conditional probability latex command code example

Example 1: how to write conditional function in latex

\[
    f(x)= 
\begin{cases}
    \frac{x^2-x}{x},& \text{if } x\geq 1\\
    0,              & \text{otherwise}
\end{cases}
\]

Example 2: conditional inclusion in latex

\documentclass{article}
%\newcommand*{\DEBUG}{}%
\begin{document}
\ifdefined\DEBUG
    DEBUG was on
\else
   DEBUG was off
\fi
\end{document}