跪求 Lanczos算法的MatLAB程序。高手顺便帮我看看这个错误

[accboy]

自编程序
A=[2,0,1;0,-2,0;1,0,1];
q(1)=[0.707 0 -0.707]';
b(0)=0;
for j=1:2
a(1)=q(1)'*A*q(1);
r(j)=A*q(j)-a(j)*q(j)-b(j-1)*q(j-1);
if r(j)~=0;
b(j)=sqrt(sum(rj^2));
q(j+1)=r(j)*(1/b(j));
a(j+1)=q(j+1)'*A*q(j+1);
else
t=j;
break;
end
end
Q=zeros(n,t);
T=zeros(t,t);
for i=1:t
Q(:,i)=q(i);
T(i,i)=a(i);
T(i,i-1)=b(i-1);
T(i,i+1)=b(i);
end