(i));
    B=subs(B,'t',a+(i-1)*h);
    C=zeros(1,m+1);
    for k=1:m
        C(k+1)=B(k)*h^k/factorial(k);
        C(1)=C(1)+C(k+1);
    end
    X(i+1)=X(i)+C(1);
end

end

When I tried to use this function, however, I got an error message. Here is the error message:

>> [X]=Taylor(a,b,h,p,m)
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/subs>inputchk (line 249)
      (length(sym(x))~=1 || length(sym(y))~=1) && isvarname(char(x))

Error in sym/subs>mupadsubs (line 154)
error(inputchk(X,Y));

Error in sym/subs (line 145)
    G = mupadsubs(F,X,Y);

Error in Taylor (line 17)
    df=subs(df0,'diff(x(t),t)','t+x(t)^2');

In line 17, I tried everything to fix this error, but none of them works. I guess I should convert my string expression to symbolic expression somewhere, but I don't know how to do. Please, can anyone help me? Thank you so much!

[Stephen Cobeldick]

" I guess I should convert my string expression to symbolic expression somewhere, but I don't know how to do."

The list "Create Symbolic Variables, Expressions, and Functions" here:

https://www.mathworks.com/help/symbolic/symbolic-variables-expressions-and-functions.html?s_tid=CRUX_lftnav

contains a link to this function:

https://www.mathworks.com/help/symbolic/str2sym.html

Did you try it?

Answer 1

[Shubham Rawat]

Hi Renjie Xiong,

You are right the error which is thrown is due to the string, so you may change the string to symbolic expression by this,

df = subs(df0, str2sym('diff(x(t),t)'), str2sym('t+x(t)^2'));

[Renjie Xiong]

It works! Thank you so much!