Why am I getting error "Array indices must be positive" and error in syms, when using eval function

Why am I getting errors when using the eval function?

syms x 
f(x) = 3.5*x*(1-x)

f(x) = 

f2 = f(f(x))

f2 = 

eqn2 = f2 == x;
sol= (solve(eqn2,x))

sol = 

eval(f2(sol(2)))

Array indices must be positive integers or logical values.

Error in sym/subsref (line 997)
R_tilde = builtin('subsref',L_tilde,Idx);

I have used the same steps to evaluate f with no errors.

Any pointers would be really appreciated.

[Stephen23]

"Why am I getting errors when using the eval function? "

Because EVAL is completely the wrong tool for the job.

Note that EVAL is not listed anywhere in the Symbolic Toolbox documentation: https://www.mathworks.com/help/symbolic/referencelist.html?type=function

The correct use of EVAL is with MATLAB code, not with objects from the Symbolic Math Toolbox:

https://www.mathworks.com/matlabcentral/answers/470366-symbolic-logical-operation-can-t-get-right-answer#answer_382067

https://www.mathworks.com/matlabcentral/answers/1582094-symbolic-solver-dot-indexing-error-and-general-understanding#comment_1824909

https://www.mathworks.com/matlabcentral/answers/490227-matlab-using-maple-symbolic-engine-suddenly-and-oddly-don-t-run-in-certain-loop#answer_403358

https://www.mathworks.com/matlabcentral/answers/444183-converting-2x1-symbolic-array-to-2x1-numeric-matrix-in-matlab#answer_360376

https://www.mathworks.com/matlabcentral/answers/253859-solve-produces-solutions-one-some-computers-but-not-others#answer_199021

Answer 1

[HighPhi]

I think it's because you need to make f, f2, and eqn2 functions of x (even though I drop eqn2)

syms x f f2 % << here

f(x) = 3.5*x*(1-x) % and see f(x) instead of just f

f(x) = 

f2(x) = f(f(x)) % etc

f2(x) = 

% eqn2(x) = f2(x) == x %% not necessary
% sol = solve(eqn2, x) %% not necessary
sol = solve(f2(x) == x, x)

sol = 

eval(f2(sol(2)))

ans = 0.4286

[Maisie Williams]

Thanks so much!