Editing Number sign (section)

=== Computing ===
* In [[Unicode]] and [[ASCII]], the symbol has a [[code point]] as {{unichar|0023}} and [[entity code]] {{code|&num;}} in [[HTML5]].<ref>HTML5 is the only version of HTML that has a named entity for the number sign, see https://www.w3.org/TR/html4/sgml/entities.html {{Webarchive|url=https://web.archive.org/web/20180401051616/http://www.w3.org/TR/html4/sgml/entities.html |date=2018-04-01 }} ("The following sections present the complete lists of character entity references.") and https://www.w3.org/TR/2014/CR-html5-20140731/syntax.html#named-character-references {{Webarchive|url=https://web.archive.org/web/20170805013240/http://www.w3.org/TR/2014/CR-html5-20140731/syntax.html#named-character-references |date=2017-08-05 }} ("num;").</ref>
* In many scripting languages and data file formats, especially ones that originated on Unix, {{code|#}} introduces a comment that goes to the end of the line.<ref name="hash character">{{cite web |title=CSS Syntax and Selectors |url=https://www.w3schools.com/css/css_syntax.asp |website=W3Schools |access-date=2019-07-15 |archive-date=2019-07-12 |archive-url=https://web.archive.org/web/20190712051145/https://www.w3schools.com/css/css_syntax.asp |url-status=live }}</ref> The combination {{code|#!}} at the start of an executable file is a "[[Shebang (Unix)|shebang]]", "hash-bang" or "pound-bang", used to tell the operating system which program to use to run the script (see [[Magic number (programming)|magic number]]). This combination was chosen so it would be a comment in the scripting languages.
** {{code|#!}} is the symbol of the CrunchBang Linux distribution.
* In the [[Perl]] programming language, {{code|#}} is used as a modifier to array syntax to return the index number of the last element in the [[Array data type|array]], e.g., an array's last element is at {{code|$array[$#array]}}. The number of elements in the array is {{code|$#array + 1}}, since Perl arrays default to using zero-based indices. If the array has not been defined, the return is also undefined. If the array is defined but has not had any elements assigned to it, e.g., {{code|1=@array = ()}}, then {{code|$#array}} returns {{code|−1}}. See the section on [[Perl language structure#Array functions|Array functions]] in the Perl language structure article.
* In both the [[C preprocessor|C]] and [[C++]] preprocessors, as well as in other syntactically C-like languages, {{code|#}} is used to start a preprocessor [[Directive (programming)|directive]]. Inside macros, after {{code|#define}}, it is used for various purposes; for example, the double pound (hash) sign {{code|##}} is used for token [[concatenation]].
* In [[Unix shell]]s, {{code|#}} is placed by convention at the end of a [[command prompt]] to denote that the user is working as [[Superuser|root]].
* {{code|#}} is used in a [[URL]] of a [[web page]] or other resource to introduce a "[[fragment identifier]]" – an id which defines a position within that resource. In HTML, this is known as an [[HTML element#Anchor|anchor link]]. For example, in the URL {{code|https://en.wikipedia.org/wiki/Number_sign#Computing}} the portion after the {{code|#}} ({{code|Computing}}) is the fragment identifier, in this case denoting that the display should be moved to show the tag marked by {{code|1=<span id="Computing">...</span>|2=html}} in the HTML.<ref>{{cite web|url=http://www.w3.org/TR/html4/intro/intro.html#fragment-uri|title=Introduction to HTML|access-date=16 September 2014|archive-date=16 August 2008|archive-url=https://web.archive.org/web/20080816014507/http://www.w3.org/TR/html4/intro/intro.html#fragment-uri|url-status=live}}</ref>
* [[Internet Relay Chat]]: on (IRC) servers, {{code|#}} precedes the name of every [[IRC channels|channel]] that is available across an entire IRC network.
* In [[blog]]s, {{code|#}} is sometimes used to denote a [[permalink]] for that particular weblog entry.
* In [[lightweight markup language]]s, such as [[wiki]]text, {{code|#}} is often used to introduce numbered list items.
* {{code|#}} is used in the [[Modula-2]] and [[Oberon (programming language)|Oberon]] programming languages designed by [[Niklaus Wirth]] and in the [[Component Pascal]] language derived from Oberon to denote the ''not equal'' symbol, as a stand-in for the mathematical unequal sign {{char|≠}}, being more intuitive than {{code|<>}} or {{code|1=!=}}. For example: {{nowrap|{{code|IF i # 0 THEN ...|cp}}}}
* In [[Rust (programming language)|Rust]], {{code|#}} is used for attributes such as in {{code|#[test]}}.
* In [[OCaml]], {{code|#}} is the operator used to call a method.
* In [[Common Lisp]],<ref>{{cite web|url=http://www.lispworks.com/documentation/HyperSpec/Body/02_dh.htm|title=Lispworks.com|access-date=16 September 2014|archive-date=10 October 2014|archive-url=https://web.archive.org/web/20141010014731/http://www.lispworks.com/documentation/HyperSpec/Body/02_dh.htm|url-status=live}}</ref> {{code|#}} is a dispatching [[Lisp reader|read macro]] character used to extend the [[S-expression]] syntax with short cuts and support for various data types ([[complex number]]s, vectors and more).
