emacs: what is C-c % doing in auctex and how can I make it behave better?

Question

C-c % is supposed to be the emacs auctex mode shortcut for commenting out stuff. (There's also C-c ; which comments out the marked region, but that one works). Now sometimes it comments out a single line, sometimes it comments out a line and the ones above it. It doesn't seem to have very consistent behaviour.

What I'd really like it to do is comment out the line the cursor is on unless it's on a begin or end tag, in which case comment out the whole environment. (Actually, I'd settle for just understanding the slightly odd behaviour of the comment macro...)

Answer 1

C-c % runs TeX-comment-or-uncomment-paragraph. For what exactly is considered a paragraph here, see the manual:

Command: TeX-comment-or-uncomment-paragraph
(C-c %) Add or remove % from the beginning of each line in the current paragraph. When removing % characters the paragraph is considered to consist of all preceding and succeeding lines starting with a %, until the first non-comment line.

---

Here's a commenting function that does more or less what you want. Uncommenting an environment only works if LaTeX-syntactic-comments is t (and not always very well even then).

(defun LaTeX-comment-environment-or-line (arg)
  "Comment or uncomment the current line.
If the current line is the \\begin or \\end line of an environment, comment
or uncomment the whole environment."
  (interactive "*P")
  (save-match-data
    (save-excursion
      (beginning-of-line)
      (cond
       ((looking-at (concat "\\s-*\\(" TeX-comment-start-regexp "\\)?\\s-*"
                            (regexp-quote TeX-esc) "begin"))
        (let ((begin (point)))
          (goto-char (match-end 0))
          (LaTeX-find-matching-end)
          (TeX-comment-or-uncomment-region begin (point) arg)))
       ((looking-at (concat "\\s-*\\(" TeX-comment-start-regexp "\\)?\\s-*"
                            (regexp-quote TeX-esc) "end"))
        (let ((end (save-excursion (end-of-line) (point))))
          (LaTeX-find-matching-begin)
          (beginning-of-line)
          (TeX-comment-or-uncomment-region (point) end arg)))
       (t
        (TeX-comment-or-uncomment-region
         (point) (save-excursion (end-of-line) (point)) arg))))))