Polyfit and polyval in Matlab

6 views (last 30 days)
Emmanuelle on 13 Aug 2012
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.
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.

Accepted Answer

Image Analyst
Image Analyst on 13 Aug 2012
Edited: Image Analyst on 13 Aug 2012
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.
Emmanuelle on 23 Aug 2012
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:
  • plot(x2, nuevo); xt= 250:0.1:2500; yt= interp1(x2, nuevo, xt, 'spline'); plot(x2, nuevo, 'o', xt, yt);*

Sign in to comment.

More Answers (0)


Find more on Creating and Concatenating Matrices in Help Center and File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!