How to use polyfit() function in Matlab?

Question

First of all i am new to Matlab, and not sure whether to use polyfit or something else for my problem.

I plotted a graph with the following matlab code:

$t=transpose(linspace(-1,1,50))$
$y=1./(1+25*t.^2)$
$n=10$
$A=fliplr(vander(t))$
$A=A(:,1:n)$
$x=A$\y
$u=linspace(-1,1,1000)$
$g=x(n)*ones(1,1000)$
for $i=(n-1):-1:1$
$g=g.*u+x(i)$ end $plot(u,g,'-',t,y,'o')$

Now my question is: plot the figure with polynomial fit of polynomial degree 2.
How should i do this?
Thanks

Answer 1

So you want to fit y as a function of t, right? Use

p = polyfit(t,y,2);

fit = polyval(p,t);

plot(u,g,'-',t,y,'o',t,fit)

The first line is the built-in polynomial fit function. The number 2 is the degree which you specify and it returns the coefficients of the polynomial in p. Note degree 2 means three coefficients. The second line then evaluates the polynomial using the coefficients in p. And then the third, plots them all together.