Patch newline (\\) in math environment using amsmath

Question

I want to patch the newline command \\ in an align environment provided by amsmath. The patched version should execute some custom code and finally call the original definition, like in the following example.

\documentclass{minimal}
\usepackage{amsmath}
\begin{document}
    \begin{align}
        \let\old\\%
        \def\\{1\old}%
        %
        a\\
        b\\
        c
\end{align}
\end{document}

This results in:

So it works only for the first call (the "b" is missing a "1"). Why is this and how can I make it work for all newlines?

Answer 1

You mention in a comment that your goal is to automatically affix a \notag instruction to all equations that aren't cross-referenced in the document. To achieve this objective, you could load the mathtools package and issue the command

\mathtoolsset{showonlyrefs}

in the preamble.

Answer 2

Although the ideas in Mico's comment probably do work, they would be very specific to one environment and subject to break if anything related changes. I was heading for a more general solution.

There are two problems:

1. As egreg points out, every line is an implicit group, so that all local definitions get deleted before the next line.
2. Every amsmath environment is executed twice, first for measuring its size and then for printing.

Because of 1., \global has to be used. Because of 2., every direct \let followed by a \def of the same command will lead to an infinite recursion in the second pass, because the \let then points to itself, so the \gdef reaches towards an infity.

With this knowledge, a fix is simple. The recursion has to be avoided by only executing the commands in the first pass. Thus, this is solution:

\documentclass{minimal}
\usepackage{amsmath}
\usepackage{etoolbox}

\begin{document}
    \begin{align}
        \ifundef{\old}{%
            \global\let\old\\%
            \gdef\\{1\old}%
        }{}
        %
        a\\
        b\\
        c
    \end{align}
\end{document}