### H.2 Using Octave Mode

If you are lucky, your sysadmins have already arranged everything so that Emacs automatically goes into Octave mode whenever you visit an Octave code file as characterized by its extension `.m`. If not, proceed as follows.

1. To begin using Octave mode for all `.m` files you visit, add the following lines to a file loaded by Emacs at startup time, typically your `~/.emacs` file:

                 (autoload 'octave-mode "octave-mod" nil t)
                 (setq auto-mode-alist
                       (cons '("\\.m$" . octave-mode) auto-mode-alist))

2. Finally, to turn on the abbrevs, auto-fill and font-lock features automatically, also add the following lines to one of the Emacs startup files:

                 (add-hook 'octave-mode-hook
                           (lambda ()
                             (abbrev-mode 1)
                             (auto-fill-mode 1)
                             (if (eq window-system 'x)
                                 (font-lock-mode 1))))

   See the Emacs manual for more information about how to customize Font-lock mode.

In Octave mode, the following special Emacs commands can be used in addition to the standard Emacs commands.

`C-h m`
  Describe the features of Octave mode.
`LFD`
  Reindent the current Octave line, insert a newline and indent the new line (`octave-reindent-then-newline-and-indent`). An abbrev before point is expanded if `abbrev-mode` is non-`nil`.
`TAB`
  Indents current Octave line based on its contents and on previous lines (`indent-according-to-mode`).
`;`
  Insert an “electric” semicolon (`octave-electric-semi`). If `octave-auto-indent` is non-`nil`, reindent the current line. If `octave-auto-newline` is non-`nil`, automagically insert a newline and indent the new line.
```
  Start entering an abbreviation (`octave-abbrev-start`). If Abbrev mode is turned on, typing ``C-h` or ``?` lists all abbrevs. Any other key combination is executed normally. Note that all Octave abbrevs start with a grave accent.
`M-LFD`
  Break line at point and insert continuation marker and alignment (`octave-split-line`).
`M-TAB`
  Perform completion on Octave symbol preceding point, comparing that symbol against Octave's reserved words and built-in variables (`octave-complete-symbol`).
`M-C-a`
  Move backward to the beginning of a function (`octave-beginning-of-defun`). With prefix argument N, do it that many times if N is positive; otherwise, move forward to the N-th following beginning of a function.
`M-C-e`
  Move forward to the end of a function (`octave-end-of-defun`). With prefix argument N, do it that many times if N is positive; otherwise, move back to the N-th preceding end of a function.
`M-C-h`
  Puts point at beginning and mark at the end of the current Octave function, i.e., the one containing point or following point (`octave-mark-defun`).
`M-C-q`
  Properly indents the Octave function which contains point (`octave-indent-defun`).
`M-;`
  If there is no comment already on this line, create a code-level comment (started by two comment characters) if the line is empty, or an in-line comment (started by one comment character) otherwise (`octave-indent-for-comment`). Point is left after the start of the comment which is properly aligned.
`C-c ;`
  Puts the comment character ‘`#`’ (more precisely, the string value of `octave-comment-start`) at the beginning of every line in the region (`octave-comment-region`). With just `C-u` prefix argument, uncomment each line in the region. A numeric prefix argument N means use N comment characters.
`C-c :`
  Uncomments every line in the region (`octave-uncomment-region`).
`C-c C-p`
  Move one line of Octave code backward, skipping empty and comment lines (`octave-previous-code-line`). With numeric prefix argument N, move that many code lines backward (forward if N is negative).
`C-c C-n`
  Move one line of Octave code forward, skipping empty and comment lines (`octave-next-code-line`). With numeric prefix argument N, move that many code lines forward (backward if N is negative).
`C-c C-a`
  Move to the `real' beginning of the current line (`octave-beginning-of-line`). If point is in an empty or comment line, simply go to its beginning; otherwise, move backwards to the beginning of the first code line which is not inside a continuation statement, i.e., which does not follow a code line ending in ‘`...`’ or ‘`\`’, or is inside an open parenthesis list.
`C-c C-e`
  Move to the `real' end of the current line (`octave-end-of-line`). If point is in a code line, move forward to the end of the first Octave code line which does not end in ‘`...`’ or ‘`\`’ or is inside an open parenthesis list. Otherwise, simply go to the end of the current line.
`C-c M-C-n`
  Move forward across one balanced begin-end block of Octave code (`octave-forward-block`). With numeric prefix argument N, move forward across n such blocks (backward if N is negative).
`C-c M-C-p`
  Move back across one balanced begin-end block of Octave code (`octave-backward-block`). With numeric prefix argument N, move backward across N such blocks (forward if N is negative).
