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# Tips and Standards

This chapter describes no additional features of Octave. Instead it gives advice on making effective use of the features described in the previous chapters.

## Writing Clean Octave Programs

Here are some tips for avoiding common errors in writing Octave code intended for widespread use:

- Since all global variables share the same name space, and all functions share another name space, you should choose a short word to distinguish your program from other Octave programs. Then take care to begin the names of all global variables, constants, and functions with the chosen prefix. This helps avoid name conflicts. If you write a function that you think ought to be added to Octave under a certain name, such as `fiddle_matrix`, don't call it by that name in your program. Call it `mylib_fiddle_matrix` in your program, and send mail to @email{bug-octave@bevo.che.wisc.edu} suggesting that it be added to Octave. If and when it is, the name can be changed easily enough. If one prefix is insufficient, your package may use two or three alternative common prefixes, so long as they make sense. Separate the prefix from the rest of the symbol name with an underscore ``_'`. This will be consistent with Octave itself and with most Octave programs.
- When you encounter an error condition, call the function `error` (or `usage`). The `error` and `usage` functions do not return. See section How Octave Reports Errors.
- Please put a copyright notice on the file if you give copies to anyone. Use the same lines that appear at the top of the function files distributed with Octave. If you have not signed papers to assign the copyright to anyone else, then place your name in the copyright notice.

## Tips for Making Code Run Faster.

Here are some ways of improving the execution speed of Octave programs.

- Avoid looping wherever possible.
- Use iteration rather than recursion whenever possible. Function calls are slow in Octave.
- Avoid resizing matrices unnecessarily. When building a single result matrix from a series of calculations, set the size of the result matrix first, then insert values into it. Write

      result = zeros (big_n, big_m)
      for i = over:and_over
        r1 = ...
        r2 = ...
        result (r1, r2) = new_value ();
      endfor

  instead of

      result = [];
      for i = ever:and_ever
        result = [ result, new_value() ];
      endfor

- Avoid calling `eval` or `feval` whenever possible, because they require Octave to parse input or look up the name of a function in the symbol table. If you are using `eval` as an exception handling mechanism and not because you need to execute some arbitrary text, use the `try` statement instead. See section The `try` Statement.
- If you are calling lots of functions but none of them will need to change during your run, set the variable `ignore_function_time_stamp` to `"all"` so that Octave doesn't waste a lot of time checking to see if you have updated your function files.

## Tips for Documentation Strings

Here are some tips for the writing of documentation strings.

- Every command, function, or variable intended for users to know about should have a documentation string.
- An internal variable or subroutine of an Octave program might as well have a documentation string.
- The first line of the documentation string should consist of one or two complete sentences that stand on their own as a summary. The documentation string can have additional lines that expand on the details of how to use the function or variable. The additional lines should also be made up of complete sentences.
- For consistency, phrase the verb in the first sentence of a documentation string as an infinitive with "to" omitted. For instance, use "Return the frob of A and B." in preference to "Returns the frob of A and B." Usually it looks good to do likewise for the rest of the first paragraph. Subsequent paragraphs usually look better if they have proper subjects.
- Write documentation strings in the active voice, not the passive, and in the present tense, not the future. For instance, use "Return a list containing A and B." instead of "A list containing A and B will be returned."
- Avoid using the word "cause" (or its equivalents) unnecessarily. Instead of, "Cause Octave to display text in boldface," write just "Display text in boldface."
- Do not start or end a documentation string with whitespace.
- Format the documentation string so that it fits in an Emacs window on an 80-column screen. It is a good idea for most lines to be no wider than 60 characters. However, rather than simply filling the entire documentation string, you can make it much more readable by choosing line breaks with care. Use blank lines between topics if the documentation string is long.
- Do not indent subsequent lines of a documentation string so that the text is lined up in the source code with the text of the first line. This looks nice in the source code, but looks bizarre when users view the documentation. Remember that the indentation before the starting double-quote is not part of the string!
- The documentation string for a variable that is a yes-or-no flag should start with words such as "Nonzero means...", to make it clear that all nonzero values are equivalent and indicate explicitly what zero and nonzero mean.
- When a function's documentation string mentions the value of an argument of the function, use the argument name in capital letters as if it were a name for that value. Thus, the documentation string of the operator `/` refers to its second argument as ``DIVISOR'`, because the actual argument name is `divisor`. Also use all caps for meta-syntactic variables, such as when you show the decomposition of a list or vector into subunits, some of which may vary.

## Tips on Writing Comments

Here are the conventions to follow when writing comments.

