Subs function not working as expected

11 vues (au cours des 30 derniers jours)
Patricio Whittingslow
Patricio Whittingslow le 25 Sep 2018
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

Réponses (1)

Bish Erbas
Bish Erbas le 25 Sep 2018
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
  1 commentaire
Patricio Whittingslow
Patricio Whittingslow le 25 Sep 2018
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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