# Long dashes for denoting omitted columns of a matrix

Mathematicians would often to like to think of a row of a m by n matrix with entries from a , say, field, as a vector in the n-dimensional vector space over the field.

Though I am not a mathematician yet, I am stuck with a situation where my proof becomes significantly simpler if I did not care about the entries of the row but just think of it as a vector.

For instance, I'd like my rows to look like how the columns look here. The code for one of the matrices there:

$P= \begin{bmatrix} \biggl |& \biggl|&\biggl|\\ x_1&x_2 &x_3\\ \biggl|&\biggl|&\biggl| \end{bmatrix}$


I'd be grateful if some one helped me.

Here's a way, inspired by \rightarrofill:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\longdash}[2em]{%
\makebox[#1]{$\m@th\smash-\mkern-7mu\cleaders\hbox{$\mkern-2mu\smash-\mkern-2mu$}\hfill\mkern-7mu\smash-$}}
\makeatother
\newcommand{\omitskip}{\kern-\arraycolsep}
\newcommand{\llongdash}[2em]{\longdash[#1]\omitskip}
\newcommand{\rlongdash}[2em]{\omitskip\longdash[#1]}

\begin{document}
$\begin{bmatrix} \llongdash & x_{1} & \rlongdash \\ \llongdash & x_{2}+y & \rlongdash \\ \llongdash & x_{n} & \rlongdash \end{bmatrix}$
\end{document}


I use \llongdash and \rlongdash to back up slightly and so to ensure the correct alignment of the dashes. If dots are needed for denoting omitted rows, one can use \hdotsfor:

\begin{bmatrix}
\llongdash & x_{1}   & \rlongdash \\
\llongdash & x_{2}+y & \rlongdash \\
\hdotsfor{3} \\
\llongdash & x_{n}   & \rlongdash
\end{bmatrix}


instead of the code above will produce Alternatively, one can use only one column

$\begin{bmatrix} \longdash\hfill x_{1} \hfill \longdash \\ \longdash\hfill x_{2} + y \hfill \longdash \\ \longdash\hfill x_{n} \hfill \longdash \\ \end{bmatrix}$


and get Here is a solution based on TikZ:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}

% possible to customize here the dash aspect
\newcommand{\mydash}{
\draw(0.3,0.5ex)--(-0.3,0.5ex);
}

\begin{document}
$P= \begin{tikzpicture}[baseline=-0.5ex] \matrix(m)[matrix of math nodes,left delimiter=(,right delimiter=),ampersand replacement=\&] { \mydash \& y_1 \& \mydash \\ \mydash \& y_2+z_2 \& \mydash \\ \mydash \& y_3 \& \mydash \\ }; \end{tikzpicture}$

\end{document}


which gives: • Thank you very much. I am not good with Tikz yet. But, I thank you for posting a solution. :) – kan May 26 '12 at 13:11