In
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 practical disciplines (includin ...
, 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 ( constituenc ...
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 l ...
that may be evaluated to determine its value. It is a combination of one or more
constants
Constant or The Constant may refer to:
Mathematics
* Constant (mathematics), a non-varying value
* Mathematical constant, a special number that arises naturally in mathematics, such as or
Other concepts
* Control variable or scientific const ...
,
variables,
functions, and
operator
Operator may refer to:
Mathematics
* A symbol indicating a mathematical operation
* Logical operator or logical connective in mathematical logic
* Operator (mathematics), mapping that acts on elements of a space to produce elements of another ...
s that the programming language interprets (according to its particular
rules of precedence and of
association) and computes to produce ("to return", in a
stateful
In information technology and computer science, a system is described as stateful if it is designed to remember preceding events or user interactions; the remembered information is called the state of the system.
The set of states a system can oc ...
environment) another value. This process, for
mathematical expressions, is called ''evaluation''.
In simple settings, the
resulting value is usually one of various
primitive types, such as numerical,
string
String or strings may refer to:
*String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects
Arts, entertainment, and media Films
* ''Strings'' (1991 film), a Canadian anim ...
,
boolean
Any kind of logic, function, expression, or theory based on the work of George Boole is considered Boolean.
Related to this, "Boolean" may refer to:
* Boolean data type, a form of data with only two possible values (usually "true" and "false" ...
,
complex data type 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 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. 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 (;) 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 attacks.
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.
See also
* Statement (computer science) (contrast)
* Boolean expression
* Expression (mathematics)
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, ...
* Evaluation strategy
In a programming language, an evaluation strategy is a set of rules for evaluating expressions. The term is often used to refer to the more specific notion of a ''parameter-passing strategy'' that defines the kind of value that is passed to the f ...
References
External links
*
{{DEFAULTSORT:Expression (Programming)
Evaluation strategy
Programming language concepts