How can I subtract 1 from the number of columns to repeat in array environment?

Question

I was trying to create an environment for an augmented matrix and came across this answer, which gave me this code

\newenvironment{amatrix}[1]{%
  \left(\begin{array}{@{}*{#1}{c}|c@{}}
}{%
  \end{array}\right)
}

used as

\begin{amatrix}{2}
   1 & 2 & 3 \\  a & b & c
 \end{amatrix}

It works great, but it drives me nuts that the argument is one fewer than the total number of columns in the augmented matrix. I tried changing the #1 to #1-1, but that does not seem to work. The closest thing I could find was the calc package, but that only works in certain circumstances, and this does not seem to be one of them.

So my question is: is there a way to either redefine the amatrix environment so that the argument matches the total number of columns in the augmented matrix? An even better solution would be one where I don't have to put in an argument at all, but I don't want to be greedy here.

Answer 1

An even better solution would be one where I don't have to put in an argument at all, but I don't want to be greedy here.

Be greedy! Here is a solution with tabularray package (and no argument at all):

\documentclass{article}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
\NewDocumentEnvironment{amatrix}{+b}{
    \begin{+pmatrix}[
        colsep=4pt,
        rowsep=0pt,
        vline{Y}={-}{solid},
        column{1}={leftsep=1pt},
        column{Z}={rightsep=1pt}
        ]
        #1
    \end{+pmatrix}%
}{}
\begin{document}
\[
\begin{amatrix}
    1 & 2 & 3 \\
    a & b & c 
\end{amatrix}
\]

\[
\begin{amatrix}
    1 & 2 & 3 & 4\\
    a & b & c & d\\
    x & y & z & w
\end{amatrix}
\]


\end{document}

(Thanks to samcarter for her suggestion).

Answer 2

This addresses your requirement of specifying the number of columns. \numexpr#1-1 removes 1 from the input:

\documentclass{article}
\newenvironment{amatrix}[1]{%
  \left(\begin{array}{@{} *{\numexpr#1-1}{c} | c @{}}
}{%
  \end{array}\right)
}
\begin{document}
[
  \begin{amatrix}{3}
    1 & 2 & 3 \
    a & b & c
  \end{amatrix}
]
\end{document}