Editing Expression (computer science)

{{Short description|Syntactic entity in a programming language with a determinable value}}
In [[computer science]], an '''expression''' is a [[Syntax (programming languages)|syntactic]] entity in a [[programming language]] that may be evaluated to determine its [[value (computer science)|value]].<ref>[[John C. Mitchell|Mitchell, J.]] (2002). Concepts in Programming Languages. Cambridge: Cambridge University Press, ''3.4.1 Statements and Expressions'', p. 26</ref> It is a combination of one or more [[Constant (programming)|constants]], [[variable (programming)|variable]]s, [[function (programming)|function]]s, and [[operator (programming)|operator]]s that the programming language interprets (according to its particular [[Order of operations|rules of precedence]] and of [[Associative property|association]]) and computes to produce ("to return", in a [[state (computer science)|stateful]] environment) another value. This process, for [[mathematical expression]]s, is called ''evaluation''.
In simple settings, the [[return type|resulting value]] is usually one of various [[primitive data type|primitive types]], such as [[string (computer science)|string]], [[boolean expression|boolean]], or numerical (such as [[integer (computer science)|integer]], [[floating-point number|floating-point]], or [[complex data type|complex]]).

Expressions are often contrasted with [[Statement (computer science)|statement]]s—[[syntactic]] entities that have no value (an instruction).

==Examples==

<code>2 + 3</code> is both an [[arithmetic expression|arithmetic]] and programming expression, which evaluates to <code>5</code>. A variable is an expression because it denotes a value in [[memory (computer)|memory]], so <code>y + 6</code> is also an expression. An example of a [[Relational operator|relational expression]] is <code>4 ≠ 4</code>, which evaluates to <code>false</code>.<ref>[https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Expressions Javascript expressions, Mozilla] {{Webarchive|url=https://web.archive.org/web/20120309174344/https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Expressions |date=2012-03-09 }} Accessed July 6, 2009</ref><ref>[https://www.cs.drexel.edu/~rweaver/COURSES/ISTC-2/TOPICS/expr.html Programming in C] {{Webarchive|url=https://web.archive.org/web/20150109162605/https://www.cs.drexel.edu/~rweaver/COURSES/ISTC-2/TOPICS/expr.html |date=2015-01-09 }} Accessed July 6, 2009</ref>

==Void as a result type==

In [[C (programming language)|C]] and most C-derived languages, a call to a function with a [[Void type|void]] return type is a valid expression, of type void.<ref>
[http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf ISO/IEC 9899:1999] section 6.3.2.2, accessed August 31, 2009
</ref>
Values of type void cannot be used, so the value of such an expression is always thrown away.

==Side effects and elimination==

In many programming languages, a function, and hence an expression containing a function, may have [[Side effect (computer science)|side effects]]. An expression with side effects does not normally have the property of [[referential transparency]]. In many languages (e.g. [[C++]]), expressions may be ended with a semicolon (<code>;</code>) to turn the expression into an expression [[Statement (programming)|statement]].  This asks the implementation to evaluate the expression for its side-effects only and to disregard the result of the expression (e.g. {{code|x+1;}}) unless it is a part of an expression statement that induces side-effects (e.g. {{code|1=y=x+1;}} or {{code|func1(func2());}}).

===Caveats===

The formal notion of a side effect is a change to the abstract state of the running program.

Another class of side effects are changes to the concrete state of the computational system, such as loading data into [[cache memory|cache memories]].  Languages that are often described as "side effect–free" will generally still have concrete side effects that can be exploited, for example, in [[side-channel attack]]s.

Furthermore, the elapsed time evaluating an expression (even one with no other apparent side effects), is sometimes essential to the correct operation of a system, as behaviour in time is easily visible from outside the evaluation environment by other parts of the system with which it interacts, and might even be regarded as the ''primary'' effect such as when performing [[Benchmark (computing)|benchmark testing]].

It depends on the particular programming language specification whether an expression with no abstract side effects can [[Program optimization|legally be eliminated]] from the execution path by the processing environment in which the expression is evaluated.

==See also==
* [[Evaluation strategy]]

==References==
{{reflist}}

==External links==
* {{foldoc|Expression}}

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[[Category:Evaluation strategy]]
[[Category:Programming language concepts]]