Editing Question mark (section)

==Computing==
In [[computing]], the question mark [[Character (computing)|character]] is represented by [[ASCII]] code 63 (0x3F hexadecimal), and is located at Unicode code-point {{unichar|003F|QUESTION MARK|html=}}. The full-width (double-byte) equivalent ('''？'''), is located at code-point {{unichar|FF1F|FULLWIDTH QUESTION MARK|html=}}.<ref>{{cite web |title=Character Codes – HTML Codes, Hexadecimal Codes & HTML Names |url=http://www.character-code.com/ |work=Character-Code.com |access-date=August 7, 2016 |archive-date=August 7, 2016 |archive-url=https://web.archive.org/web/20160807130637/http://www.character-code.com/ |url-status=live}}</ref>

The [[Inverted question mark and exclamation point|inverted question mark]] ('''¿''') corresponds to Unicode code-point {{unichar|00BF|INVERTED QUESTION MARK|html=}}, and can be accessed from the keyboard in [[Microsoft Windows]] on the default US layout by holding down the [[Alt key|<kbd>Alt</kbd>]] and typing either <kbd>1 6 8</kbd> (ANSI) or <kbd>0 1 9 1</kbd> (Unicode) on the numeric keypad. In [[GNOME]] applications on [[Linux]] operating systems, it can be entered by typing the hexadecimal Unicode character (minus leading zeros) while holding down both [[Ctrl key|<kbd>Ctrl</kbd>]] and [[Shift key|<kbd>Shift</kbd>]], i.e.: <kbd>Ctrl Shift B F</kbd>. In recent [[XFree86]] and [[X.Org Server|X.Org]] incarnations of the [[X Window System]], it can be accessed as a compose sequence of two straight question marks, i.e. pressing [[Compose key|<kbd>Compose</kbd>]]<kbd> ? ?</kbd> yields <samp>¿</samp>. In [[classic Mac OS]] and [[Mac OS X]] (macOS), the key combination [[Option key|<kbd>Option</kbd>]]<kbd> Shift ?</kbd> produces an inverted question mark.

In shell and scripting languages, the question mark is often utilized as a [[wildcard character]]: a symbol that can be used to substitute for any other character or characters in a [[String (computer science)|string]]. In particular, [[glob (programming)|filename globbing]] uses "?" as a substitute for any one character, as opposed to the [[asterisk]], "*", which matches zero or more characters in a string.

The question mark is used in ASCII renderings of the [[International Phonetic Alphabet]], such as [[SAMPA]], in place of the [[glottal stop]] symbol, {{IPA|'''ʔ'''}}, (which resembles "'''?'''" without the dot), and corresponds to Unicode code point {{unichar|0294|LATIN LETTER GLOTTAL STOP|html=}}.

In [[computer programming]], the symbol "'''?'''" has a special meaning in many [[programming language]]s. In [[C (programming language)|C]]-descended languages, <code>?</code> is part of the [[?:|<code>?:</code>]] operator, which is used to evaluate simple [[Boolean domain|boolean conditions]]. In [[C Sharp (programming language)|C#]] 2.0, the <code>?</code> modifier is used to handle [[Nullable type|nullable data types]] and <code>??</code> is the [[null coalescing operator]]. In the [[POSIX]] syntax for [[regular expression]]s, such as that used in [[Perl]] and [[Python (programming language)|Python]], <code>?</code> stands for "zero or one instance of the previous subexpression", i.e. an optional element. It can also make a quantifier like <code>{x,y}</code>, <code>+</code> or <code>*</code> match as few characters as possible, making it lazy, e.g. <code lang="js">/^.*?px/</code> will match the substring <code>165px</code> in <code>165px 17px</code> instead of matching <code>165px 17px</code>.{{efn|The [[Perl Compatible Regular Expressions]] library implements the <code>U</code> flag, which reverses behavior of quantifiers: these become lazy by default, and <code>?</code> can make them greedy.|name="pcre-qm-Uflag"}} In certain implementations of the [[BASIC]] programming language, the <code>?</code> character may be used as a shorthand for the "print" function; in others (notably the [[BBC BASIC]] family), <code>?</code> is used to address a single-byte memory location. In [[OCaml]], the question mark precedes the label for an optional parameter. In [[Scheme (programming language)|Scheme]], as a convention, symbol names ending in <code>?</code> are used for predicates, such as <code>odd?</code>, <code>null?</code>, and <code>eq?</code>. Similarly, in [[Ruby (programming language)|Ruby]], method names ending in <code>?</code> are used for predicates. In [[Swift (programming language)|Swift]] a type followed by <code>?</code> denotes an [[option type]]; <code>?</code> is also used in "optional chaining", where if an option value is nil, it ignores the following operations. Similarly, in [[Kotlin (programming language)|Kotlin]], a type followed by <code>?</code> is [[Nullable type|nullable]] and functions similar to option chaining are supported. In [[APL (programming language)|APL]], <code>?</code> generates random numbers or a random subset of indices. In [[Rust (programming language)|Rust]], a <code>?</code> suffix on a function or method call indicates error handling. In [[SPARQL]], the question mark is used to introduce variable names, such as <code>?name</code>. In [[MUMPS]], it is the pattern match operator.

In the [[Xbase]] family of programming languages, which includes [[dBase]] and [[FoxPro]], either one or two question marks at the start of a line of code serve as a shorthand for the Print function. The effect is to evaluate the following expression(s) and to send the result(s) either to the screen or a printer. A single question mark sends a carriage return and line feed before the output; this is not the case with a double question mark.<ref>{{cite web |title=The Xbase++ Reference Documentation |url=https://doc.alaska-software.com/ |website=Alaska Software |access-date=23 March 2025}}</ref><ref>{{cite web |title=Visual FoxPro Reference |url=https://hackfox.github.io/section4/ |website=Hacker’s Guide to Visual FoxPro |publisher=Hentzenwerke Publishing |access-date=23 March 2025}}</ref>

In many [[Web browser]]s and other computer programs, when converting text between encodings, it may not be possible to map some characters into the target [[character set]]. In this situation it is common to replace each unmappable character with a question mark <samp>?</samp>, inverted question mark <samp>¿</samp>, or the Unicode [[replacement character]], usually rendered as a white question mark in a black diamond: {{unichar|FFFD|REPLACEMENT CHARACTER}}. This commonly occurs for apostrophes and quotation marks when they are written with software that uses its own proprietary non-standard code for these characters, such as [[Microsoft Office]]'s [[Quotation marks in English#Smart quotes|"smart quotes"]].

The generic [[Uniform Resource Locator|URL]] syntax allows for a [[query string]] to be appended to a resource location in a Web address so that additional information can be passed to a script; the query mark, <code>?</code>, is used to indicate the start of a query string. A query string is usually made up of a number of different field/value pairs, each separated by the [[ampersand]] symbol, <code>&</code>, as seen in this URL:

<code><nowiki>http://www.example.com/search.php?query=testing&database=English</nowiki></code>

Here, a script on the page <samp>search.php</samp> on the server <samp>www.example.com</samp> is to provide a response to the query string containing the pairs <samp>query=</samp><kbd>testing</kbd> and <samp>database=</samp><kbd>English</kbd>.