LaTeX.org

I don't think the environment array is being defined in TeX, can someone help me to translate the following LaTeX to TeX please?
\[
f(n) = \left\{
\begin{array}{l l}
n/2 & \quad \text{if $n$ is even}\\
-(n+1)/2 & \quad \text{if $n$ is odd}\\
\end{array} \right.
\]

Thank you!

It's done, Thanks very much Stefan!!

Thanks Stefan,I wonder if I can use it in pdfTex?

Hi, I have been trying to use $\frac{abc}{xyz}$ in TeXworks, but I failed to do so. Can someone help me? the error message is:
This is pdfTeX, Version 3.1415926-1.40.11 (MiKTeX 2.9)
entering extended mode
(C:\Users\Eugene\Desktop\untitled-2.tex
! Undefined control sequence.
<recently read> \frac ...