Optional argument to an environment inside align

Question

I have lots of matrices so I want to define a mat environment. I sometimes need the matrix to have right-aligned columns so I am using mathtools and I want to have an optional argument that is usually c but that I can make r when I want.

A small working example is below. The definition works great in lots of cases, including inside the equation* but inside of align* it fails. Specifically, I get ! Extra }, or forgotten $. <recently read> } l.22 \end{align*}.

If I add [c] or [r] as optional arguments to the mat inside the align* then compilation will succeed. Adding a \ (slash, space) to the first line of the mat, before the &\vdots, also works.

To me this looks like a problem with LaTeX trying to read the optional argument when I don't want it to. I've tried putting %'s in various places in the \begin{pmatrix*}[#1] line but to no avail. Any suggestions?

\documentclass{article}
\RequirePackage{mathtools}
\newenvironment{mat}[1][c]{\begin{pmatrix*}[#1]
      }{\end{pmatrix*}}

\begin{document}
\begin{equation*}
  \begin{mat}
           &\vdots                    \\
    \cdots &ra_{j,i}+sb_{j,i} &\cdots \\
           &\vdots
  \end{mat}
\end{equation*}

\begin{align*}
  \begin{mat}
           &\vdots                    \\
    \cdots &ra_{j,i}+sb_{j,i} &\cdots \\
           &\vdots
  \end{mat}
  &= 0
\end{align*}
\end{document}

Answer 1

It's an expansion time problem. A way out comes from xparse:

\usepackage{mathtools,xparse}
\NewDocumentEnvironment{mat}{O{c}}
  {\begin{pmatrix*}[#1]}
  {\end{pmatrix*}}

Solution without xparse

Redefine \env@matrix, but not with \renewcommand:

\makeatletter
\def\env@matrix{\new@ifnextchar[\env@m@trix{\env@m@trix[c]}}
\def\env@m@trix[#1]{%
 \hskip-\arraycolsep\let\@ifnextchar\new@ifnextchar\array{*\c@MaxMatrixCols#1}}
\makeatother

This will allow you to use an optional argument to the Xmatrix environments; however you can't define an abbreviation with \newenvironment. This will work (with or without the optional argument)

\begin{align*}
  \begin{pmatrix}[r]
           &\vdots                    \\
    \cdots &ra_{j,i}+sb_{j,i} &\cdots \\
           &\vdots
  \end{pmatrix}
  &= 0
\end{align*}

If you want an abbreviation, then the magic formula

\expandafter\let\expandafter\mat\csname pmatrix*\endcsname
\expandafter\let\expandafter\endmat\csname endpmatrix*\endcsname

will allow you to write

\begin{align*}
  \begin{mat}
           &\vdots                    \\
    \cdots &ra_{j,i}+sb_{j,i} &\cdots \\
           &\vdots
  \end{mat}
\end{align}

with or without the optional argument.

Solution with minimal impact

(Thanks to Bruno Le Floch)

Write

\begin{mat}
\relax & \vdots                    \\
\cdots & ra_{j,i}+sb_{j,i} &\cdots \\
       & \vdots
\end{mat}

Answer 2

I defined a new command for easy matrix typing:

\documentclass{article}
\RequirePackage{mathtools}

\newcommand{\pma}[2][r]{\begin{pmatrix*}[#1] #2 \end{pmatrix*} } %pmatrix shortcut

\begin{document}
    \begin{align*}
    \pma{
    &\vdots                    \\
    \cdots &ra_{j,i}+sb_{j,i} &\cdots \\
    &\vdots
    }
    &= 0
    \end{align*}
\end{document}

And I have no problems with align

Answer 3

put the mat environment into braces:

\begin{align*}
 { \begin{mat}
           &\vdots                    \\
    \cdots &ra_{j,i}+sb_{j,i} &\cdots \\
           &\vdots
  \end{mat} }
  &= 0
\end{align*}