### C.4 Tips for Documentation Strings

As noted above, documentation is typically in a commented header block on an Octave function following the copyright statement. The help string shown above is an unformatted string and will be displayed as is by Octave. 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,” just write “Display text in boldface.”
- Use two spaces between the period marking the end of a sentence and the word which opens the next sentence. This convention has no effect for typeset formats like TeX, but improves the readability of the documentation in fixed-width environments such as the Info reader.
- Do not start or end a documentation string with whitespace.
- Format the documentation string so that it fits within 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!
- When choosing variable names try to adhere to the following guidelines.
    vectors : x,y,z,t,w
    matrices : A,B,M
    strings : str,s
    filenames : fname
    cells,cellstrs : c,cstr
- 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.

Octave also allows extensive formatting of the help string of functions using Texinfo. The effect on the online documentation is relatively small, but makes the help string of functions conform to the help of Octave's own functions. However, the effect on the appearance of printed or online documentation will be greatly improved.

The fundamental building block of Texinfo documentation strings is the Texinfo-macro `@deftypefn`, which takes three arguments: The class the function is in, its output arguments, and the function's signature. Typical classes for functions include `Function File` for standard Octave functions, and `Loadable Function` for dynamically linked functions. A skeletal Texinfo documentation string therefore looks like this

         -*- texinfo -*-
         @deftypefn{Function File} {@var{ret} =} fn (...)
         @cindex index term
         Help text in Texinfo format.  Code samples should be marked
         like @code{sample of code} and variables should be marked
         as @var{variable}.
         @seealso{fn2}
         @end deftypefn

This help string must be commented in user functions, or in the help string of the `DEFUN_DLD` macro for dynamically loadable functions. The important aspects of the documentation string are

-*- texinfo -*-
  This string signals Octave that the following text is in Texinfo format, and should be the first part of any help string in Texinfo format.
@deftypefn{class} ... @end deftypefn
  The entire help string should be enclosed within the block defined by deftypefn.
@cindex index term
  This generates an index entry, and can be useful when the function is included as part of a larger piece of documentation. It is ignored within Octave's help viewer. Only one index term may appear per line but multiple @cindex lines are valid if the function should be filed under different terms.
@var{variable}
  All variables should be marked with this macro. The markup of variables is then changed appropriately for display.
@code{sample of code}
  All samples of code should be marked with this macro for the same reasons as the @var macro.
@seealso{function2}
  This is a comma separated list of function names that allows cross referencing from one function documentation string to another.

Texinfo format has been designed to generate output for online viewing with text terminals as well as generating high-quality printed output. To these ends, Texinfo has commands which control the diversion of parts of the document into a particular output processor. Three formats are of importance: info, HTML and TeX. These are selected with

         @ifinfo
         Text area for info only
         @end ifinfo

         @ifhtml
         Text area for HTML only
         @end ifhtml

         @tex
         Text area for TeX only
         @end tex

Note that often TeX output can be used in HTML documents and so often the `@ifhtml` blocks are unnecessary. If no specific output processor is chosen, by default, the text goes into all output processors. It is usual to have the above blocks in pairs to allow the same information to be conveyed in all output formats, but with a different markup. Currently, most Octave documentation only makes a distinction between TeX and all other formats. Therefore, the following construct is seen repeatedly.

         @tex
         text for TeX only
         @end tex
         @ifnottex
         text for info, HTML, plaintext
         @end ifnottex

Another important feature of Texinfo that is often used in Octave help strings is the `@example` environment. An example of its use is

         @example
         @group
         @code{2 * 2}
         @result{} 4
         @end group
         @end example

which produces

         2 * 2
         ⇒ 4

The `@group` block prevents the example from being split across a page boundary, while the `@result{}` macro produces a right arrow signifying the result of a command. If your example is larger than 20 lines it is better NOT to use grouping so that a reasonable page boundary can be calculated.

In many cases a function has multiple ways in which it can be called, and the `@deftypefnx` macro can be used to give alternatives. For example

         -*- texinfo -*-
         @deftypefn {Function File} {@var{a} =} fn (@var{x}, ...)
         @deftypefnx{Function File} {@var{a} =} fn (@var{y}, ...)
         Help text in Texinfo format.
         @end deftypefn

Many complete examples of Texinfo documentation can be taken from the help strings for the Octave functions themselves. A relatively complete example of which is the `nchoosek` function. The Texinfo documentation string for `nchoosek` is

         -*- texinfo -*-
         @deftypefn  {Function File} {@var{c} =} nchoosek (@var{n}, @var{k})
         @deftypefnx {Function File} {@var{c} =} nchoosek (@var{set}, @var{k})

         Compute the binomial coefficient or all combinations of a set of items.

         If @var{n} is a scalar then calculate the binomial coefficient
         of @var{n} and @var{k} which is defined as
         @tex
         $$
          {n \choose k} = {n (n-1) (n-2) \cdots (n-k+1) \over k!}
                        = {n! \over k! (n-k)!}
         $$
         @end tex
         @ifnottex

         @example
         @group
          /   \
          | n |    n (n-1) (n-2) @dots{} (n-k+1)       n!
          |   |  = ------------------------- =  ---------
          | k |               k!                k! (n-k)!
          \   /
         @end group
         @end example

         @end ifnottex
         @noindent
         This is the number of combinations of @var{n} items taken in groups of
         size @var{k}.

         If the first argument is a vector, @var{set}, then generate all
         combinations of the elements of @var{set}, taken @var{k} at a time, with
         one row per combination.  The result @var{c} has @var{k} columns and
         @w{@code{nchoosek (length (@var{set}), @var{k})}} rows.

         For example:

         How many ways can three items be grouped into pairs?

         @example
         @group
         nchoosek (3, 2)
            @result{} 3
         @end group
         @end example

         What are the possible pairs?

         @example
         @group
         nchoosek (1:3, 2)
            @result{}  1   2
                1   3
                2   3
         @end group
         @end example

         @code{nchoosek} works only for non-negative, integer arguments.  Use
         @code{bincoeff} for non-integer and negative scalar arguments, or for
         computing many binomial coefficients at once with vector inputs
         for @var{n} or @var{k}.

         @seealso{bincoeff, perms}
         @end deftypefn

which demonstrates most of the concepts discussed above.