[Tex/LaTex] missing $inserted Error when not putting$ before and after array environment

amsmatharrayserrorsmath-mode

Considering this answer I added $ before and after the array, i. e. before \begin{array} and after \end{array} in the following code. Then the former missing$ inserted Error was gone.

\documentclass[a4paper,10pt]{article}
\usepackage{amsmath}
\begin{document}
\begin{center}
$\begin{array}{cc} z_{ij} = 1 & \quad \text{if } i, j \in C \\ z_{ij} = 0 & \quad \text{otherwise} \end{array}$
\end{center}
\end{document}


However, why is that necessary? I thought the array environment switches LaTex to math mode itself.

the array environment does not automatically switch into math mode; math needs to be applied explicitly.

a better approach than embedding it in a center environment is to put it into a math display environment; either equation* or $...$ will suffice.

also, the \text{...} string in the first line would be better if it includes the in-line math expression at the end.

finally, the right-hand column of the array is better left-aligned.

making all these changes, here's a recommended alternative.

$\begin{array}{cl} z_{ij} = 1 & \quad \text{if  i, j \in C } \\ z_{ij} = 0 & \quad \text{otherwise} \end{array}$


For completeness, here's @DavidCarlisle's suggestion:

$z_{ij} = \begin{cases} 1 & \text{if } i, j \in C \\ 0 & \text{otherwise} \end{cases}$