while loop not performing

A=[-0.2 0.1; 0.1 -2];
vm=[1; 1];

while (1)
    term=A*vm;
    nterm=norm(term);
    vm1=term/nterm;
    ea=abs((vm1-vm)/vm1)*100;
    vm=vm1;
    if ea<=.0001, break, end
end

disp(vm1)

I wrote this code to perform the power method for finding eigenvectors. The while loop is not working though. It only goes through the program once. Does it have to do with the fact it's dealing with arrays? (Also, if you see a problem with this in regards to the power method feel free to correct that as well.)

Answer 1

[Matt J]

A=[-0.2 0.1; 0.1 -2];
vm=[1; 1];

vm=vm/norm(vm);
while (1)
    term=A*vm;
    
    nterm=norm(term);

    ea= nterm-abs(dot(term,vm));

    if ea<=1e-6, break, end
    
     vm=term/nterm;
     
end

eigval=vm\(A*vm)

eigval = -2.0055

eig(A)

ans = 2×1

-2.0055 -0.1945

Answer 2

[Walter Roberson]

ea=abs((vm1-vm)/vm1)*100;

Are you certain that you want to use matrix right divide between two vectors that are each 2x1 ? The results would be 2x2 and when the fit is perfect the bottom right result would be 1.0

Perhaps you want element by element division, which is the ./ operation

Answer 3

[Image Analyst]

Not sure what you're after but it never gets to the break and doesn't go through the loop once. In fact it gets in an infinite loop. I applied a failsafe to fix that but I still don't know when you'd ever expect it to hit the break.

A = [-0.2 0.1; 0.1 -2];
vm = [1; 1];
loopCounter = 1;
maxIterations = 1000;
while loopCounter < maxIterations
    term = A * vm; % A 2x1 column vector.
    nterm = norm(term); % A scalar
    vm1 = term/nterm; % A 2x1 column vector.
    ea = abs((vm1 - vm) / vm1) * 100;  % A 2x1 column vector.
    vm = vm1;	 % A 2x1 column vector.
    if ea <= 0.0001  % Huh???!!!  You're comparing two values in a column vector to a scalar.
        fprintf('Hit the break!\n');
        break % Never gets here.
    end
    loopCounter = loopCounter + 1;
end
if loopCounter == maxIterations
    fprintf('Loop exited early because of failsafe - hit max number of iterations (%d).\n', maxIterations)
else
    fprintf('Loop broke after %d iterations (%d).\n', loopCounter)
end

Loop exited early because of failsafe - hit max number of iterations (1000).

% Show values
ea

ea = 2×2

0 11.0770 0 200.0000

vm1

vm1 = 2×1

0.0553 -0.9985