Could someone show me how to do the following proof?

Consider the set A such that A={p/(2^n) where p is a natural number, n is a natural number, and p/(2^n) is between 0 and 1 (not inclusive).

Let f(x)=0 if x is in A.

Let f(x)=1 if x is in [0,1] - A.

Prove that limit (as x approaches a) of f(x) does not exist when a is in [0,1].

Any help here would be appreciated.