computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...

, an expression is a

syntactic In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure (constituency), ...

entity in a

programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...

that may be evaluated to determine its value. It is a combination of one or more constants,

variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...

function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...

s, and operators that the programming language interprets (according to its particular rules of precedence and of

association Association may refer to: *Club (organization), an association of two or more people united by a common interest or goal *Trade association, an organization founded and funded by businesses that operate in a specific industry *Voluntary associatio ...

) and computes to produce ("to return", in a stateful environment) another value. This process, for

mathematical expression In mathematics, an expression or mathematical expression is a finite combination of symbols that is well-formed according to rules that depend on the context. Mathematical symbols can designate numbers ( constants), variables, operations, f ...

s, is called ''evaluation''. In simple settings, the resulting value is usually one of various

primitive types In computer science, primitive data types are a set of basic data types from which all other data types are constructed. Specifically it often refers to the limited set of data representations in use by a particular processor, which all compiled pr ...

, such as numerical, string, boolean,

complex data type Some programming languages provide a complex data type for complex number storage and arithmetic as a built-in (primitive) data type. In some programming environments the term ''complex data type'' (in contrast to primitive data types) is a synony ...

or other types. Expression is often contrasted with statement—a syntactic entity that has no value (an instruction).

Examples

For example, 2 + 3 is both an arithmetic and programming expression, which evaluates to 5. A variable is an expression because it denotes a value in memory, so y + 6 is also an expression. An example of a relational expression is 4 ≠ 4, which evaluates to false.

Void as a result type

In C and most C-derived languages, a call to a function with a

void Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a ...

return type is a valid expression, of type void.ISO/IEC 9899:1999
section 6.3.2.2, accessed August 31, 2009 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 effects In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...

. An expression with side effects does not normally have the property of

referential transparency In computer science, referential transparency and referential opacity are properties of parts of computer programs. An expression is called ''referentially transparent'' if it can be replaced with its corresponding value (and vice-versa) withou ...

. In many languages (e.g. C++), expressions may be ended with a semicolon (;) to turn the expression into an expression statement. This asks the implementation to evaluate the expression for its side-effects only and to disregard the result of the expression (e.g. ) unless it is a part of an expression statement that induces side-effects (e.g. or ).

Caveats

Note that 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 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 In computer security, a side-channel attack is any attack based on extra information that can be gathered because of the fundamental way a computer protocol or algorithm is implemented, rather than flaws in the design of the protocol or algori ...

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

References

External links

* {{DEFAULTSORT:Expression (Programming) Evaluation strategy Programming language concepts