Polynomial Approximation, is it possible in matlab?
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I have four points on the graph with the following coordinates.
x1 - 1
y1 - 3.5
x2 - 2
y2 - 14/3
x3 - 3
y3 - 14
x4 - 4
y4 - 28
Is it possible using the Lagrange approximation polynomial coefficient calculation method to find the polynomial / function given by the four points?
I don't know the algorithm very well and I don't have the strongest matlab knowledge. I'm learning.
Thank you.
MATLAB Version: 8.5.0.197613 (R2015a)
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Answers (1)
Luna
on 3 Jan 2019
Edited: Luna
on 3 Jan 2019
Hi,
Try below (it uses least squares):
For lagrange you can look at that link:
x1 = 1;
y1 = 3.5;
x2 = 2;
y2 = 14/3;
x3 = 3;
y3 = 14;
x4 = 4;
y4 = 28;
x1Array = [x1,x2,x3,x4];
y1Array = [y1,y2,y3,y4];
n = 1; % polynomial degree (you can change it as you wish)
p = polyfit(x1Array,y1Array,n); % p is coefficient of your polynomial: P(X) = P(1)*X^N + P(2)*X^(N-1) +...+ P(N)*X + P(N+1) descending order.
newY = polyval(p,x1Array); % function results
plot(x1Array,y1Array, 'bo',x1Array,newY,'-r');
grid;
legend('Data','Fitted Data');
11 Comments
Torsten
on 4 Jan 2019
But you know that using the interpolating spline method to calculate the coefficients is not what you are supposed to do in your homework (it says something about "Lagrange interpolation polynomial", doesn't it) ??
Luna
on 4 Jan 2019
Yes, it says lagrange that's why I gave the lagrangepoly link from fileexchange in my answer.
Please check the file and use. It does the same thing with only Lagrange method.
It also explains with examples.
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