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C.5 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.

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} {} nchoosek (@var{n}, @var{k})

Compute the binomial coefficient or all combinations of 
@var{n}.  If @var{n} is a scalar then, calculate the
binomial coefficient of @var{n} and @var{k}, defined as

@tex
$$
 {n \choose k} = {n (n-1) (n-2) \cdots (n-k+1) \over k!}
$$
@end tex
@ifnottex

@example
@group
 /   \
 | n |    n (n-1) (n-2) … (n-k+1)
 |   |  = -------------------------
 | k |               k!
 \   /
@end group
@end example
@end ifnottex

If @var{n} is a vector, this generates all combinations
of the elements of @var{n}, taken @var{k} at a time,
one row per combination.  The resulting @var{c} has size
@code{[nchoosek (length (@var{n}),@var{k}), @var{k}]}.

@code{nchoosek} works only for non-negative integer arguments; use
@code{bincoeff} for non-integer scalar arguments and for using vector
arguments to compute many coefficients at once.

@seealso{bincoeff}
@end deftypefn

which demonstrates most of the concepts discussed above.


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