Drawing boxes in matrices

Question

In my document I have defined

\newcommand\bigzero{\makebox(0,0){\text{\huge0}}}

and using this the following matrix:

\begin{equation}\begin{pmatrix}
0&1\\
1&0&&&&\bigzero\\
&&0&1\\
&&1&0\\
&&&&\ddots\\
&\bigzero&&&&0&1\\
&&&&&1&0
\end{pmatrix}\end{equation}

How can I draw boxes around the three blocks which build the matrix?

score 6 · Answer 1 · 2018-09-11T18:43:19.447

6

This is only a fun answer. I am trying out LoopSpace's great update of the tikzmark package, which has now a command in that I was always looking for: \tikzmarknode. And it just works. (Yes, using tikz here may be an overkill, as I said, this is just for fun and to learn something new.) EDIT: Moved the brackets away from the boxes, big thanks to @manooooh!

\documentclass{article}
\usepackage{amsmath,tikz}
\usetikzlibrary{tikzmark,fit}
\newcommand\bigzero{\makebox(0,0){\text{\huge0}}}
\begin{document}
\begin{equation}
\left(~\begin{matrix}
\tikzmarknode{1A}{0}&1\\
1&\tikzmarknode{1B}{0}&&&&\bigzero\\
&&\tikzmarknode{2A}{0}&1\\
&&1&\tikzmarknode{2B}{0}\\
&&&&\ddots\\
&\bigzero&&&&\tikzmarknode{3A}{0}&1\\
&&&&&1&\tikzmarknode{3B}{0}
\end{matrix}~\right)
\begin{tikzpicture}[overlay, remember picture]
\foreach \X in {1,2,3}
{\node[inner sep=1pt,draw,fit=(\X A)(\X B)]{};}
\end{tikzpicture}
\end{equation}
\end{document}

edited Sep 11 '18 at 18:43

answered Sep 09 '18 at 20:51

+1, but I think the boxes are too stick to the braces of the matrix. Maybe using someting like \left(\;\begin{matrix}...\end{matrix}\;\right) would be ok? – manooooh Sep 09 '18 at 20:54
1

@manooooh You are absolutely right. Yet this is really just for fun and to pay tribute to LoopSpace's updated package. In this case, you can easily produce a very simple \fbox only solution by placing the 2x2 matrices in \fboxes. I was actually not expecting to get any votes for that, just wanted to draw the attention to the updated library such that LoopSpace and I have company when testing it. ;-) – Sep 09 '18 at 20:58
Thank you! This works for me exept what @manooooh said. Would you fix this or is this too much work? I don't know anything about fbox'es. – Sep 09 '18 at 21:00
@marmot lol I didn't know that you were paying to LoopSpace's package. With fboxes would be fantastic! – manooooh Sep 09 '18 at 21:02
1

@manooooh No, I am now on a promotion tour. Undoubtedly someone else will come and say this is a sledge hammer and it is possible to achieve the same with \fboxes (which is true in this case, but not when the boxes should become a tiny bit more fancy). ;-) – Sep 09 '18 at 21:08
1

Very nice, but why is the bottom zero of the middle box misaligned? – Max Sep 10 '18 at 05:44
2

The misalignment is actually from the previous line where you have ~~ after the 1 which adds a bit of space after the 1, thus seeming to push the 0 further out due to the centring of the matrix entries. – Andrew Stacey Sep 11 '18 at 18:39
@LoopSpace You're absolutely right. I fixed it. So far I do not have any issues. – Sep 11 '18 at 18:43

score 4 · Accepted Answer · answered Sep 09 '18 at 21:33

A solution with basic tools:

\documentclass{article}

\usepackage{amsmath}
\usepackage{hhline}
\newcommand\bigzero{\makebox(0,0){\text{\huge0}}}
\newcommand{\fblock}[1]{\begin{array}{|@{\:}cc@{\:}|}
\hhline{|--|}#1 \\
\hhline{|--|}
\end{array}}

\begin{document}

\begin{equation}
\begin{pmatrix}
\,\fblock{0&1\\ 1&0 }&&\\
&\mkern-9mu\fblock{0&1\\ 1&0}&\multicolumn{2}{c}{\raisebox{3ex}{\bigzero}} \\
&&\makebox[0pt]{$ \ddots $}\\[-0.5ex]
\multicolumn{2}{c}{\raisebox{4ex}{\bigzero}} &&\fblock{0&1\\ 1&0}\,
\end{pmatrix}\end{equation}

\end{document}

Drawing boxes in matrices

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