% interp_poly.m
% Using the Matlab polyfit and polyval functions
% to calculate interpolating polynomial

x = [1,2,3,4] % nodes
y = [2,4,3,5] % values at the nodes

% Determine coefficients defining the degree 3
% interpolating polynomial. Note that this
% corresponds to the polynomial
% a(1)*x^3 + a(2)*x^2 + a(3)*x + a(4)

a = polyfit(x,y,3) % polynomial coefficients

% plot the points and the polynomial

xp = [1:0.01:4];
yp = polyval(a,xp);

plot(x,y,'O', xp,yp, '-')