# Jac might be unused, but I am using the Jac value.

2 views (last 30 days)
Victor Jimenez Carrillo on 15 Sep 2021
Answered: Voss on 18 Dec 2021
function Test2
clear
n=2;
Jac=zeros(n); fv=zeros(n,1); f=zero(n,1); Cd=zeros(n,1); C=zeros(n,1);
C=[0.3;0.5];
Ca0=1;V=100; Q=50; k1=1; k2=1; al=0.5;
Rel_dx=0.01;
itr_max=100; tol=10^-4;
for k=1:itr_max
Jac=J(C);
fv=fx(C);
f=-1.*fv';
d=Jac\f;
C=C+d;
Rerr=d./C;
max_err=max(abs(Rerr));
fprintf('CA1=%f CB=%f, CC=%f, CD=%f max_err=%f\n', C, max_err)
if max_err<tol; break; end
end
if k==itr_max
fprintf('Newtons Method Failed to converge in %d iterations\n', itr_max)
end
if i<itr_max
fprintf('The solution is CA1=%f and CA2=%f in %d iterations\n',C,k)
end
function Jac=J(C)
fv=fx(C);
for j=1:n
C(j)=C(j)*(1+Rel_dx);
for i=1:n
Jac(i,j)=(fvd(i)-fv(i))/C(j)/Rel_dx;
end
C(j)=C(j)/(1+Rel_dx);
end
end
function fv=fx(C)
fv(1)=Q*Ca0-Q*C(1)-k1*C(1)^2*(al*V);
fv(2)=Q*C(1)-Q*C(2)-k2*C(2)^2*(1-al)*V;
end
end
Rik on 16 Sep 2021
You also have a function with the same name as a variable.

Voss on 18 Dec 2021
The reason for the warning "Jac might be unused" is that Jac is initialized and then later overwritten, and it is not used for anything between the time it's initialized and the time it's overwritten.
It is true that Jac is also a variable in a nested function, which ordinarily is seen by the parent function, but in this case Jac is an output argument from the nested function, so the two Jac's are different. That is to say, your thinking would be right (i.e., the warning is mistaken) if Jac were not used as an output argument from the nested function J.
In this case, the warning is correct and initializing Jac to zeros up top has no effect.

R2020b

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