In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program is formed by a sequence of one or more statements. A statement will 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 declaring an identifier. A distinction can also be made between simple and compound statements; the latter may contain statements as components.
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Kinds of statements
The following are the major generic kinds of statements with examples in typical imperative languages:
Simple statements
- assignment:
A := A + 1 - call:
CLEARSCREEN() - return:
return 5; - goto:
goto 1 - assertion:
assert(ptr != NULL);
Compound statements
- block:
begin integer NUMBER; WRITE('Number? '); READLN(NUMBER); A:= A*NUMBER 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 - do-loop:
do { computation(&i); } while (i < 10); - for-loop:
for A:=1 to 10 do WRITELN(A) end
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 which 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.
Programming languages
The syntax and semantics of statements is specified by the definition of the programming language.
Most programming languages do not allow self-modifying code. That is, most programming languages do not allow new statements to be created during program execution (Snobol 4 is a language that does) or existing statements to be modified (Lisp is an exception).
See also
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