Using \underbrace with table columns

Question

I have the following code:

\begin{displaymath}
\begin{array}{|c|c|c|c}
   p
 & q
 & p\lor{}q
 & p\land{}q \\
\hline
0 & 0 & 0 & 0 \\
0 & 1 & 1 & 0 \\
1 & 0 & 1 & 0 \\
1 & 1 & 1 & 1 \\
\hline
\end{array}
\end{displaymath}

I'd like to place an \underbrace under the last two columns, but can't find any TeX/LaTex manuals indicating how to do this. Any ideas?

Answer 1

There is also \upbracefill (and \downbracefill) which uses \leaders to build an extending brace. For example, \hbox to 5cm{\upbracefill} would "fill" that five centimeter length with the brace.

\documentclass{article}
\begin{document}
\[
  \begin{array}{|c|c|c|c}
     p
   & q
   & p\lor{}q
   & p\land{}q \\
  \hline
  0 & 0 & 0 & 0 \\
  0 & 1 & 1 & 0 \\
  1 & 0 & 1 & 0 \\
  1 & 1 & 1 & 1 \\
  \hline
  \noalign{\vspace{-3pt}}
  \multicolumn{2}{c}{}&
    \multicolumn{2}{@{}c@{}}{${\upbracefill}$} \\
  \multicolumn{2}{c}{}&
    \multicolumn{2}{c}{\scriptstyle\ell} \\
  \end{array}
\]
\end{document}

Answer 2

You need to play around with the spaces used inside of array:

\documentclass{article}
\begin{document}
\[
  \begin{array}{|c|c|c|c}
      p
    & q
    & p\lor{}q
    & p\land{}q \\
    \hline
    0 & 0 & 0 & 0 \\
    0 & 1 & 1 & 0 \\
    1 & 0 & 1 & 0 \\
    1 & 1 & 1 & 1 \\
    \hline
    \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{2}{@{}l@{}}{%
      \raisebox{.5\normalbaselineskip}{%
      \rlap{$\underbrace{\hphantom{\mbox{$p\lor{}q$\hspace*{\dimexpr4\arraycolsep+\arrayrulewidth}$p\land{}q$}}}_{\ell}$}}%
    }
  \end{array}
\]
\end{document}

The last two columns have the combined width of

• $p\lor{}q$ (third column heading)
• $p\land{}q$ (fourth column heading)
• 4\arraycolsep (two on either side of the two headings, totalling 4)
• \arrayrulewidth (between the two headings)

The addition of \multicolumns is to remove the vertical rules inserted with your column specification |c|c|c|c|. Finally, \raisebox{.5\normalbaselineskip} raises the entire \underbrace up half a line.

Answer 3

You can also use the usual overkill solution with tikz.

Note:

• This does require two runs. First one to determine the locations, and the second to do the drawing.

References:

• This is a tweaked version of Overbrace in amsmath align environment. A \VerticalOffset have been added to shift the brace to compensate for the \hline.
• The brace is adapted from adding a large brace next to a body of text

Code:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{decorations.pathreplacing}
\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}
\newcommand{\BraceAmplitude}{0.5em}%  Can be tweaked if
\newcommand{\VerticalOffset}{1.2ex}%  necessary.
\newcommand{\HorizontalOffset}{0.8em}%  necessary.
\newcommand{\InsertUnderBrace}[4][]{%
    \begin{tikzpicture}[overlay,remember picture]
\draw [decoration={brace,amplitude=\BraceAmplitude},decorate, thick,draw=blue,text=black,#1]
        ($(#3)+(\HorizontalOffset,-\VerticalOffset)$) -- 
        ($(#2)+(-\HorizontalOffset,-\VerticalOffset)$)
        node [below=\VerticalOffset, midway] {#4};
    \end{tikzpicture}%
}%
\begin{document}
[
\begin{array}{|c|c|c|c}
   p
 & q
 & p\lor{}q
 & p\land{}q \
\hline
0 & 0 & 0 & 0 \
0 & 1 & 1 & 0 \
1 & 0 & 1 & 0 \
1 & 1 & \tikzmark{StartBrace}1 & 1\tikzmark{EndBrace} \
\hline
\end{array}
]
\InsertUnderBrace[draw=red,text=blue]{StartBrace}{EndBrace}{Explanation}
\end{document}

Answer 4

Here is another tikz solution, this time using a matrix of math nodes. There is some trickery with pic to put the underbrace in the right place, but the end result is:

Of course, you can add colour and styling to your heart's content! Here's the code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,calc}
\newdimen\colL
\newdimen\colR

\tikzset{%
  pics/underbrace/.style args={#1,#2,#3,#4}{%
    % a pic for putting an underbrace under certain columns
    % #1 = left column
    % #2 = right column index
    % #3 = index of the row with the underbrace
    % #4 = label text
    code = {
       \pgfextractx\colL{\pgfpointanchor{M-#3-#1}{south west}}  % x-coords of columns
       \pgfextractx\colR{\pgfpointanchor{M-#3-#2}{south east}}
       % draw the delimiter with the correct width
       \node[rectangle, below delimiter={\}}, minimum width=\dimexpr\colR-\colL-1pt]
              at ($ (M-#3-#1)!0.5!(M-#3-#2)+(0,-0.1) $){};
       % the label
       \node at ($ (M-#3-#1.south)!0.5!(M-#3-#2.south) $)[below=2mm]{$\scriptstyle #4$};
       }
   },
}

\begin{document}

  \begin{center}
    \begin{tikzpicture}
      \matrix (M)[matrix of math nodes,
                  nodes={anchor=center,rectangle,
                         minimum width=9mm, minimum height=5mm,
                         inner sep=0mm
                  }
      ]{
          p & q & p\lor{}q & p\land{}q \\
          0 & 0 & 0 & 0 \\
          0 & 1 & 1 & 0 \\
          1 & 0 & 1 & 0 \\
          1 & 1 & 1 & 1 \\
      };
      % vertical lines
      \foreach \c in {1,...,4} {
         \draw[thick](M-1-\c.north west)--(M-5-\c.south west);
      }
      % horizontal lines
      \foreach \r in {1,5} {
        \draw[thick](M-\r-1.south west)--(M-\r-4.south east);
      }
      % and, finally, the underbrace!
      \pic{underbrace={3,4,5,\ell}};
    \end{tikzpicture}
  \end{center}

\end{document}