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.

## Best Answer

the

`array`

environment doesnotautomatically 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.

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