* In [[Scheme (programming language)|Scheme]], {{code|#}} is the prefix for certain syntax with special meaning.
* In [[Standard ML]], {{code|#}}, when prefixed to a field name, becomes a projection function (function to access the field of a record or tuple); also, {{code|#}} prefixes a [[string literal]] to turn it into a character literal.
* In [[Wolfram Mathematica|Mathematica]] syntax, {{code|#}}, when used as a variable, becomes a pure function (a placeholder that is mapped to any variable meeting the conditions).
* In [[LaTeX]], {{code|#}}, when prefixing a number, references an arguments for a user defined command. For instance <syntaxhighlight lang="tex" inline>\newcommand{\code}[1]{\texttt{#1}}</syntaxhighlight>.
* In [[Javadoc]],<ref>{{cite web|url=http://download.oracle.com/javase/1.5.0/docs/tooldocs/windows/javadoc.html#@see|title=Oracle.com|access-date=16 September 2014|archive-date=28 October 2011|archive-url=https://web.archive.org/web/20111028092204/http://download.oracle.com/javase/1.5.0/docs/tooldocs/windows/javadoc.html#@see|url-status=live}}</ref> {{code|#}} is used with the {{code|@see}} tag to introduce or separate a field, constructor, or method member from its containing class.
* In [[Core War|Redcode]] and some other dialects of [[assembly language]], {{code|#}} is used to denote immediate mode addressing, e.g., {{code|LDA #10}}, which means "load accumulator A with the value 10" in [[MOS 6502]] assembly language.
* in [[HTML]], [[Cascading Style Sheets|CSS]], [[Scalable Vector Graphics|SVG]], and other computing applications {{code|#}} is used to identify a color specified in [[hexadecimal]] format, e.g., {{code|#FFAA00}}. This usage comes from [[X11]] color specifications, which inherited it from early assembler dialects that used {{code|#}} to prefix hexadecimal constants, e.g.: ZX Spectrum [[Zilog Z80|Z80]] assembly.<ref>{{cite web|url=http://www.worldofspectrum.org/pub/sinclair/games-info/h/HiSoftDevpacV3.pdf|title=HISOFT DEVPAC ZX Spectrum Programmer's Manual|website=worldofspectrum.org|access-date=2017-10-03|archive-date=2018-11-12|archive-url=https://web.archive.org/web/20181112231113/http://www.worldofspectrum.org/pub/sinclair/games-info/h/HiSoftDevpacV3.pdf|url-status=live}}</ref>
* In [[Be-Music Script]], every command line starts with {{code|#}}. Lines starting with characters other than "#" are treated as comments.
* The use of the hash symbol in a [[hashtag]] is a phenomenon conceived by [[Chris Messina (open-source advocate)|Chris Messina]], and popularized by social media network [[Twitter]], as a way to direct conversations and topics amongst users. This has led to an increasingly common tendency to refer to the symbol itself as "hashtag".<ref name="Time">{{Cite magazine|url=https://time.com/2870942/hashtag-oed-oxford-english-dictionary/|title=You'll Never Guess the Real Name for a Hashtag|last=Nicks|first=Denver|date=June 13, 2014|magazine=TIME|access-date=May 5, 2016|archive-date=May 11, 2016|archive-url=https://web.archive.org/web/20160511235410/http://time.com/2870942/hashtag-oed-oxford-english-dictionary/|url-status=live}}</ref>
* In programming languages like PL/1 and Assembler used on IBM mainframe systems, as well as JCL (Job Control Language), the {{code|#}} (along with {{code|$}} and {{code|@}}) are used as additional letters in identifiers, labels and data set names.
* In [[J (programming language)|J]], {{code|#}} is the ''Tally'' or ''Count'' function,<ref>{{cite web|url=https://code.jsoftware.com/wiki/Vocabulary/number|title=Vocabulary/number|access-date=November 20, 2019|website=J NuVoc|archive-date=February 14, 2020|archive-url=https://web.archive.org/web/20200214105150/https://code.jsoftware.com/wiki/Vocabulary/number|url-status=live}}</ref> and similarly in [[Lua (programming language)|Lua]], {{code|#}} can be used as a shortcut to get the length of a table, or get the length of a string. Due to the ease of writing "#" over longer function names, this practice has become standard in the Lua community.
* In Dyalog [[APL (programming language)|APL]], {{code|#}} is a reference to the root [[namespace]] while {{code|##}} is a reference to the current space's parent namespace.
* In [[Ada (programming language)|Ada]], the {{code|#}} character is used in based integer literals, which take the form {{code|base#digits#}}, where {{code|base}} is an integer from 2 to 16 specifying the radix, and {{code|digits}} are the digits valid in that base (0-9, optionally A-F for bases above 10).