I am messing up Eigenvalues in a loop...Help me please.

clear

g=0.1; E0=9.8; A=1; B=0;

E = 5:0.01:15;

for i=1:length(E);

H=[E(i),g/sqrt(2),g/sqrt(2);g/sqrt(2),E0,A;g/sqrt(2),B,E0];

[EIGVEC,EIGVAL]=eig(H);

EIGVAL1(i)=EIGVAL(1); EIGVAL2(i)=EIGVAL(5); EIGVAL3(i)=EIGVAL(9); 

end

figure

plot(E,real(EIGVAL1),'g',E,real(EIGVAL2),'r',E,real(EIGVAL3),'b')

Matlab is calculating the eigenvalues for every i. The calculated valus are allright the problem is that i am not able to give them any order. If you watch the plot you can see that Matlab is switching between eigenvalue number 1,2,3.

If you want to see the final plot without mistakes i have made it allready with other methods, but i need the working code for following physics problems (and for learning how to Matlab of course).

Answer 1

[Philipp Marx]

Just replacing eig() with eigs() solves the problem.

[John D'Errico]

NO. IT DOES NOT SOLVE THE PROBLEM. Eigs is not designed to solve that problem. You got lucky, and the different order that eigs might return happens to work for you.

Answer 2

[John D'Errico]

Download eigenshuffle . It is designed to solve exactly this problem.

http://www.mathworks.com/matlabcentral/fileexchange/22885-eigenshuffle

[Philipp Marx]

How can i make this working with my code? I am a totally Matlab beginner. I dont get it running.