MATLAB: How to break from the following for loop when beta( J+1) become zero

Question

for j=2:inf
    w = A*V(:,j) - beta(j)*V(:,j-1);
    alpha(j) = w'*V(:,j);
    w = w - alpha(j)*V(:,j);
    beta(j+1) = norm(w,2);  
    V(:,j+1) = w/beta(j+1);
    loopcnt = loopcnt + 1;
end

Answer 1

Put this inside your loop:

if beta(j+1) == 0
    break
end

You might not want to check for exact equality, because of possible floating point error. Instead, you could check like this

if abs(beta(j+1)) < 1.e-8
    break
end

or use some other suitably small tolerance.