\char<number> is a TeX primitive that results in the character of ASCII code <number> with category code 12. However, it isn't expandable, unlike it's many cousins \romannumeral, \number, etc.
From my understanding, \char does not look up upper/lowercase/catcode tables, like \uppercase and \lowercase, so I'm confused as to why it unexpandable. (I'm not really sure if this is related at all, but I remember from the top of my head that it's the case for \upper/lowercase)
luatex and xetex provide \Uchar that is expandable
Note that tex has a built in mapping between external character encodings and the internal character codes, and \char access the latter so if \char expanded to a character token and files were written out and read back (for tables of contents etc) then the behaviour would be different to the current non-expandable behaviour as the characters would be re-mapped (incorrectly, most likely)
\char_generate:nn of expl3 that also works with pdftex.
– egreg
May 20 '21 at 12:57
Only for additional information.
You can define expandable \echar{number} by this code in pdftex:
\def\hex#1{\ifcase\numexpr#1\relax 0\or 1\or 2\or 3\or 4%
\or 5\or 6\or 7\or 8\or 9\or a\or b\or c\or d\or e\or f\fi}
\newcount\tmpa \newcount\tmpb
\def\echar{}
\def\addto#1#2{\expandafter\def\expandafter#1\expandafter{#1#2}}
\def\xstring{\expandafter\xstringA\string} \def\xstringA#1{}
{\endlinechar=-1
\loop
\edef\tmp{\noexpand\xstring\xstring\\^\string^\hex\tmpb\hex\tmpa}
\scantokens\expandafter{\expandafter\edef\expandafter\tmp\expandafter{\tmp}}
\global\expandafter\addto\expandafter\echar\expandafter{\tmp\noexpand\or}
\advance\tmpa by1\relax
\ifnum\tmpa=16 \tmpa=0 \advance\tmpb by1 \fi
\ifnum\tmpb<16 \repeat
}
\edef\echar #1{\noexpand\ifcase\noexpand\numexpr#1\relax\echar\noexpand\fi}
This code creates \echar as macro, roughly speaking:
\def\echar#1{\ifcase#1^^00\or ^^01\or ^^02\or ... \or ^^fe\or ^^ff\fi}
where all tokens between \or have category like after \string.
\char_generate:nn (the second argument specifying a catcode)
– David Carlisle
May 27 '21 at 08:55
\Uchar and \Ucharcat and there is no reason to use pdftex today. My code above shows that things are more complicated if somebody sticks on pdftex.
– wipet
May 27 '21 at 12:29
David Carlisle already has explained why \char should be unexpandable.
However, there are cases in which an expandable version is handy and expl3 has the facility available. Here is a plain TeX version (needs an e-TeX engine such as pdftex, xetex or luatex).
\input expl3-generic
\ExplSyntaxOn
\cs_new:Npn \expchar #1 { \char_generate:nn { #1 } { 12 } }
\cs_new:Npn \xexpchar #1 #2 { \char_generate:nn { #1 } { #2 } }
\ExplSyntaxOff
\edef\test{\expchar{92}}
{\tt\meaning\test}
{\tt\test}
\catcode`/=13 \def/{abc}
\xexpchar{47}{13}
\bye
With LaTeX one has a better facility (the category code can be given as an optional argument)
\documentclass{article}
\ExplSyntaxOn
\NewExpandableDocumentCommand{\expchar}{O{12}m}
{
\char_generate:nn { #2 } { #1 }
}
\ExplSyntaxOff
\begin{document}
\ttfamily
\edef\test{\expchar{92}}
\meaning\test
\test
\begingroup
\catcode`/=13 \def/{abc}
\edef\test{\expchar[13]{47}}
\meaning\test
\test
\endgroup
\end{document}
\expandafter. See also https://tex.stackexchange.com/questions/467360/is-there-a-list-of-expandable-tex-primitives-latex-e-tex-others – Henri Menke May 20 '21 at 11:10\characcesses a slot in a font and the glyphs at these slots are not necessarily representable in 8-bit encoding in a meaningful way. It's different for Unicode, where all characters are representable in UTF-8 encoding which is why\Ucharin LuaTeX is expandable. – Henri Menke May 20 '21 at 11:14