# Creating straight line in matrix

I need to add some straight lines inside a matrix to indicate labeling of rows, like this:

How do I do that?

-

Leaders is optimal for this situation. amsmath provides \hdotsfor{<cols>} that draws a dotted leader across <cols> columns. It is also possible to overlay a row number by setting that specific row twice (once with \hdotsfor and once with a row number, say). I've also added \numberrowwithline{<cols>}{<stuff>} that draws a line (instead of dots) as a dash leader, and inserts <stuff> in the middle of <cols> columns. The default space on either side is \fboxsep.

The following is a modified example, taken from Herbert's mathmode document:

\documentclass{article}
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\usepackage{amsmath}% http://ctan.org/pkg/amsmath

\newcommand{\numberrowwithline}[2]{%
\multicolumn{#1}c%
#2\hspace*{\fboxsep}%
}%
}

\begin{document}
$$\underline{A}=\left[\begin{array}{ccccccc} a_{11} & a_{12} & 0 & \ldots & \ldots & \ldots & 0 \\ a_{21} & a_{22} & a_{23} & 0 & \ldots & \ldots & 0 \\ 0 & a_{32} & a_{33} & a_{34} & 0 & \ldots & 0 \\ \hdotsfor{7} \\ \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \vdots \\ \hdotsfor{7} \\[-\normalbaselineskip] \multicolumn{7}{c}{\colorbox{white}{\ R_n\ }} \\ \vdots & \vdots & \vdots & \vdots & \vdots & \vdots & \vdots \\ \numberrowwithline{7}{R_i} \\ 0 & \ldots & 0 & a_{n-2,n-3} & a_{n-2,n-2} & a_{n-2,n-1} & 0 \\ 0 & \ldots & \ldots & 0 & q_{n-1,n-2} & a_{n-1,n-1} & a_{n-1,n} \\ 0 & \ldots & \ldots & \ldots & 0 & a_{n,n-1} & a_{nn} \end{array}\right]$$
\end{document}


With the addition of xhfill you can go crazy with the colours and types of leaders. See the xhfill documentation for the different styles of lines as a sample.

-
  \multicolumn{3}{c}{\hrulefill R_2 \hrulefill}


if it is a 3 column array

-