Defining repeated tabular/array columns using a computed value to specify the number

Question

Background:

I just recently saw that one could define multiple columns in a tabular or array using the syntax:

*{num}{<align>} 

So, for example \begin{tabular}{r*{3}{l}r} is the identical to \begin{tabular}{rlllr}, and similarly for array.

Problem:

I used this to produce a solution to Best way to create an system of equations environment? where the number used was provided as a parameter. This works fine if the specified parameter is one less than the number of variables.

So, I attempted to modify this to accept the actual number of variables, and set that to a counter value, decrement it by one, and use the new value of the counter to specify how many of these were to be used. I suspect that this is an expansion issue (which I haven't quite gotten the hang of).

Alternatively, if I could somehow perform a decrement operation and apply that value to the column specification would be fine as well.

Incorrect Result:

Currently, the code below produces:

Desired Result:

I would like to use something equivalent to the intent of the commented \begin{array} that uses a value of one less than the provided parameter. So when this works as desired, the output should be (note the shift in the right vertical bar):

Code:

\documentclass[border=5pt]{standalone}
\usepackage{xcolor}
\usepackage{array}
\usepackage[paperwidth=4.7in]{geometry}
\newcounter{NumberOfRepeatedColumns}
\newenvironment{MySystem}[2]{% <num+1> <r|l|c>
    \setcounter{NumberOfRepeatedColumns}{#1}
    \addtocounter{NumberOfRepeatedColumns}{-1}
    \begin{array}{c| #1{#2} | c}% want to replace this with something equivalent to line below 
    %\begin{array}{c| {\the\value{NumberOfRepeatedColumns}}{#2}}%
        \the\value{NumberOfRepeatedColumns}\
}{%
    \end{array}%
}
\newcommand{\data}{123 & abc & de  & f \1   & a   & def & fg}%
\begin{document}
\color{red}
$\begin{MySystem}{4}{r}
    \data
\end{MySystem}$
\hspace{0.5in}
$\begin{MySystem}{4}{l}
    \data
\end{MySystem}$
\color{blue}
$\begin{MySystem}{3}{r}
    \data
\end{MySystem}$
\hspace{0.5in}
$\begin{MySystem}{3}{l}
    \data
\end{MySystem}$
\end{document}

Answer 1

You can use

\newcounter{NumberOfRepeatedColumns}
\newenvironment{MySystem}[2]{% <num+1> <r|l|c>
    \setcounter{NumberOfRepeatedColumns}{#1}
    \addtocounter{NumberOfRepeatedColumns}{-1}
    \begin{array}{c| *{\theNumberOfRepeatedColumns}{#2} | c}
}{%
    \end{array}%
}

or even better, without the extra counter, and \numexpr:

\newenvironment{MySystem}[2]{% <num+1> <r|l|c>
    \begin{array}{c| *{\numexpr#1-1\relax}{#2} | c}
}{%
    \end{array}%
}