Statement (computer science)
In computer programming, a statement is the smallest standalone element of an imperative programming language that expresses some action to be carried out. It is an instruction written in a high-level language that commands the computer to perform a specified action.[1] A program written in such a language is formed by a sequence of one or more statements. A statement may have internal components (e.g., expressions).
Many languages (e.g. C) make a distinction between statements and definitions, with a statement only containing executable code and a definition instantiating an identifier, while an expression evaluates to a value only. A distinction can also be made between simple and compound statements; the latter may contain statements as components.
Kinds of statements
The following are the major generic kinds of statements with examples in typical imperative languages:
Simple statements
- assertion:
assert(ptr != NULL); - assignment:
A:= A + 5 - goto:
goto next; - return:
return 5; - call:
CLEARSCREEN()
Compound statements
- block:
begin integer NUMBER; WRITE('Number? '); READLN(NUMBER); A:= A*NUMBER end - do-loop:
do { computation(&i); } while (i < 10); - for-loop:
for A:=1 to 10 do WRITELN(A) end - if-statement:
if A > 3 then WRITELN(A) else WRITELN("NOT YET"); end - switch-statement:
switch (c) { case 'a': alert(); break; case 'q': quit(); break; } - while-loop:
while NOT EOF DO begin READLN end - with-statement:
with open(filename) as f: use(f)
Syntax
The appearance of statements shapes the look of programs. Programming languages are characterized by the type of statements they use (e.g. the curly brace language family). Many statements are introduced by identifiers like if, while or repeat. Often statement keywords are reserved such that they cannot be used as names of variables or functions. Imperative languages typically use special syntax for each statement, which looks quite different from function calls. Common methods to describe the syntax of statements are Backus–Naur form and syntax diagrams.
Semantics
Semantically many statements differ from subroutine calls by their handling of parameters. Usually an actual subroutine parameter is evaluated once before the subroutine is called. This contrasts to many statement parameters that can be evaluated several times (e.g. the condition of a while loop) or not at all (e.g. the loop body of a while loop). Technically such statement parameters are call-by-name parameters. Call-by-name parameters are evaluated when needed (see also lazy evaluation). When call-by-name parameters are available a statement like behaviour can be implemented with subroutines (see Lisp). For languages without call-by-name parameters the semantic description of a loop or conditional is usually beyond the capabilities of the language. Therefore standard documents often refer to semantic descriptions in natural language.
Expressions
In most languages, statements contrast with expressions in that statements do not return results and are executed solely for their side effects, while expressions always return a result and often do not have side effects at all. Among imperative programming languages, Algol 68 is one of the few in which a statement can return a result. In languages that mix imperative and functional styles, such as the Lisp family, the distinction between expressions and statements is not made: even expressions executed in sequential contexts solely for their side effects and whose return values are not used are considered 'expressions'. In purely functional programming, there are no statements; everything is an expression.
This distinction is frequently observed in wording: a statement is executed, while an expression is evaluated. This is found in the exec and eval functions found in some languages: in Python both are found, with exec applied to statements and eval applied to expressions.
Extensibility
Most languages have a fixed set of statements defined by the language, but there have been experiments with extensible languages that allow the programmer to define new statements.
See also
- Comparison of Programming Languages - Statements
- Control flow
- Expression (contrast)
- Extensible languages
References
- ↑ "statement". webopedia. Retrieved 2015-03-03.