How do I expandably extract the length of a box

Question

I thought that the construction \wd\bopxcontrolsequence was expandable. But in mucking around in things, it seems not to be. At least in the following, the code demonstrates that \wd\aetmpbox never gets fully expanded in the definition of \aewidth.

\documentclass{article}
\usepackage{pgffor}
\newsavebox\aetmpbox
\begin{document}

\def\aewidth{0pt}%%
\foreach \myitem in {first,a,a bit longer,shorter,very very very very very long}
{\savebox\aetmpbox{\myitem}%%
 \ifdim\wd\aetmpbox>\aewidth\relax
    \xdef\aewidth{\wd\aetmpbox}%%
    \typeout{---> \detokenize\expandafter{\aewidth} :L: \myitem }%%
  \else 
    \typeout{---> \detokenize\expandafter{\aewidth} :s: \myitem }%%
  \fi
}

hello

\end{document}

What I did not want to have to create a new length (for whatever reason). I just wanted to get the value for the width of my box to be saved in the macro \aewidth for later use.

\xdef\aewidth{\the\wd\aetmpbox} perhaps? If I apply this way, I can use \aewidth in \setlength .... — , Mar 15 '16 at 15:35
You have to build the box anyway, so it can't be fully expandable. — egreg, Mar 15 '16 at 15:38
@egreg I thought \savebox\aetmpbox{\myitem} was building the box. — A.Ellett, Mar 15 '16 at 15:39

score 5 · Accepted Answer · edited Apr 13 '17 at 12:36

5

\wd is an unexpandable primitive; \aetempbox is a chardef token, so it's unexpandable. Hence

\xdef\aewidth{\wd\aetmpbox}

is exactly the same as \def\aewidth{\wd\aetmpbox}

However, \the is expandable!

\xdef\aewidth{\the\wd\aetmpbox}

See The \the command

edited Apr 13 '17 at 12:36

Community

1

answered Mar 15 '16 at 15:43

egreg

1,121,712

How do I expandably extract the length of a box

1 Answers1