Problem with a function: "??? Undefined function or variable 'x'"

I'm trying to write a generic algorithm to use the Newton-Raphson method of finding the roots of a function, but my algorithm is having troubles with the call function system, giving the error: ??? Undefined function or variable 'x'.

Here's my algorithm, it doesn't have mistakes, because when I try to substitute the f, x and tol variables without calling the function it works perfectly.

function [x,y,err]=raphson(f,a,tol)
  %syms x
  %f=2*x.^3+log(x)-5
  %a=2
  %tol=10^-7
  syms x
  f1=diff(f);
  x=a;
  err=1;
  while err>tol;
    y=subs(f,x);
    y1=subs(f1,x);
    xk1=x-(y/y1);
    err=y;
    x=xk1;
  end
end

The comentaries in the beginning of the algorythm are my attempts to atribute a value to the variables without using the function. I've tried everything and I can't get it to work.

Answer 1

[Chaya N]

The input argument f is a symbolic expression. You are probably seeing the error because the variable x has not been defined as a symbolic variable before calling your function. Simply declare syms x before calling your function and it should work.