Re: A question about category name in ELisp

[Eli Zaretskii]

> Date: Sat, 5 Oct 2013 18:43:27 +0800
> From: Xue Fuqiao <xfq.free@gmail.com>
> 
> I have a question about category and category table.  In (info
> "(elisp) Categories"):
> 
>     Each category has a name, which is an ASCII printing character in
>   the range ‘ ’ to ‘~’.  You specify the name of a category when you
>   define it with ‘define-category’.
>     The category table is actually a char-table (*note Char-Tables::).
>   The element of the category table at index C is a "category set"—a
>   bool-vector—that indicates which categories character C belongs to.
>   In this category set, if the element at index CAT is ‘t’, that means
>   category CAT is a member of the set, and that character C belongs to
>   category CAT.
> 
> >From the second paragraph, my impression about a category table is
> something like this:
> 
> |     | a   | -   | .   | "   |
> |-----+-----+-----+-----+-----|
> | bv1 | bv2 | bv3 | bv4 | bv5 |
> 
> (The first row contains the indices of the char-table (i.e., the
> "index C" above), and the second row contains the elements at index
> C1, C2, ... (i.e., it's a category set, which is a bool-vector.))
> 
> As with all arrays, bool-vector indices start from 0, so CAT should be
> an integer.  But according to my understanding, CAT is a category
> name, which should be an ASCII printing character (instead of just an
> integer, from the first paragraph).  Why?  Am I missing something?

You need to read about char-table, your assumption about its structure
is wrong.  In particular, a char-table is not a flat array, and it is
indexed by characters, not by zero-based integers.