How to get width of parameter in plain TeX?

Question

I am trying to get the width of a macro parameter using

\def\getwidthof#1{%
    \newdimen\myl%
    \settowidth\myl{#1}%
    \the\myl%
}

where \settowidth is defined as

\catcode`\@=11
\newbox\@tempboxa
\def\@settodim#1#2#3{\setbox\@tempboxa\hbox{{#3}}#2#1\@tempboxa
   \setbox\@tempboxa\box\voidb@x}
\def\settoheight{\@settodim\ht}
\def\settodepth {\@settodim\dp}
\def\settowidth {\@settodim\wd}
\catcode`\@=12

using egreg's answer here. I would like the code \getwidthof{some text} to print out the width of some text, which in this case would be 42.55563pt. The code

\newdimen\myl
\settowidth\myl{some text}
\the\myl

works, but breaks when a parameter is used. Is it because #1 is not expanded when put in \settowidth\myl{#1}?

To start with, newdimen inside a command definition is almost certainly wrong. — user202729, Jul 19 '22 at 01:44
By the way next time include the error message in the question itself. Easier for answerers to spot the issue — user202729, Jul 19 '22 at 01:45
See also: tex core - Why are \new... defined as \outer in the plain TeX format? - TeX - LaTeX Stack Exchange — user202729, Jul 19 '22 at 01:47

score 3 · Accepted Answer · answered Jul 19 '22 at 08:56

Error or not, your code is wrong anyway. You're allocating a new \dimen register each time you're calling \getwidthof and wasting resources.

Let's see with an error-free definition:

\def\getwidthof#1{%
    \csname newdimen\endcsname\myl%
    \settowidth\myl{#1}%
    \the\myl%
}
\catcode`\@=11
\newbox\@tempboxa
\def\@settodim#1#2#3{\setbox\@tempboxa\hbox{{#3}}#2#1\@tempboxa
   \setbox\@tempboxa\box\voidb@x}
\def\settoheight{\@settodim\ht}
\def\settodepth {\@settodim\dp}
\def\settowidth {\@settodim\wd}
\catcode`\@=12
\getwidthof{abc}
\getwidthof{def}
\getwidthof{ghij}
\bye

The log file will have

\@tempboxa=\box16
\myl=\dimen16
\myl=\dimen17
\myl=\dimen18

Fixed code.

\catcode`\@=11
\newbox\@tempboxa
\def\@settodim#1#2#3{\setbox\@tempboxa\hbox{{#3}}#2#1\@tempboxa
   \setbox\@tempboxa\box\voidb@x}
\def\settoheight{\@settodim\ht}
\def\settodepth {\@settodim\dp}
\def\settowidth {\@settodim\wd}
\catcode`\@=12
\newdimen\myl
\def\printwidthof#1{%
  \settowidth{\myl}{#1}%
  \the\myl
}
\printwidthof{abc}
\printwidthof{def}
\printwidthof{ghil}
\bye

score 1 · Answer 2 · answered Jul 19 '22 at 09:52

This also will helps:

\newbox\mywidthbox%
\newdimen\mywidthdimen%
\def\getwidthof#1{%
\setbox\mywidthbox=\hbox{#1}%
\mywidthdimen=\wd\mywidthbox%
}
\getwidthof{test}
\showthe\mywidthdimen
\getwidthof{long test}
\showthe\mywidthdimen
\bye

16.16669pt.

37.8334pt.

How to get width of parameter in plain TeX?

2 Answers2