牛顿迭代法的MATLAB程序，为什么无论输什么初始值，得到的结果f都为零呀？

function [x,f,n]=shuzhiniudun(x0,escep,nita,N)
%x0给定初始值,escep根的容许误差,nita是函数的容许误差,N最大迭代次数
syms X
fun=X^3+X^2-3*(X+1);
g=diff(fun,X);
gx=double(subs(g,X,x0));
f=double(subs(fun,X,x0));
x1=x0-f/gx;
n=1;
while(abs(f)>nita||abs(x1-x0)>escep)
x0=x1;
gx=double(subs(g,X,x0));
f=double(subs(fun,X,x0));
x1=x0-f/gx;
n=n+1;
if(gx==0||n>N)
display('算法失败');
break;
end
end
x=x1;

f=double(subs(fun,X,x));

end

牛顿法不就是，求零点的x值吗