Polyfit and polyval in Matlab

Question

Emmanuelle 2012-8-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/45881-polyfit-and-polyval-in-matlab

Hi!I have a problem with polyfit and polyval. I have two vectors (x and y). You can see it in the attached image (the red line). I want to create a polynom that allow to create the blue line (imagine that it doesn't exist) only with the values that I have.

http://imageshack.us/photo/my-images/6/graphrw.jpg/

I've tried with polyfit and polyfit but it doesn't work at all.

x2=x'; %I have changed the direction because if not, it doesn't do the polyfit.
coeffs1=polyfit(x2, msensiv_graf,1);
coeffs3=polyfit(x2, msensiv_graf,3);
coeffs5=polyfit(x2, msensiv_graf,5);
coeffs12=polyfit(x2, msensiv_graf,12);
new_y1 = polyval(coeffs1, x2);
new_y3 = polyval(coeffs3, x2);
new_y5 = polyval(coeffs5, x2);
new_y12 = polyval(coeffs12, x2);
   plot(x2, msensiv_graf, 'r');
   hold on
   plot(x2, new_y1, 'r');
   plot(x2, new_y3, 'b');
   plot(x2, new_y5, 'g'); 
   plot(x2, new_y12, 'y');

And the error is:

Warning: Polynomial is badly conditioned. Add points with distinct X values, reduce the degree of the polynomial, or try centering and scaling as described in HELP POLYFIT.

Does somebody know what can I do? I also think to try interpolation to get the blue line.

Thanks a lot in advance.

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Answer 1

Image Analyst 2012-8-13

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编辑：Image Analyst 2012-8-13

Try using the S and mu outputs of polyfit(), like it (almost) says. I once had the same error and that helped me. Just get them and pass them into polyval() - it was as simple as that.

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Image Analyst 2012-8-23

Did you read my prior comment about supplying input data we can work with?

Emmanuelle 2012-8-23

Image Analyst, thanks for your reply. Yes,I've changed but it doesn't work at all. Now I'm trying directly with interpolation... (this is a capture of my result)

And this is what I want to get, like the example in Matlab's web:

My code: