How can I define an environment similar to alignat, but calculates the number of columns automatically like align?

Question

I've read several posts explaining the difference between align and alignat such as this TeX SE answer. I want to make a new environment with the capabilities of align (create as many as desired based on the environment's content without having to input a <number> argument) but with the "no horizontal spaces" property of alignat.

Is this possible? If so how can I define such an environment, (let's say altalign)?

Thank you.

Answer 1

A simple approach is to make a new environment that forwards 99 to alignat: _{(remark, this is the "standard" method to redefine environment; nevertheless it will not work for some particular environments that hard codes the environment name. Fortunately it works with this one)}

\newenvironment{alignatz}{\alignat{99}}{\endalignat}
\catcode*=11 \newenvironment{alignatz*}{\alignat*{99}}{\endalignat*} \catcode*=12

Usage:

%! TEX program = pdflatex
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\newenvironment{alignatz}{\alignat{99}}{\endalignat}
\catcode*=11 \newenvironment{alignatz*}{\alignat*{99}}{\endalignat*} \catcode*=12
\begin{alignatz}
    a &= b \quad & c &= d \
    a &= b & c &= d
\end{alignatz}
\begin{alignatz}
    a &= b \quad & c &= d \
    a &= b & c &= d
\end{alignatz}
\end{document}