How do I properly plot Lagrange Interpolation ?

x=[-2 1 3 5 ];
y=[0 -1.8 -1.2 -1.6];
sum=0;
for i=1:length(x)
    p=1;
    for j=1:length(x)
        if j~=i
            c = poly(x(j))/(x(i)-x(j));
            p = conv(p,c);
        end
    end
    term = p*y(i);
    sum= sum + term;
end
disp(sum);
xx = linspace(min(sum), max(sum), 100); % Bunch of new samples for a smoother plot
yy = sum(1)*xx.^3 + sum(2)*xx.^2 + sum(3)*xx + sum(4);


plot(x, y, '-s')
hold on 
plot(xx,yy,'-r')
%plot(sum,'-o')
xlabel('x'), ylabel('y'), title('Lagrange Polynomial Fit ')
legend('Data Collection', 'Lagrange Polynomial Fit', 'Location', 'northeast')

[Yo mama]

I know that y(2) is -1.5 and where I went wrong was in xx and yy i just dont really know how to do it at all lol

Answer 1

[Sriram Tadavarty]

Hi Daniel,

Update the variable xx with the limits of x rather than sum, and you would see a clear plot. Something like this

xx = linspace(-2, 5, 1000); % Bunch of new samples for a smoother plot
% -2 and 5 are minimum and maximum values of x

Hope this helps.

Regards,

Sriram

[Yo mama]

does this now affect my "yy"? I did this change and now my plots dont work

[Sriram Tadavarty]

May i know what are you referring here about plots not working, xx has just increased it's span.

It would give a plot as attached.

[Yo mama]

Never mind I just needed to open it in a new window, thank you sriram