Why do two functions in my plot "switch" positions when plotted?

Question

Donald 2015-9-25

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/244953-why-do-two-functions-in-my-plot-switch-positions-when-plotted

编辑： Walter Roberson 2015-9-25

Here is my code:

clear
clc
m1 = 1;
m2 = 2;
m3 = 1;
k1 = 2000:8000;
k2 = 2000;
k3 = 6000;
M = [ m1 0 0; 0 m2 0; 0 0 m3]
for n = 1:length(k1);
    K = [k1(n)+k2 -k2 0; -k2 k2+k3+3 -k3; 0 -k3 k3];
      EV = eig(-inv(M)*K);
      w1(n) = sqrt(abs(EV(1)));
      w2(n) = sqrt(abs(EV(2)));
      w3(n) = sqrt(abs(EV(3)));
end
figure(1)
plot(k1,w1/(2*pi),k1,w2/(2*pi),k1,w3/(2*pi))
legend w1 w2 w3
title('Varying k1')
ylabel('Frequency (Hz)')
xlabel('k1')

my plot looks like the picture attached. I cannot figure out why w2 and w3 seem to jump to one another after about 7000.

How can I resolve this?

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Answer 1

Steven Lord 2015-9-25

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https://ww2.mathworks.cn/matlabcentral/answers/244953-why-do-two-functions-in-my-plot-switch-positions-when-plotted#answer_193656

EIG doesn't guarantee it always returns the eigenvalues in any particular order. If you want them to always be returned in increasing order or always in decreasing order, SORT them.

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Donald 2015-9-25

I'm not too familiar with using the EIG function, after going back and using the sort function to obtain the correct corresponding eigenvalues, the plot was fixed.

Thank you!

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