Editing Octave style guide

A lot of GNU Octave is written in the Octave language itself.  This document
details the Octave style used by the GNU Octave project.

Being part of the GNU project, Octave inherits the [https://www.gnu.org/prep/standards/standards.html#Writing-C GNU coding standards].  However, those were written
with C in mind and can't always apply to Octave code.

See also the GNU Octave [[C++ style guide]].

== Formatting ==

=== Line Length ===

Keep the length of source lines to 79 characters or less, for maximum
readability in the widest range of environments.  This is inherited from
the [https://www.gnu.org/prep/standards/standards.html#Formatting GNU Coding Standards].

=== Indentation ===

Use only spaces, and indent 2 spaces at a time.

We use spaces for indentation. Absolutely do not use tabs in your code.
You should probably set your editor to emit spaces when you hit the tab key.

=== Whitespace ===

When calling functions, put spaces after commas and before the calling
parentheses, like this:

<pre>x = max (sin (y + 3), 2);</pre>

An exception are matrix or cell constructors:

<pre>
[sin(x), cos(x)]
{sin(x), cos(x)}
</pre>

Here, putting spaces after {{codeline|sin}}, {{codeline|cos}} would result in a
parse error.

For indexing expressions, do ''not'' put a space after the
identifier (this differentiates indexing and function calls nicely).
The space after a comma is not necessary if index expressions are simple,
i.e., you may write

<pre>A(:,i,j)</pre>

but

<pre>A([1:i-1;i+1:n], XI(:,2:n-1))</pre>

When constructing matrices, prefer using the comma rather than the space to
distinguish between columns.

<pre>
  M = [1, 2, 3
       4, 5, 6];
</pre>

However, if the matrix is large or the indentation makes it clear the comma
may be dropped.

<pre>
  prices = [ 1.01  2.02  3.03
            44.04 55.05  6.06];
</pre>

Do include spaces around all binary arithmetic operators, for example

<pre>
  x = 1 / (1 + y) ^ 2;
</pre>

An exception is for extremely simple expressions like <pre>n+1</pre>, in
particular when used as an argument to a function or as part of an indexing
expression. For example, you may write

<pre>
  x(1:end-1)
</pre>

Another exception is for complex arithmetic expressions. It may improve
readability to omit spaces around higher precedence operators, for example

<pre>
  z = cat (dim, (x2.*y3 - x3.*y2), (x3.*y1 - x1.*y3), (x1.*y2 - x2.*y1));
</pre>

== Error messages ==

When you encounter an error condition, call the function {{codeline|error}}
(or {{codeline|print_usage}}).  The {{codeline|error}} and {{codeline|print_usage}} functions
do not return.  It is customary to prefix the error message
with the name of the function that generated it.  For example:

<pre>error ("my_cool_function: input A must be a matrix");</pre>

Octave has several functions that produce error messages according
to the Octave guidelines.  Consider using {{codeline|inputParser}}
and {{codeline|validateattributes}}.

== Naming ==

Use lowercase names if possible.  Uppercase is acceptable for variable
names consisting of 1-2 letters.  Do not use mixed case names.  Function
names must be lowercase.  Function names are global, so choose them
wisely.

=== General naming functions ===

=== Function names ===

For most public functions we are limited by Matlab compatibility.  Use
whatever name Matlab choose.

For functions that are not present in Matlab favour the use of underscores.
For example, {{codeline|base64_decode}}, {{codeline|common_size}}, or
{{codeline|compare_versions}}.  There are exceptions to this:

; Matching C functions
: If the function exists elsewhere with a common name, use it.  For example, {{codeline|dup2}}, {{codeline|waitpid}}, {{codeline|printf}}, {{codeline|argv}}, or {{codeline|getopt}}.
; Matching similar functions
: If there are similarly named functions, consider using same style.  For example, {{codeline|fftconvn}} and {{codeline|histthresh}}, match the naming of {{codeline|fftconv2}} and {{codeline|graythresh}}.

=== Variable names ===

Avoid reusing the names of other functions as local variable names.  For
example, try to avoid naming local variables {{codeline|abs}},
{{codeline|log}}, or {{codeline|pow}}.  These functions may be used in a
later change and may lead to confusing errors.

