TeX equivalent for LaTeX's \boxed

Question

Does anyone have any suggestions for recreating the \boxed{...} function from LaTeX in plain TeX? I'm pretty sure it's part of the amsmath package for LaTeX (not actually sure because I include amsmath it in all of my LaTeX files).

Answer 1

You can recycle the definition of \cstok that you can find in manmac.tex:

\def\boxed#1{% from \cstok in manmac.tex
  \leavevmode\thinspace
  \hbox{\vrule\vtop{\vbox{\hrule\kern1pt
        \hbox{\thinspace$\displaystyle{#1}$\thinspace}}
      \kern1pt\hrule}\vrule}\thinspace}

$$
a = \boxed{1\over2} = {2\over 4} = \boxed{\root3\of{8^{-1}}}
$$

\bye

Adjust the spacing to suit your taste.

Answer 2

I have done Werner's exercise. With the additional braces for use with \over as in egreg's answer.

Edit for my first ever non zero use of \mathsurround. But sub-formulas (in \hboxes) then should be aware and set their own, as is systematic I think in LaTeX/amsmath.

\newdimen\fboxsep  \fboxsep3pt
\newdimen\fboxrule \fboxrule.4pt
\newbox\boxedbox

\long\def\boxed #1{\leavevmode
  \setbox \boxedbox = 
  %%\hbox{\kern\fboxsep {\mathsurround0pt $\displaystyle {#1}$}\kern\fboxsep}%
  \hbox{{\mathsurround\fboxsep $\displaystyle {#1}$}}%
  \dimen0=\fboxrule\advance\dimen0\fboxsep\advance\dimen0\dp\boxedbox
  \hbox{\lower \dimen0
           \hbox {%
            \vbox {\hrule height \fboxrule 
             \hbox {\vrule width \fboxrule 
              \vbox {\vskip \fboxsep \box \boxedbox \vskip \fboxsep }%
                    \vrule width \fboxrule }%
                   \hrule height \fboxrule }}}}

$$
a = \boxed{1\over2} = {2\over 4} = \boxed{\root3\of{8^{-1}}}
$$

\bye

The second being the output of

\documentclass{article}
\usepackage{amsmath}

\begin{document}\thispagestyle{empty}

$$
a = \boxed{{1\over2}} = {2\over 4} = \boxed{\root3\of{8^{-1}}}
$$

\end{document}