$\displaystyle f: R->R$ is defined by $\displaystyle f(x)=x$ if $\displaystyle x$ is rational, and $\displaystyle f(x)=0$ is irrational.

I need to show f does not have a limit at c if c is not equal to 0. I've shown this true if c is rational, but I'm having a problem with if it's irrational, any help?