Statement (computer science)
In computer programming, a statement is a syntactic unit of an imperative programming language that expresses some action to be carried out. 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 programming languages (e.g. Ada, Algol 60, C, Java, Pascal) make a distinction between statements and definitions/declarations. A definition or declaration specifies the data on which a program is to operate, while a statement specifies the actions to be taken with that data. Statements which cannot contain other  statements are "simple"; those which can contain other statements are "compound". The appearance of a statement (and indeed a program) is determined by its syntax or grammar. The meaning of a statement is determined by its semantics. Simple statements. Simple statements are complete in themselves; these include assignments, subroutine calls, and a few statements which may significantly affect the program flow of control (e.g. goto, return, stop/halt). In some languages, input and output, assertions, and exits are handled by special statements, while other languages use calls to predefined subroutines. Compound statements. Compound statements may contain (sequences of) statements, nestable  to any reasonable depth, and generally involve tests to decide whether or not to obey or repeat these contained statements. Many compound statements are loop commands or choice commands. In theory only one of each of these types of commands is required. In practice there are various special cases which occur quite often; these may make a program easier to understand, may make programming easier, and can often be implemented much more efficiently. There are many subtleties not mentioned here; see the linked articles for details. Syntax. Apart from assignments and subroutine calls, most languages start each statement with a  special word (e.g. goto, if, while, etc.) as shown in the above examples. Various methods have been used to describe the form of statements in different languages; the more formal methods tend to be more precise: BNF uses recursion to express repetition, so various extensions have been proposed to allow direct indication of repetition. Statements and keywords. Some programming language grammars reserve keywords or mark them specially, and do not allow them to be used as identifiers. This often leads to grammars which are easier to parse, requiring less lookahead. No distinguished keywords. Fortran and PL/1 do not have reserved  keywords, allowing statements like: Flagged words. In Algol 60 and Algol 68, special tokens were distinguished explicitly: for publication, in boldface e.g. codice_56; for programming, with some special marking, e.g., a flag (codice_57), quotation marks (codice_58), or underlined (codice_56 on the Elliott 503). This is called "stropping". Tokens that are part of the language syntax thus do not conflict with programmer-defined names. Reserved keywords. Certain names are reserved as part of the programming language and can not be used as programmer-defined names. The majority of the most popular programming languages use reserved keywords. Early examples include FLOW-MATIC  (1953) and COBOL (1959). Since 1970 other examples include Ada, C, C++, Java, and Pascal. The number of reserved words depends on the language: C has about 30 while COBOL has about 400. Semantics. Semantics is concerned with the meaning of a program. The standards documents for many programming languages use BNF or some equivalent to express the syntax/grammar in a fairly formal and precise way, but the semantics/meaning of the program is generally described using examples and English prose. This can result in ambiguity. In some language descriptions the meaning of compound statements is defined by the  use of 'simpler' constructions, e.g. a while loop can be defined by a combination of tests, jumps, and labels, using codice_60 and codice_61. The semantics article describes several mathematical/logical formalisms which have been used to specify semantics in a precise way; these are generally more complicated than BNF, and no single approach is generally accepted as the way to go. Some approaches effectively define an interpreter for the language, some use formal logic to reason about a program, some attach affixes to syntactic entities to ensure consistency, etc. Expressions. A distinction is often made between statements, which are executed , and expressions, which are evaluated. The value obtained from an expression is often used as part of a statement e.g. assignment codice_62 Some programming languages (e.g. C, C++) allow some statements to provide a result (technically all statements provide a result, but this result is often of type 'void' and can't be used for anything). The most useful statement which provides a result is an assignment, the result being the value just assigned. This can be useful for multiple initialisation: It can also result in simple finger trouble completely changing the meaning of some code: Some languages allow multiple  assignment (Algol 60) but don't allow assignments to appear in expressions (Algol 60, Pascal). 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.