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

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Maisie Williams
Maisie Williams 2022-2-9
编辑： Stephen23 2022-2-10
Ran in:
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.
  1 个评论
Stephen23
Stephen23 2022-2-10
编辑：Stephen23 2022-2-10
"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

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Highphi
Highphi 2022-2-9
Ran in:
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

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