Question
Let $f(x) = \begin{cases} x, & \text{if } x \text{ is irrational}, \\ 0, & \text{if } x \text{ is rational}. \end{cases}$ then $f$ is __________.
Let $f(x) = \begin{cases} x, & \text{if } x \text{ is irrational}, \\ 0, & \text{if } x \text{ is rational}. \end{cases}$ then $f$ is __________.