Subs function not working as expected

Question

Patricio Whittingslow 2018-9-25

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https://ww2.mathworks.cn/matlabcentral/answers/420658-subs-function-not-working-as-expected

评论： Patricio Whittingslow 2018-9-25

I've come across an unexpected result. Symbolic equation substitution is yielding wrong result. Below is my code.I've tried reset(symengine) but it doesen't seem to help.

syms y real
Tsym = ...
(204800*y)/7 - (19342813113834066795298816*y^2)/661131307601750375 + 60;
Tfunc=@(y) ...
(204800*y)/7 - (19342813113834066795298816*y^2)/661131307601750375 + 60;
H=0.0005;
y=H;
symval=subs(Tsym);
funcval=Tfunc(H);
if funcval~=symval
    fprintf('Something is wrong. \n')
end

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Answer 1

Bish Erbas 2018-9-25

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Code below works for me. You defined the H and y variables after defining the symbolic equations. Move the variable definition at the beginning of your code.

syms y real
H=0.0005;
y=H;
Tsym = ...
(204800*y)/7 - (19342813113834066795298816*y^2)/661131307601750375 + 60;
Tfunc=@(y) ...
(204800*y)/7 - (19342813113834066795298816*y^2)/661131307601750375 + 60;
symval=subs(Tsym);
funcval=Tfunc(H);
if funcval~=symval
    fprintf('Something is wrong. \n')
end

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Patricio Whittingslow 2018-9-25

Hey there Bish! It seems your solution does return the correct value. However, I lose the symbolic function in the process. Tsymb (the equation is now equal to a value, not an equation). What's the best practice to use subs if I want to continue evaluating the same function for N values of y?

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