# don't understand a particular error message I'm getting

When I type

$$A_n\ =\ \left\{\begin{array}{ccc} [\hskip.017in 0,1\hskip.017in] & {} & n\ {\rm is\ even} \\ [\hskip.017in -1,0\hskip.017in] & {} & n\ {\rm is\ odd} \\ \end{array}\right.\ ,$$


I get an error message claiming "illegal unit of measure" and "missing number." I can't find anything wrong with the code. What am I missing?

The problem is that the first [ follows \\ and is mistaken for introducing the optional argument. Use \relax or, better, use the proper tool.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$$A_n\ =\ \left\{\begin{array}{ccc} [\hskip.017in 0,1\hskip.017in] & {} & n\ {\rm is\ even} \\ \relax [\hskip.017in -1,0\hskip.017in] & {} & n\ {\rm is\ odd} \\ \end{array}\right.\ ,$$

$A_n=\begin{cases} [0,1] & \text{n is even,} \\ [-1,0] & \text{n is odd,} \end{cases}$

\end{document}


I have little doubts about which rendering is better both for input and output. If you want to align the two intervals to the right, add \hphantom{-}:

$A_n=\begin{cases} \hphantom{-}[0,1] & \text{n is even,} \\ [-1,0] & \text{n is odd,} \end{cases}$


I'd avoid the comma hanging from nowhere.

Note that \rm has been deprecated for about 30 years and  since LaTeX version 1, about 40 years ago. • You might want to use \textup, just in case this is used within an italic environment like a theorem. Jun 16 at 23:21 • @barbarabeeton I'm not sure why the text should be different from the context. Jun 16 at 23:21 • Okay, but it's nice to know that \text, all by itselt, follows the style of the surrounding environment. Jun 16 at 23:24 • Another option would be [\hphantom{-}0,1], so that the left brackets align. I don't know if that would be better. Jun 17 at 13:39 the [is being seen as an optional argument, which takes a length, \\[5pt] so use ....\\ \relax [....]  to hide the [ from \\  Note  and\hskip are TeX primitives that should not be used in LaTeX, use \[ and \hspace (but avoid \hspacein math) \rm has not been defined by default in latex since 1993, use \textrm{ is odd}