Codigo Spline SNOOPY - Alex Rodrigo Chavez Reyes

function [a,b,c,d]=splinesE(X)

n=length(X(1,:));

for i=1:n;
a(i)=X(2,i);
end

for i=1:n-1;
h(i)=X(1,i+1)-X(1,i);
end

for i=2:n-1;
alfa(i)=3/h(i)*(a(i+1)-a(i))-3/h(i-1)*(a(i)-a(i-1));
end

l(1)=1;
mu(1)=0;
z(1)=0;

for i=2:n-1;
l(i)=2*(X(1,i+1)-X(1,i-1))-h(i-1)*mu(i-1);
mu(i)=h(i)/l(i);
z(i)=(alfa(i)-h(i-1)*z(i-1))/l(i);
end

l(n)=1;
z(n)=0;
c(n)=0;

for i=n-1:-1:1;
c(i)=z(i)-mu(i)*c(i+1);
b(i)=(a(i+1)-a(i))/h(i)-h(i)*(c(i+1)+2*c(i))/3;
d(i)=(c(i+1)-c(i))/(3*h(i));
end

for i=1:n-1;
x=X(1,i):0.1:X(1,i+1);
y=a(i)+b(i)*(x-X(1,i))+c(i)*(x-X(1,i)).^2+d(i)*(x-X(1,i)).^3;
hold on;
plot(x,y,'b');
end

for i=1:n;
hold on;
plot (X(1,i),X(2,i),'*','MarkerEdgeColor','r','LineWidth',1);
title('Interpolación por "splines" de orden 3.');
end