Expression (computer science)

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Expression (programming)
)

In

mathematical expressions
, is called evaluation. In simple settings, the
floating-point, or complex
).

Expressions are often contrasted with statements—syntactic entities that have no value (an instruction).

Examples

2 + 3 is both an

memory, so y + 6 is also an expression. An example of a relational expression is 4 ≠ 4, which evaluates to false.[3][4]

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.[5] 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

statement
. This asks the implementation to evaluate the expression for its side-effects only and to disregard the result of the expression (e.g. x+1;) unless it is a part of an expression statement that induces side-effects (e.g. y=x+1; or func1(func2());).

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 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

References

  1. ^ Mitchell, J. (2002). Concepts in Programming Languages. Cambridge: Cambridge University Press, 3.4.1 Statements and Expressions, p. 26
  2. ^ Maurizio Gabbrielli, Simone Martini (2010). Programming Languages - Principles and Paradigms. Springer London, 6.1 Expressions, p. 120
  3. ^ Javascript expressions, Mozilla Archived 2012-03-09 at the Wayback Machine Accessed July 6, 2009
  4. ^ Programming in C Archived 2015-01-09 at the Wayback Machine Accessed July 6, 2009
  5. ^ ISO/IEC 9899:1999 section 6.3.2.2, accessed August 31, 2009

External links

  • This article is based on material taken from Expression at the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.