MatLab / Lagr

function Lagr(X, X1, f)

for i=1:1:length(X)
    F(i)=subs(f,X(i));
end

for k = 1:1:length(X1)
    A = ones(length(X), length(X)); 
for i = 1:1:length(X)
    for j = 1:1:length(X)
        if (j ~= i)
            A(i,j) = ((X1(k) - X(j))/(X(i) - X(j)));
        end
    end
end
P(k) = F*prod(A,2);
end
plot(X1, P, '-k')
vpa(P, 4)
end