How to change comment character from # (hash) to % (percent sign) for Octave major mode

Question

By default, comment-region inserts # for comments in Octave major mode:

# foo

I want to modify the behavior so that it inserts the percent sign

% foo 

I used the following

(setq octave-mode-hook
      (lambda () (setq octave-comment-char ?%) ))
(modify-syntax-entry ?% "<"  octave-mode-syntax-table)

But it gives me this error:

Symbol's value as variable is void: octave-mode-syntax-table

I don't know about elisp so well, so how can I solve this?

Edit

Here is the configuration needed to make octave-mode the default one for .m files and change commenting behaviour to that of MATLAB's. Note that for Octave, both % and # are valid for commenting.

(add-to-list 'auto-mode-alist '("\\.m\\'" . octave-mode))

(add-hook 'octave-mode-hook
    (lambda () (progn (setq octave-comment-char ?%)
                      (setq comment-start "% ")
                      (setq comment-add 0))))

Answer 1

Inside your hook put this: (setq comment-start "% "). Also, this will explain why:

comment-region is an interactive compiled Lisp function.

(comment-region BEG END &optional ARG)

Comment or uncomment each line in the region. With just C-u prefix arg, uncomment each line in region BEG .. END. Numeric prefix ARG means use ARG comment characters. If ARG is negative, delete that many comment characters instead.

The strings used as comment starts are built from comment-start and comment-padding; the strings used as comment ends are built from comment-end and comment-padding.

By default, the comment-start markers are inserted at the current indentation of the region, and comments are terminated on each line (even for syntaxes in which newline does not end the comment and blank lines do not get comments). This can be changed with comment-style.

Answer 2

The octave-comment-char was added specifically for your kind of use case, so all you should need is:

(setq octave-comment-char ?%)

No need to change the syntax table, since % is already recognized as a comment starter. The important detail is that the above setq needs to be executed before octave.el is loaded (so its values is correctly propagated to other vars such as comment-start).