Operator (computer programming): Difference between revisions

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Many operators differ syntactically from user-defined functions. In most languages, a function is [[prefix notation]] with fixed [[Order of operations|precedence]] level and associativity and often with compulsory [[parentheses]] (e.g. <code>Func(a)</code> or <code>(Func a)</code> in [[Lisp (programming language)|Lisp]]). In contrast, many operators are infix notation and involve different use of delimiters such as parentheses.

In general, an operator may be prefix, infix, postfix, [[matchfix]], [[circumfix]] or bifix,<ref>{{Cite web|url=https://reference.wolfram.com/language/tutorial/OperatorInputForms.html.en|title=Operator Input Forms—Wolfram Language Documentation|website=reference.wolfram.com}}</ref><ref>{{Cite web|url=httphttps://maxima.sourceforge.net/docs/manual/maxima_7.html|title=Maxima 5.42.0 Manual: 7. Operators|website=maxima.sourceforge.net}}</ref><ref>{{Cite web|url=https://mythryl.org/my-Prefix__Postfix_and_Circumfix_Operators.html|title=Prefix, Postfix and Circumfix Operators|website=mythryl.org}}</ref><ref>{{Cite web|url=http://doc.perl6.org/language/operators#___top|title=Operators|website=doc.perl6.org}}</ref><ref name=Pribavkina>{{cite book conference|ref= Pribavkina| last1 = Pribavkina| last2= Rodaro| date= August 2010| title= State Complexity of Prefix, Suffix, Bifix and Infix Operators on Regular Languages |journal= Lecture Notes in Computer Science| type= Conference Article| series= Developments in Language Theory| language= English| editionconference= 14th International Conference on Developments in Language Theory| ___location= London Ontario| publisher= Springer| publication-place= Germany| publication-date= 2010| issue= 6224| pages= 376–377 | doi= 10.1007/978-3-642-14455-4_34| isbn= 978-3-642-14454-7| issn= 0302-9743}}</ref> and the syntax of an [[expression (computer science)|expression]] involving an operator depends on its [[arity]] (number of [[operand]]s), precedence, and (if applicable), [[Operator associativity|associativity]]. Most programming languages support [[binary operator]]s and a few [[unary operation|unary operators]], with a few supporting more operands, such as the [[?:]] operator in C, which is ternary. There are prefix unary operators, such as unary minus <code>-x</code>, and postfix unary operators, such as [[post-increment]] <code>x++</code>; and binary operations are infix, such as <code>x + y</code> or <code>x = y</code>. Infix operations of higher arity require additional symbols, such as the [[ternary operator]] ?: in C, written as <code>a ? b : c</code> – indeed, since this is the only common example, it is often referred to as ''the'' ternary operator. Prefix and postfix operations can support any desired arity, however, such as <code>1 2 3 4 +</code>.

=== Semantics ===

The semantics of an operator may significantly differ from that of a normal function. For reference, addition is evaluated like a normal function. For example, <code>x + y</code> can be equivalent to a function <code>add(x, y)</code> in that the arguments are evaluated and then the functional behavior is applied. However, [[Assignment_(computer_science)|assignment]] is different. For example, given <code>a = b</code> the target <code>a</code> is ''not'' evaluated. Instead its value is replaced with the value of <code>b</code>. The [[scope resolution operator|scope resolution]] and element access operators (as in <code>Foo::Bar</code> and <code>a.b</code>, respectively, in the case of e.g. [[C++]]) operate on identifier names; not values.

In C, for instance, the array indexing operator can be used for both read access as well as assignment. In the following example, the [[increment and decrement operators|increment operator]] reads the element value of an array and then assigns the element value.

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== ad hoc polymorphic ==

{{further|Ad hoc polymorphism}}

Some languages provide operators that are '''ad hoc polymorphic''' {{endash}} inherently overloaded. For example, in [[Java (programming language)|Java]] the {{code|+}} operator sums [[number]]s or [[concatenate]]s [[String (computer science)|strings]].

== Customization ==

Some languages support user-defined [[operator overloading|overloading]] (such as [[C++]] and [[Fortran]]). An operator, defined by the language, can be [[function overloading|overloaded]] to behave differently based on the type of input.

Some languages (e.g. C, C++ and [[PHP]]) define a fixed set of operators, while others (e.g. [[Prolog]],<ref>{{Cite web|url=https://www.swi-prolog.org/pldoc/man?predicate=op/3|title=SWI-Prolog -- op/3|website=www.swi-prolog.org}}</ref> [[Seed7]],<ref>{{Cite web|url=httphttps://seed7.sourceforge.net/examples/operator.htm|title=Declare an operator|website=seed7.sourceforge.net}}</ref> [[F Sharp (programming language)|F#]], [[OCaml]], [[Haskell]]) allow for user-defined operators. Some programming languages restrict operator symbols to special characters like {{mono|1='''[[Addition|+]]'''}} or {{mono|1='''[[Assignment (computer science)|:=]]'''}} while others allow names like <code>[[Integer_division#Division_of_integers|div]]</code> (e.g. [[Pascal (programming language)|Pascal]]), and even arbitrary names (e.g. [[Fortran]] where an upto 31 character long operator name is enclosed between dots<ref name="IntelFortran">{{cite web |title=Defined Operations |url=https://www.intel.com/content/www/us/en/docs/fortran-compiler/developer-guide-reference/2023-0/defined-operations.html |publisher=Intel |access-date=6 May 2025}}</ref>).

Most languages do not support user-defined operators since the feature significantly complicates parsing. Introducing a new operator changes the arity and precedence [[lexical specification]] of the language, which affects phrase-level [[lexical analysis]]. Custom operators, particularly via runtime definition, often make correct [[static analysis]] of a program impossible, since the syntax of the language may be Turing-complete, so even constructing the syntax tree may require solving the halting problem, which is impossible. This occurs for [[Perl]], for example, and some dialects of [[Lisp (programming language)|Lisp]].

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|{{code|1=() [] ! ~ ++ -- + - * & / % << >> < <= > >= == != ^ <nowiki>|</nowiki> [[?:]] = =+ =- =* =/ =% =& =^ =<nowiki>|</nowiki>}}<ref>{{Cite web |title=A TUTORIAL INTRODUCTION TO THE LANGUAGE B |url=https://www.bell-labs.com/usr/dmr/www/btut.html |access-date=2024-08-03 |archive-date=2017-04-03 |archive-url=https://web.archive.org/web/20170403063756/https://www.bell-labs.com/usr/dmr/www/btut.html |url-status=dead }}</ref>

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