MATLAB: I want to create a 12×6 Matrix from six 12×1 eigenvectors; what’s wrong with this code

eigenvectorfor loop

When I execute this code, instead of an intended 12×6 matrix "u" made from u(1), u(2), u(3), etc., I end up with a single 12×1 vector "u", as u gets overwritten each pass.
 for i = 1:12;
      o = omega(i);
M = [(-o^2*m(1)+k(1)+k(2)), -k(2), 0, 0, 0, 0; -k(2), (-o^2*m(2)+k(2)+k(3)), -k(3), 0, 0, 0; 0, -k(3), (-o^2*m(3)+k(3)+k(4)), -k(4), 0, 0; 0, 0, -k(4), (-o^2*m(4)+k(4)+k(5)), -k(5), 0;0, 0, 0, -k(5), (-o^2*m(5)+k(5)+k(6)), -k(6); 0, 0, 0, 0, -k(6), (-o^2*m(6)+k(6)+k(7))];
u(i)= eig(M);
end
where omega is a 12×1 vector.
What's wrong with the code?

Best Answer

I'm not sure how you avoided getting a dimension mismatch error there. The thing you're missing is a colon:
u = zeros(length(m),length(omega));
for i = 1:length(omega);
  o = omega(i);
  M = [(-o^2*m(1)+k(1)+k(2)), -k(2), 0, 0, 0, 0; ...
      -k(2), (-o^2*m(2)+k(2)+k(3)), -k(3), 0, 0, 0; ...
      0, -k(3), (-o^2*m(3)+k(3)+k(4)), -k(4), 0, 0; ...
      0, 0, -k(4), (-o^2*m(4)+k(4)+k(5)), -k(5), 0; ...
      0, 0, 0, -k(5), (-o^2*m(5)+k(5)+k(6)), -k(6); ...
      0, 0, 0, 0, -k(6), (-o^2*m(6)+k(6)+k(7))];
  u(:,i)= eig(M);
end
Note that I have included a few good programming practices: initializing omega, using length(omega) and length(m) in place of the meaningless numbers 12 and 6, and formatting the code so it is easy to understand.
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