Matlab second derivative approximation

[splelvis]

clear all;
nx=50;
ny=30;
hx=pi/nx;
x=linspace(0,pi,nx+1);
y=linspace(0,pi,ny+1);
x_plus_h=x+hx.*ones(1,nx+1);
x_minus_h=x-hx.*ones(1,nx+1);
for i=1:nx+1
for j=1:ny+1
f_xx(j,i)=(f9(x_plus_h(i),y(j))-2*f9+f9(x_minus_h(i),y(j)))./(hx.^2);
end;
end;
[xx,yy]=meshgrid(x,y);
surf(xx,yy,f_xx);
title('partial derivative w.r.t.xx')
,

and the other new document,f9,
function l=f9(x,y)
l=exp(sin(x-y));and then the system said have error,

? Input argument "x" is undefined.

Error in ==> f9 at 2
l=exp(sin(x-y));
Error in ==> partial_derivxx at 11
f_xx(j,i)=(f9(x_plus_h(i),y(j))-2*f9+f9(x_minus_h(i),y(j)))./(hx.^2);where should i corrected to make it work?
thanks

anyone can help

 

[splelvis]

i have find my error,
f9should be f9(x(i),y(j))