Polyfit 4th degree unexpected behavior

Question

Terence O'Brien 2020-7-29

0
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评论： Terence O'Brien 2020-7-29

exampleData.mat

在 MATLAB Online 中打开

I'm getting strange results plotting a 4th degree polynomial.

Here is the resultant graph:

I don't understand why there appear to be 2 lines. The first degree polynomial is just a single line, and 2nd degree and beyond comes up with the two lines like this graph.

The data is a 420x3 matrix attached.

here is the code:

p1=polyfit(tp(:,1),tp(:,2),4);    %4th degree polynomial
f=polyval(p1,tp(:,1));
figure(1)
plot(tp(:,1),tp(:,2),'*',tp(:,1),f)

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Answer 1

Steven Lord 2020-7-29

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It looks to me like tp(:, 1) is not sorted. You can check this with issorted. If it's not sorted, MATLAB may draw a line from around x = 27 back to x = 13 then have to draw a straight line back to x = 25.

Try sorting tp(:, 1) before using it to evaluate the fitted polynomial and before plotting.