How to replace \\ with \\& in math mode?

Question

I am having the following isssue: I have to write a mathematical paper/book with many different optimization problems formulated.

To be consistent all over I'd like to create an environment to typeset these opt problems. I'll give you a minimal running example:

\documentclass[a4paper,10pt]{scrartcl}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{environ}
\begin{document}

\NewEnviron{OptEnv}[1]{%
\begin{align}
\text{min. } & #1 \\
\text{s.t. } & \BODY
\end{align}
}

\begin{OptEnv}{a}
a \leq 1 \\
& b \leq a \\
& 3 \leq b
\end{OptEnv}

\end{document}

My question now: Is there an easy way to omit the & in the environemnt and append in the definition of the environment on each \\?

Thanks

Answer 1

Better method with expl3

If you're sure that every \\ in the environment has to be replaced, you can do it with a regex replacement:

\documentclass[a4paper,10pt]{scrartcl}
\usepackage{amsmath,amssymb}
\ExplSyntaxOn
\NewDocumentEnvironment{OptEnv}{mb}
 {
  \tl_set:Nn \l_tmpa_tl { #2 }
  \regex_replace_all:nnN { \c{\} } { \c{\} \cT& } \l_tmpa_tl
  \begin{align}
  \text{min.~} & #1 \
  \text{s.t.~} & \tl_use:N \l_tmpa_tl
  \end{align}
}
\ExplSyntaxOff
\begin{document}
\begin{OptEnv}{a}
a \leq 1 \
b \leq a \
3 \leq b
\end{OptEnv}
\end{document}

Old answer

If you're sure that every \\ in the environment has to be replaced, you can do it with xstring:

\documentclass[a4paper,10pt]{scrartcl}
\usepackage{amsmath,amssymb}
\usepackage{environ,xstring}
\NewEnviron{OptEnv}[1]{%
  \noexpandarg % don't expand arguments
  % with \expandafter get the expansion of \BODY
  \expandafter\StrSubstitute\expandafter{\BODY}{\}{\&}[\BODY]
  \begin{align}
  \text{min. } & #1 \
  \text{s.t. } & \BODY
  \end{align}
}
\begin{document}
\begin{OptEnv}{a}
a \leq 1 \
b \leq a \
3 \leq b
\end{OptEnv}
\end{document}

Answer 2

If you encase \BODY in a top-aligned, left-justified array environment, there's no further need to provide & alignment indicators:

\documentclass[a4paper]{scrartcl}
\usepackage{amsmath,amsfonts,amssymb,environ}
\setlength\textwidth{3in} % just for this example
\NewEnviron{OptEnv}[1]{%
  \begin{align}
  \min\  & #1 \\
  \text{s.t.\ } & \begin{array}[t]{@{}l} \BODY \end{array}
  \end{align}
  }
\begin{document}
\begin{OptEnv}{a}
  a \leq 1 \\
  b \leq a \\
  3 \leq b
\end{OptEnv}
\end{document}

If you would rather have the conditions centered vertically, just leave off the [t] specifier.

A consequence of this setup is that the condition, or conditions, associated with the minimization problem will be assigned a single equation number instead of one number per condition. This may, or may not, be to your liking.

Answer 3

hacking a bit into amsmath one can avoid loading further packages:

\documentclass[a4paper,10pt]{scrartcl}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{environ}

\makeatletter
\NewEnviron{OptEnv}[1]{%
  \def\Let@{\def\\{\math@cr &}}%
  \begin{align}
  \text{min. } & #1 \math@cr
  \text{s.t. } & \BODY
  \end{align}
}
\makeatother

\begin{document}\thispagestyle{empty}

\begin{OptEnv}{a}
a \leq 1 \\
b \leq a \\
3 \leq b
\end{OptEnv}

\end{document}