Expression (computer science): Difference between revisions

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An '''expression''' in a [[programming language]] is a combination of explicit [[value (computer science)|value]]s, [[Constant (programming)|constants]], [[variable (programming)|variable]]s, [[operator (programming)|operator]]s, and [[function (programming)|function]]s that are interpreted (''[[Evaluation (disambiguation)|evaluated]]'') according to the particular [[Order of operations|rules of precedence]] and of association for a particular programming language, which computes and then produces (''returns'', in a [[state (computer science)|stateful]] environment) another value. TheThis process, like for [[mathematical expression]]s, is saidcalled toevaluation. ''evaluateThe to''value thatcan value{{Factbe of various [[Data type|datetype]]s, =such Julyas 2009}}numerical, string, and logical.

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 beis 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>

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.

As in [[Expression (mathematics)|math]], the expression ''is'' (or can be said to ''have'') its evaluated value; the expression is a representation of that value. So, in mathematics, an expression is a representation of a value{{fact|date = July 2009}}.

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}}.