Minimum and Maximum value of fitted curve

I am trying to find the minimum and maximum value of fitted curve. I will use the min and max point in another calculation. I used 'poly4' fit and its gives 4th order polynomial function. I tried polyder but couldnt go further. How can find the maximum and minimum point of fitted curve?

Answer 1

[Image Analyst]

So I assume you have the red, fitted curve since you plotted it (if not see first 2 lines below), so why can't you just simply do

coefficients = polyfit(x, y, 4); % fit to a 4th order polynomial.
yfit = polyval(coefficients, x); % Evaluate the fit at the same x values.
minFit = min(yFit); % Find min of fitted curve in the range we have fit it over.
maxFit = max(yFit); % Find max of fitted curve in the range we have fit it over.

If you want, you can use an x in polyval() that has finer resolution than the original x. Something like

numSamplePoints = 2000; % However many you need to get the resolution you want.
xFit = linspace(min(x), max(x), numSamplePoints);
yfit = polyval(coefficients, xFit); % Evaluate the fit at the same x values.

Of course you could get the exact value of the apex just by computing the derivative from the coefficients vector.

[Zack Trahem]

Thank you so much thats exactly what i looking for:) .just one more thing, that is coresponded x values of for min and max data.

https://www.mathworks.com/help/curvefit/evaluate-a-curve-fit.html i checked this one but nothing.

scatter(Bw(:,1),Bw(:,2))

f=fit(length,Bw(:,2),'poly4')

plot(f,length,w0)

c=coeffvalues(f);

yfit=polyval(c,length);

minfit=min(yfit)

maxfit=max(yfit)

for now i made this way and gives the values Thank you again.

[Zack Trahem]

I solved it thanks a lot!

minindex=find(yfit==minfit);

maxindex=find(yfit==maxfit);

maxPoint=xFit(minindex);

minPoint=xFit(maxindex);

Answer 2

[Torsten]

p = polyfit(x,y,4);
dp=polyder(p);
ddp=polyder(dp);
r=roots(dp);
r=r(abs(imag(r) ) < 1e-6);
maxima=r(polyval(ddp,r))<=1e-6 )
minima=r(polyval(ddp,r))>=1e-6 )

Extrema at the boundaries are not accounted for.

Answer 3

[Nipun]

Use the curve of best fit to find the maximum and minimum flow rates separately during the inflow and the outflow