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

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 2

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 3

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. The \upbracefill starts math-mode, and when already in math-mode with the array, I need to escape the array's math-mode for those columns (lest TeX gets into display-math-mode). I use primitives for this here because I don't know the LaTeX equivalents.

After \omitting the column definition(s), I need to reinstate the \strut on the line, so that the brace-row doesn't touch the previous \hrule. I use a slightly smaller \mathstrut for this. I am also \omitting the first couple of columns for the brace-row and \ell-row because otherwise the vertical rules would extend to the last two rows.

\documentclass{article}
\begin{document}
\[
  \begin{array}{|c|c|c|c}
     p
   & q
   & p\lor{}q
   & p\land{}q \\
  \hline
  0 & 0 & 0 & 0 \\
  0 & 1 & 1 & 0 \\
  1 & 0 & 1 & 0 \\
  1 & 1 & 1 & 1 \\
  \hline
  \omit & \omit & \omit\span\omit\mathstrut\upbracefill \\
  \omit & \omit & \multicolumn{2}{c}{\scriptstyle\ell} \\
  \end{array}
\]
\end{document}

(One could also use \multispan<no of cols> <col contents> to emit \omit\span\omit..., so the first row could be \multispan2 & \multispan2\mathstrut\upbracefill.)