I modified the code more than once and every time there was an error, can someone tell me where the problem is and how to solve it?

function out = Crank_Niclson( D, u)

beta=0.5;

% where D = alpha * dt / dx^2

A=zeros(length(u),length(u));

B=u;

B(2:end-1)=B(2:end-1)*(1-2*(1-beta)*D);

for i=1:length(u)

%[I,J]=xxx(size,i);

I=i;

if i==1 ||i==length(u)

A(i,i)=1;

else

for j=1:length(u)

%[I1,J1]=xxx(size,j);

I1=j;

A(i,i)=1+2*beta*D;

if abs(I1-I) >1

A(i,j)=0;

elseif I1-I==+1

B(i)=B(i)+(1-beta)*D*u(j);

A(i,j)=-beta*D;

elseif I1-I==-1

B(i)=B(i)+(1-beta)*D*u(j);

A(i,j)=-beta*D;

end

end

end

end

out = (A\B')';

end

[Mathieu NOE]

hello

you have to post a fully working code and not only the function so we can better help you