In
computer science
Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, 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.
Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...
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
operators that the programming language interprets (according to its particular
rules of precedence and of
association) and computes to produce ("to return", in a
stateful environment) another value. This process, for
mathematical expression
In mathematics, an expression is a written arrangement of symbols following the context-dependent, syntactic conventions of mathematical notation. Symbols can denote numbers, variables, operations, and functions. Other symbols include punct ...
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 ...
, such as
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, or numerical (such as
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
,
floating-point
In computing, floating-point arithmetic (FP) is arithmetic on subsets of real numbers formed by a ''significand'' (a Sign (mathematics), signed sequence of a fixed number of digits in some Radix, base) multiplied by an integer power of that ba ...
, or
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
).
Expressions are often contrasted with
statements—
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 ...
entities that have no value (an instruction).
Examples
2 + 3 is both an
arithmetic
Arithmetic is an elementary branch of mathematics that deals with numerical operations like addition, subtraction, multiplication, and division. In a wider sense, it also includes exponentiation, extraction of roots, and taking logarithms.
...
and programming expression, which evaluates to
5. A variable is an expression because it denotes a value in
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
, 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
In medicine, a side effect is an effect of the use of a medicinal drug or other treatment, usually adverse but sometimes beneficial, that is unintended. Herbal and traditional medicines also have side effects.
A drug or procedure usually used ...
. An expression with side effects does not normally have the property of referential transparency
In analytic philosophy and computer science, referential transparency and referential opacity are properties of linguistic constructions, and by extension of languages. A linguistic construction is called ''referentially transparent'' when for an ...
. 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
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 a type of security exploit that leverages information inadvertently leaked by a system—such as timing, power consumption, or electromagnetic or acoustic emissions—to gain unauthorized access to ...
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.
See also
* 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 ...
References
External links
*
{{DEFAULTSORT:Expression (Programming)
Evaluation strategy
Programming language concepts