``#'`
  Comments that start with a single sharp-sign, ``#'`, should all be aligned to the same column on the right of the source code. Such comments usually explain how the code on the same line does its job. In the Emacs mode for Octave, the `M-;` (`indent-for-comment`) command automatically inserts such a ``#'` in the right place, or aligns such a comment if it is already present.
``##'`
  Comments that start with two semicolons, ``##'`, should be aligned to the same level of indentation as the code. Such comments usually describe the purpose of the following lines or the state of the program at that point.

The indentation commands of the Octave mode in Emacs, such as `M-;` (`indent-for-comment`) and `TAB` (`octave-indent-line`) automatically indent comments according to these conventions, depending on the number of semicolons. See section `Manipulating Comments' in The GNU Emacs Manual.

## Conventional Headers for Octave Functions

Octave has conventions for using special comments in function files to give information such as who wrote them. This section explains these conventions.

The top of the file should contain a copyright notice, followed by a block of comments that can be used as the help text for the function. Here is an example:

    ## Copyright (C) 1996, 1997 John W. Eaton
    ##
    ## This file is part of Octave.
    ##
    ## Octave is free software; you can redistribute it and/or
    ## modify it under the terms of the GNU General Public
    ## License as published by the Free Software Foundation;
    ## either version 2, or (at your option) any later version.
    ##
    ## Octave is distributed in the hope that it will be useful,
    ## but WITHOUT ANY WARRANTY; without even the implied
    ## warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
    ## PURPOSE.  See the GNU General Public License for more
    ## details.
    ##
    ## You should have received a copy of the GNU General Public
    ## License along with Octave; see the file COPYING.  If not,
    ## write to the Free Software Foundation, 59 Temple Place -
    ## Suite 330, Boston, MA 02111-1307, USA.

    ## usage: [IN, OUT, PID] = popen2 (COMMAND, ARGS)
    ##
    ## Start a subprocess with two-way communication.  COMMAND
    ## specifies the name of the command to start.  ARGS is an
    ## array of strings containing options for COMMAND.  IN and
    ## OUT are the file ids of the input and streams for the
    ## subprocess, and PID is the process id of the subprocess,
    ## or -1 if COMMAND could not be executed.
    ##
    ## Example:
    ##
    ##  [in, out, pid] = popen2 ("sort", "-nr");
    ##  fputs (in, "these\nare\nsome\nstrings\n");
    ##  fclose (in);
    ##  while (isstr (s = fgets (out)))
    ##    fputs (stdout, s);
    ##  endwhile
    ##  fclose (out);

Octave uses the first block of comments in a function file that do not appear to be a copyright notice as the help text for the file. For Octave to recognize the first comment block as a copyright notice, it must match the regular expression

    ^ Copyright (C).*\n\n This file is part of Octave.

or

    ^ Copyright (C).*\n\n This program is free softwar

(after stripping the leading comment characters). This is a fairly strict requirement, and may be relaxed somewhat in the future.

After the copyright notice and help text come several header comment lines, each beginning with ``## header-name:'`. For example,

    ## Author: jwe
    ## Keywords: subprocesses input-output
    ## Maintainer: jwe

Here is a table of the conventional possibilities for header-name:

``Author'`
  This line states the name and net address of at least the principal author of the library.

      ## Author: John W. Eaton <jwe@bevo.che.wisc.edu>

``Maintainer'`
  This line should contain a single name/address as in the Author line, or an address only, or the string ``jwe'`. If there is no maintainer line, the person(s) in the Author field are presumed to be the maintainers. The example above is mildly bogus because the maintainer line is redundant. The idea behind the ``Author'` and ``Maintainer'` lines is to make possible a function to "send mail to the maintainer" without having to mine the name out by hand. Be sure to surround the network address with ``<...>'` if you include the person's full name as well as the network address.
``Created'`
  This optional line gives the original creation date of the file. For historical interest only.
``Version'`
  If you wish to record version numbers for the individual Octave program, put them in this line.
``Adapted-By'`
  In this header line, place the name of the person who adapted the library for installation (to make it fit the style conventions, for example).
``Keywords'`
  This line lists keywords. Eventually, it will be used by an apropos command to allow people will find your package when they're looking for things by topic area. To separate the keywords, you can use spaces, commas, or both.

Just about every Octave function ought to have the ``Author'` and ``Keywords'` header comment lines. Use the others if they are appropriate. You can also put in header lines with other header names--they have no standard meanings, so they can't do any harm.

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