How to display two-rows bracket in Latex?

Question

Does anyone know how to modify the following string in order to display the two-lines bracket?

str = '$$c_i =\{\begin{array}{l l} 1  \quad L\left(Q_i\right) < 0 \\ 0 \quad L\left(Q_i\right) \geq 0 \\ \end{array}$$';

The current output is the following:

The sign '{' has to embrace both rows (1 and 0).

Answer 1

$$c_i =\begin{cases} 1 & L\left(Q_i\right) < 0 \\
                     0 &  L\left(Q_i\right) \geq 0
       \end{cases}$$

The tex file should have \usepackage{amsmath} in the preamble.

Answer 2

This is derived from Niall Murphy's answer, "tidied up" a bit:

\[
c_i =
\begin{cases} 
1 & L (Q_i) < 0 \\ 
0 & L (Q_i) \geq 0 
\end{cases}
\] 

Note that the "\" becomes "\\", and I've removed the \left and \right parenthesis modifiers, which introduce unwanted (I think) space between L and (.