An exception is the use of {{codeline|i}} and {{codeline|j}} as loop indices.
If a function has nothing to do with complex arithmetic, it is common and
acceptable to use {{codeline|i}} and {{codeline|j}} as local variables in
for loops.

== Quoted Strings ==

Always use double quotes for strings and characters rather than the Matlab single quote convention. Both quote types are accepted by Octave, but double quoted strings are interpreted slightly differently (see [https://www.gnu.org/software/octave/doc/interpreter/Strings.html Strings] in the manual for details).

'''Do:'''

<pre>
a = "Hello, world";
b = "x";
disp ("This \"string\" contains a\nnewline");
</pre>

'''Don't:'''

<pre>
s = 'Hello, world';
if (x(1) == 'c')
  disp ('Don''t quote one character this way, even if you''re a C programmer');
endif
</pre>

There are a few edge cases where single quoted strings may be preferable, and are permitted as exceptions under this style guide.

; String containing double quotes
: A string that contains many double quote characters itself, where escaping all of them with backslashes becomes inconvenient, may be easier with single quotes.
; String containing backslashes
: A string that contains literal backslashes, in particular a regular expression pattern, where doubly escaping certain character sequences is both inconvenient and harder to read, is usually better done with single quotes.
; Argument interpreted differently
: A string argument to the regexp family of functions may be interpreted differently depending on whether it is a double quoted or single quoted string. Certain escape sequences are interpreted only in a single quoted string for Matlab compatibility.

== ending blocks ==

Always use a specific end-of-block statement (like {{codeline|endif}},
{{codeline|endswitch}}) rather than the generic {{codeline|end}}.

Enclose the condition of an {{codeline|if}}, {{codeline|while}}, {{codeline|until}}, or
{{codeline|switch}} statement in parentheses, as in C:

<pre>
if (isvector (a))
  s = sum (a);
endif
</pre>

Do not do this, however, with the iteration counter portion of a {{codeline|for}}
statement.  Write:

<pre>
for i = 1:n
  b(i) = sum (a(:,i));
endfor
</pre>

== ! operator ==

The Octave operator {{codeline|!}} should be used for logical negation, rather than
{{codeline|~}}.  The negation operator is written with a space between the operator
and its target, e.g., {{codeline|! A}}.

Use {{codeline|!=}} instead of {{codeline|~=}}.

== Comments ==

=== # or % ===

Always use {{Codeline|#}} to write comments.

Absolutely do not use {{codeline|%#}} or mix {{codeline|%}} and {{codeline|#}}
in the same file.

=== Block and Inline comment ===

Use a single {{codeline|#}} for inline comments.  Use double {{codeline|##}}
for block comments.

Comments that start with a single sharp-sign, {{codeline|#}}, are used to explain
the code on the same line as the comment itself.  These comments should
all be aligned to the same column to the right of the source code.  In
the Emacs mode for Octave, the {{codeline|M-;}} (@code{indent-for-comment})
command automatically inserts such a {{codeline|#}} in the right place, or
aligns such a comment if it is already present.  Example:

<pre>
C = 2 * pi * r;    # formula for circumference of a circle
</pre>

Comments that start with a double sharp-sign, {{codeline|##}}, are stand-alone
comments that occupy an entire line.  These comments 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.  Example:

<pre>
## Calculate area and volume of a sphere
A = 4 * pi * r^2;
V = 4/3 * pi * r^3;
</pre>

=== Commenting out code ===

Do not comment code out.  If the code is no longer used, remove it.  We use
version control, we can always bring it back.

=== %! for test and demo blocks ===

Any demos or Built-In Self Tests (BIST) using the {{codeline|%!demo}} or
{{codeline|%!test}} syntax should begin two lines after the {{codeline|endfunction}}
keyword.  Demo blocks should be listed before test blocks.

See the section Writing tests on the [[Tests]] page.

=== FIXME notes ===

The preferred comment mark for places that may need further attention is
with {{codeline|FIXME:}} comments.