Vectorisation of a Lagrange polynomial

I am trying to vectorise this function of a lagrange polynomial without a for loop. The for loop code gives the right answer but my vectorised code does not. Any help would be appreciated.

function f is the lagrange polynomial to be approximated and x is a set of sample points (10 points).

Vectorised code

function dfdx = diff1(f, x)
b = length(x);
dfdx = nan(size(f));
%initial value
dfdx(1) = ...

%vectorisation to get 2 -> b-1 have formula
j = 2:1:b-1;
dfdx(j) = f(j-1).*(x(j)+x(j+1))/((x(j-1)+x(j)).*(x(j-1)+x(j+1)))...
    +f(j).*(2.*x(j)+x(j-1)-x(j+1))/((x(j-1)-x(j-1)).*(x(j)+x(j+1)))...
    +f(j+1).*((x(j)+x(j-1))/((x(j+1)+x(j)).*(x(j)-x(j+1))));

%end value
dfdx(b) = ...
end

For loop code code

function dfdx = diff1(f, x)
b = length(x);
dfdx = nan(size(f));
%initial value
dfdx(1) = ...

%vectorisation to get 2 -> b-1 have formula
for j = 2:b-1
dfdx(j) = f(j-1).*(x(j)+x(j+1))/((x(j-1)+x(j)).*(x(j-1)+x(j+1)))...
    +f(j).*(2.*x(j)+x(j-1)-x(j+1))/((x(j-1)-x(j-1)).*(x(j)+x(j+1)))...
    +f(j+1).*((x(j)+x(j-1))/((x(j+1)+x(j)).*(x(j)-x(j+1))));
end

%end value
dfdx(b) = ...
    end

Calling code

x = ...
f = 1 + x + x.^2;
dfdx = diff1(f,x);
absError = abs((1 + 2*x - diff1(f, x)))

Answer 1

[Alan Stevens]

In your expression

dfdx(j) = f(j-1).*(x(j)+x(j+1))./((x(j-1)+x(j)).*(x(j-1)+x(j+1)))...
    +f(j).*(2.*x(j)+x(j-1)-x(j+1))./((x(j-1)-x(j-1)).*(x(j)+x(j+1)))...
    +f(j+1).*((x(j)+x(j-1))./((x(j+1)+x(j)).*(x(j)-x(j+1))));

You need ./ as well as .*