[Tex/LaTex] How to get the category code of a character that is the value of a control sequence

catcodestex-core

How can I get the category code of a character that is the value of a control sequence?

If I do this

The catcode for A is \the\catcode`A.

I get

The catcode for A is 11.

If I do this

\let\abc=A
The catcode for \abc\ is \the\catcode\abc.

I get "Missing number, treated as zero."

I think I understand it like this:
\abc is replaced with the character code, category code pair <65, 11>, but TeX expects just a number after \the\catcode, so it inserts a 0, then leaves my A hanging around.

Best Answer

Sadly Knuth in his wisdom gave only few clues in the TeXBook for cases like this.

In order to understand the error we need to firstly understand the definition of \the, which is for the case of catcode \the<codename><8-bit number>, where <codename> stands for either \catcode, \mathcode, \lccode etc...

So clearly in this case catcode TeX expects a number and hence the error generated in the example provided by the OP.

All solutions provided by the other posts revolve around changing the definition one way or another to produce the required character code number and which I am demonstrating here with some different examples:

The example below will produce the right answer in both cases,

% results category 11
\makeatletter
\def\ABC{`@ }
\the\catcode\ABC

% results category 12
\makeatother
\def\ABC{`@ }
\the\catcode\ABC

While comparing two tokens using \ifcat things become a bit more complicated, if you want to compare two active characters you have to say \noexpand - Knuth says so somewhat obscurely in Exercise 20.11!

Consider the following definitions

\catcode`[=13 \catcode`]=13
\def[{*}

The following will result True since we comparing [ with ']' both now being category 13

\ifcat\noexpand[\noexpand]  True \else False \fi

Also \ifcat[* True \else False \fi is True

Since now we have established two facts \the\catcode needs a number and if you use an active character, implicitly or explicitly we can understand why the following will all work!

\def\abc{`A}
\chardef\abc=65

or Joseph's suggestion for comparisons:

\let\abc=A
   \ifcat\noexpand\abc A%
     \TRUE
   \else
     \FALSE
   \fi

Related Solutions

[Tex/LaTex] How to print and assign category codes

There's really no category code 16. This code is assigned to control sequences (not \let to a character) only for the purposes of \ifcat.

Similarly, character code 256 is assigned to the same tokens for the purposes of \if.

You can get category codes 9 and 15 with

\the\catcode`\^^@
\the\catcode`\^^?

(these are the only characters that have those category codes in the default setting, bytes 0 and 127).

Notice also that \ifcat doesn't compare a token with a number, but two tokens:

\ifcat ab
\ifcat a1

The first evaluates to true, the second to false.

Quoting from the TeXbook, page 209:

• \ifcat<token1><token2> (test if category codes agree)
TeX will expand macros following \if until two unexpandable tokens are found. If either token is a control sequence, TeX considers it to have character code 256 and category code 16, unless the current equivalent of that control sequence has been \let equal to a non-active character token.

The key word is considers. Look at page 38:

A token is either (a) a single character with an attached category code, or (b) a control sequence.

Section 506 of "TeX, the program" tells the truth about this: if the next token is not an active character or is a control sequence (not let to a character), then the variable *cur_chr* gets the value 256.

[Tex/LaTex] Why can’t TeX show us the actual “undefined control sequence”

Generally you can find the undefined control sequence if you look in the right place. Here's a simple beamer document:

\documentclass{beamer}
\begin{document}
\begin{frame}
\frametitle{A title}
\foo
\end{frame}
\end{document}

And the error it generates:

./undefined.tex:6: Undefined control sequence.
\beamer@doifinframe ...\frametitle {A title} \foo 
                                                  \end {beamer@frameslide}
l.6 \end{frame}

The undefined control sequence is the last control sequence in the first line of code reported by the error. This is in fact fairly consistent, and if you ask TeX for help on the error (by entering h in the console), this is exactly what it tells you:

The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.

So you can tell the actual offending undefined control sequence, you just need to know where to look. Unfortunately many editing environments hide or otherwise filter the console output, so perhaps this is why it appears not to report the offending command.

This is not to say that all errors are as easy to diagnose in TeX; they're not, because often the line that the error gets noticed is after the line that causes the error. But for undefined control sequences, this is not the case.

Best Answer

Related Solutions

[Tex/LaTex] How to print and assign category codes

[Tex/LaTex] Why can’t TeX show us the actual “undefined control sequence”

Related Question