Content deleted Content added
No edit summary |
No edit summary |
||
| (84 intermediate revisions by 63 users not shown) | |||
Line 1:
{{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).
For example 2+3 is an arithmetic and programming expression which evaluates to 5. The assignment expression x=2+3 also evaluates to 5 and is a programming expression, but is an equation in mathematics and not a [[mathematical expression]]. A variable is an expression because it is a pointer to a value in memory, so y+6 is an expression. An example of a programming relational expression would be 4==4 which evaluates to true.<ref> [https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Expressions Javascript expressions, Mozilla]Accessed July 6, 2009</ref><ref> [https://www.cs.drexel.edu/~rweaver/COURSES/ISTC-2/TOPICS/expr.html Programming in C]Accessed July 6, 2009</ref>▼
==Examples==
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.▼
▲
==Void as a result type==
Expressions may or may not have [[Side effect (computer science)|side effect]]s. An expression with side effects does not normally have the property of [[referential transparency (computer science)|referential transparency]]. In many languages (e.g. C++) statements may be ended with a semicolon ';' to turn the expression into an expression [[Statement (programming)|statement]]. This asks the implementation to evaluate the expression for its side-effects only, and disregard the result of the expression{{fact|date = July 2009}}.▼
▲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
[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==
▲
===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==
Line 21 ⟶ 39:
* {{foldoc|Expression}}
{{DEFAULTSORT:Expression (Programming)}}
[[Category:
[[Category:Programming language concepts]]
| |||