4

According to the TeXbook's* description of the

\if<token1><token2>

construct on p. 209 (the emphasis is mine)

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. In this way, each token specifies a (character code, category code) pair.

What if one, or both, of the unexpandable tokens is an active character, e.g.

\catcode`@=13%
\if\noexpand @ ...

What is an active character's effective character code in the "eyes" of an \if construct?


* 20th printing, Addison-Wesley 1991

7

If you suppress expansion with \noexpand then it acts like a command name so is a character code if it is let to a character,

If you suppress expansion with \string then \if sees a non-active token of catcode 12, with the character code of the original character.

\catcode`@=13
\let@=X

\if\noexpand @X yes \else no\fi


\if\noexpand @ \string@ yes \else no\fi


\bye

makes

yes no

as \noexpand tests equal to X here as it is \let to X note however that if the character is \let to a character as here, then it isn't expandable so the \noexpand isn't needed.

  • What if the active character isn't \let to anything? – Evan Aad Aug 15 '17 at 11:34
  • @EvanAad it acts like a csname all non-character control sequences are equal to each other but not equal to any character – David Carlisle Aug 15 '17 at 11:35
  • Then why does the following manuscript outputs 'no'? \catcode`@=13\if @\relax yes\else no\fi\bye – Evan Aad Aug 15 '17 at 11:38
  • @EvanAad you expanded the if before assigning the catcode – David Carlisle Aug 15 '17 at 11:40
  • 4
    put a space after the 13, it is expanding \if while trying to finish the number to assign the catcode \count0=13\ifx ab 2\else 4\fi assigns count0 to be 134 – David Carlisle Aug 15 '17 at 11:44

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