[Tex/LaTex] Text column in array

Question

I am trying to do something that seems relatively simple but I can't get it to work. Basically, I would like one of the columns in my math mode array to be text, so I am trying to use pre and post processing >{} and <{} like so

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    \begin{align}
    \begin{array}{{>{\text\bgroup}c<{\egroup}}@{:}c@{=}c}
    a & x^2 & x^2 \\
    b & y^2 & y^2 \\
    c & z^2 &  z^2
    \end{array}
    \end{align}
\end{document}

This leads to an "illegal character in array arg" error. What is the right way to do this?

Answer 1

You're close. In the modified example below, I'm using the array environment and define a special column type, L', that's automatically in text mode. The reason this works is because l, c, and r are automatically in math mode in an array environment.

Note the second intercolumn specifier, @{{}={}}: The pairs of {} curly braces before and after the = sign are there to inform TeX that the = sign is to be treated as a so-called mathrel object. In case this sounds a bit cryptic: mathord, mathbin, and mathrel objects each have different amounts of whitespace before and after them. An easy way to make TeX treat a = symbol as a mathrel object even though it occurs in a non-equation setting is to pre- and post-fix it with (empty) math "atoms", viz., {}.

Note also that it's not necessary to use the align environment as there is really only one "equation" in the example.

\documentclass{article}
\usepackage{array,amsmath}
\newcolumntype{L}{>$l<$}
\begin{document}
\begin{equation*}
    \begin{array}{L@{\quad}c@{{}={}}c}
       some text:            & x^2 & x^2 \\
       more thoughts:        & y^2 & y^2 \\
       really deep thoughts: & z^2 & z^2
    \end{array}
\end{equation*}
\end{document}