Polyfit 4th degree unexpected behavior
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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)
回答（1 个）
Steven Lord
2020-7-29
2 个投票
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.
4 个评论
John D'Errico
2020-7-29
编辑：John D'Errico
2020-7-29
The data may be sordid, but it is definitely not sorted. :) In fact, I did check the file, and tp(:,1) is not in a sorted sequence.
Terence O'Brien
2020-7-29
Awesome, thanks so much.
The data is not sorted. I tried that with sort(tp) yesterday and I end up with this:
So now my data is inverted. I'm sure there is an easy fix... might you have the answer?
Terence O'Brien
2020-7-29
John Jr
2024-10-28
I have the same problem, were you able to solve it, how did you do it?
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2020-7-29
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