Fix spacing in custom matrix environment

Question

I have made a custom matrix environment for using Gauss Jordan elimination, I need a vertical bar between the two halves of my matrix.

Here is what I have slopped together

\newenvironment{gjmatrix}[1] {
    \left[\begin{array} { @{}*{#1}{c}|@{}*{#1}{c}@{} }
} {
    \end{array}\right]
}

This is almost perfect. But it doesn't give me enough spacing after the bar.

Could someone help me out? How do I adjust this so it has proper and uniform spacing with everything else.

Answer 1

I suggest you change |@{} to |, i.e., get rid of the @{} particle that follows |. If keeping | is a bit too terse for you taste, you could also write |@{\hspace{\arraycolsep}}, to make explicit which operation is supposed to take place immediately after |.

If you are surmising that @{} means "don't insert whitespace or anything else" and that omitting @{} means "perform the default operation" (which happens to be to insert whitespace, in the amount of \arraycolsep), you would be entirely correct. :-)

\documentclass{article}
\usepackage{array} % optional, but recommended
\newenvironment{gjmatrix}[1]{% 
   \left[ \begin{array}{ @{} *{#1}{c} | *{#1}{c} @{} }}{%  
   \end{array} \right]}

\begin{document}
\[
\begin{gjmatrix}{3}
  -1 &  2 & -2 & 1 & 0 & 0 \\
   0 &  1 & -1 & 0 & 1 & 0 \\
  -3 & -1 &  0 & 0 & 0 & 1
\end{gjmatrix}
\]
\end{document}

Answer 2

You have @{} after |, so you're suppressing the standard intercolumn space before the column following the vertical line.

I suggest a slightly different approach, because you can also use Gauss-Jordan also on augmented matrices having fewer (or even more) columns after the line than before it.

\documentclass{article}

\NewDocumentEnvironment{gjmatrix}{mO{#1}}
 {\left[\begin{array}{ @{} *{#1}{c} | *{#2}{c} @{} }}
 {\end{array}\right]}

\begin{document}
\[
\begin{gjmatrix}{3}
  -1 &  2 & -2 & 1 & 0 & 0 \\
   0 &  1 & -1 & 0 & 1 & 0 \\
  -3 & -1 &  0 & 0 & 0 & 1
\end{gjmatrix}
\qquad
\begin{gjmatrix}{3}[2]
  -1 &  2 & -2 & 1 & 0 \\
   0 &  1 & -1 & 0 & 1 \\
  -3 & -1 &  0 & 0 & 0
\end{gjmatrix}
\]

\[
\begin{gjmatrix}{4}
  -1 &  2 & -2 & 1 & 1 & 0 & 0 & 0\\
   0 &  1 & -1 & 0 & 0 & 1 & 0 & 0\\
  -1 &  2 & -2 & 0 & 0 & 0 & 1 & 0\\
  -3 & -1 &  0 & 0 & 0 & 0 & 0 & 1
\end{gjmatrix}
\qquad
\begin{gjmatrix}{4}[1]
  -1 &  2 & -2 & 1 & 0 \\
   0 &  1 & -1 & 0 & 1 \\
  -1 &  2 & -2 & 0 & 0 \\
  -3 & -1 &  0 & 0 & 0
\end{gjmatrix}
\]

\end{document}

The trailing optional argument is the number of columns after the line and defaults to the same number as in the mandatory argument.