`C-c M-C-d`
  Move forward down one begin-end block level of Octave code (`octave-down-block`). With numeric prefix argument, do it that many times; a negative argument means move backward, but still go down one level.
`C-c M-C-u`
  Move backward out of one begin-end block level of Octave code (`octave-backward-up-block`). With numeric prefix argument, do it that many times; a negative argument means move forward, but still to a less deep spot.
`C-c M-C-h`
  Put point at the beginning of this block, mark at the end (`octave-mark-block`). The block marked is the one that contains point or follows point.
`C-c ]`
  Close the current block on a separate line (`octave-close-block`). An error is signaled if no block to close is found.
`C-c f`
  Insert a function skeleton, prompting for the function's name, arguments and return values which have to be entered without parentheses (`octave-insert-defun`).
`C-c C-h`
  Search the function, operator and variable indices of all info files with documentation for Octave for entries (`octave-help`). If used interactively, the entry is prompted for with completion. If multiple matches are found, one can cycle through them using the standard ‘`,`’ (`Info-index-next`) command of the Info reader.

  The variable `octave-help-files` is a list of files to search through and defaults to `'("octave")`. If there is also an Octave Local Guide with corresponding info file, say, `octave-LG`, you can have `octave-help` search both files by

                (setq octave-help-files '("octave" "octave-LG"))

  in one of your Emacs startup files.

A common problem is that the <RET> key does not indent the line to where the new text should go after inserting the newline. This is because the standard Emacs convention is that <RET> (aka `C-m`) just adds a newline, whereas <LFD> (aka `C-j`) adds a newline and indents it. This is particularly inconvenient for users with keyboards which do not have a special <LFD> key at all; in such cases, it is typically more convenient to use <RET> as the <LFD> key (rather than typing `C-j`).

You can make <RET> do this by adding

         (define-key octave-mode-map "\C-m"
           'octave-reindent-then-newline-and-indent)

to one of your Emacs startup files. Another, more generally applicable solution is

         (defun RET-behaves-as-LFD ()
           (let ((x (key-binding "\C-j")))
             (local-set-key "\C-m" x)))
         (add-hook 'octave-mode-hook 'RET-behaves-as-LFD)

(this works for all modes by adding to the startup hooks, without having to know the particular binding of <RET> in that mode!). Similar considerations apply for using <M-RET> as <M-LFD>. As Barry A. Warsaw bwarsaw@cnri.reston.va.us says in the documentation for his `cc-mode`, “This is a very common question. `:-)` If you want this to be the default behavior, don't lobby me, lobby RMS!”

The following variables can be used to customize Octave mode.

`octave-auto-indent`
  Non-`nil` means auto-indent the current line after a semicolon or space. Default is `nil`.
`octave-auto-newline`
  Non-`nil` means auto-insert a newline and indent after semicolons are typed. The default value is `nil`.
`octave-blink-matching-block`
  Non-`nil` means show matching begin of block when inserting a space, newline or ‘`;`’ after an else or end keyword. Default is `t`. This is an extremely useful feature for automatically verifying that the keywords match—if they don't, an error message is displayed.
`octave-block-offset`
  Extra indentation applied to statements in block structures. Default is 2.
`octave-continuation-offset`
  Extra indentation applied to Octave continuation lines. Default is 4.
`octave-continuation-string`
  String used for Octave continuation lines. Normally ‘`\`’.
`octave-mode-startup-message`
  If `t` (default), a startup message is displayed when Octave mode is called.

If Font Lock mode is enabled, Octave mode will display

- strings in `font-lock-string-face`
- comments in `font-lock-comment-face`
- the Octave reserved words (such as all block keywords) and the text functions (such as ‘`cd`’ or ‘`who`’) which are also reserved using `font-lock-keyword-face`
- the built-in operators (‘`&&`’, ‘`==`’, ...) using `font-lock-reference-face`
- and the function names in function declarations in `font-lock-function-name-face`.

There is also rudimentary support for Imenu (currently, function names can be indexed).

You can generate TAGS files for Emacs from Octave `.m` files using the shell script `octave-tags` that is installed alongside your copy of Octave.

Customization of Octave mode can be performed by modification of the variable `octave-mode-hook`. If the value of this variable is non-`nil`, turning on Octave mode calls its value.

If you discover a problem with Octave mode, you can conveniently send a bug report using `C-c C-b` (`octave-submit-bug-report`). This automatically sets up a mail buffer with version information already added. You just need to add a description of the problem, including a reproducible test case and send the message.