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computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as anal ...
, a statement 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) ...
unit of an imperative programming language that expresses some action to be carried out. A
program Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * Program, a part of planning Arts and entertainment Audio * Programm ...
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 Ada may refer to: Places Africa * Ada Foah, a town in Ghana * Ada (Ghana parliament constituency) * Ada, Osun, a town in Nigeria Asia * Ada, Urmia, a village in West Azerbaijan Province, Iran * Ada, Karaman, a village in Karaman Province, T ...
,
Algol 60 ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a ...
, C,
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
,
Pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, Frenc ...
) 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''.Revised ALGOL 60 report section. 4.1. The appearance of a statement (and indeed a program) is determined by its
syntax 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 ( constituenc ...
or grammar. The meaning of a statement is determined by its
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...
.


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 GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function c ...
,
return Return may refer to: In business, economics, and finance * Return on investment (ROI), the financial gain after an expense. * Rate of return, the financial term for the profit or loss derived from an investment * Tax return, a blank document or t ...
, stop/halt). In some languages, input and output, assertions, and exits are handled by special statements, while other languages use calls to predefined subroutines. * assignment **Fortran: ''variable'' = ''expression'' **Pascal, Algol 60, Ada: ''variable'' := ''expression''; **C, C#, C++, PHP, Java: ''variable'' = ''expression''; *
call Call or Calls may refer to: Arts, entertainment, and media Games * Call, a type of betting in poker * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage Music and dance * Call (band), from Lahore, Paki ...
**Fortran: CALL ''subroutine name''(''parameters'') **C, C++, Java, PHP, Pascal, Ada: ''subroutine name''(''parameters''); * assertion **C, C++, PHP: assert(''relational expression''); **Java: assert ''relational expression''; *
goto GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function c ...
**Fortran: GOTO numbered-label **Algol 60: goto ''label''; **C, C++, PHP, Pascal: goto ''label''; *
return Return may refer to: In business, economics, and finance * Return on investment (ROI), the financial gain after an expense. * Rate of return, the financial term for the profit or loss derived from an investment * Tax return, a blank document or t ...
**Fortran: RETURN ''value'' **C, C++, Java, PHP: return ''value''; * stop/halt/exit **Fortran: STOP ''number'' **C, C++: exit(''expression'') **PHP: exit ''number'';


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. ::Notation for the following examples: ::* is any single statement (could be simple or compound). ::* is any sequence of zero or more ::Some programming languages provide a general way of grouping statements together, so that any single can be replaced by a group: ::* Algol 60: begin end ::* Pascal: begin end ::* C, PHP, Java: ::Other programming languages have a different special terminator on each kind of compound statement, so that one or more statements are automatically treated as a group: ::* Ada: if test then end if; 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. * count-controlled loop: ** Algol 60: for index := 1 step 1 until limit do ; ** Pascal: for index := 1 to limit do ; ** C, Java: for ( index = 1; index <= limit; index += 1) ; ** Ada: for index in 1..limit loop end loop ** Fortran 90: * condition-controlled loop with test at start of loop: ** Algol 60: for index := expression while test do ; ** Pascal: while test do ; ** C, Java: while (test) ; ** Ada: while test loop end loop ** Fortran 90: * condition-controlled loop with test at end of loop: ** Pascal: repeat until test; ** C, Java: do while (test) ; ** Ada: loop exit when test; end loop; * condition-controlled loop with test in the middle of the loop: ** C: do while (true) ; ** Ada: loop exit when test; end loop; * if-statement simple situation: ** Algol 60:if test then ; ** Pascal:if test then ; ** C, Java: if (test) ; ** Ada: if test then end if; ** Fortran 77+: * if-statement two-way choice: ** Algol 60:if test then else ; ** Pascal:if test then else ; ** C, Java: it (test) else ; ** Ada: if test then else end if; ** Fortran 77+: * case/switch statement multi-way choice: ** Pascal: ** Ada: ** C, Java: *
Exception handling In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
: ** Ada: begin ''protected code'' except when ''exception specification'' => ''exception handler'' ** Java: try catch (''exception specification'') finally ** Python: try: ''protected code'' except ''exception specification'': ''exception handler'' else: ''no exceptions'' finally: ''cleanup''


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: * Algol 60 used
Backus–Naur form In computer science, Backus–Naur form () or Backus normal form (BNF) is a metasyntax notation for context-free grammars, often used to describe the syntax of languages used in computing, such as computer programming languages, document format ...
(BNF) which set a new level for language grammar specification.Revised ALGOL 60 report section 1.1. * Up until Fortran 77, the language was described in English prose with examples,ANSI FORTRAN 66 standard From Fortran 90 onwards, the language was described using a variant of BNF.ANSI FORTRAN 95 standard * Cobol used a two-dimensional metalanguage.Cobol manual. * Pascal used both syntax diagrams and equivalent BNF.Pascal User Manual and Report Appendix D. BNF uses recursion to express repetition, so various
extensions Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * Ex ...
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 An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable ...
. This often leads to
grammars In linguistics, the grammar of a natural language is its set of structural constraints on speakers' or writers' composition of clauses, phrases, and words. The term can also refer to the study of such constraints, a field that includes doma ...
which are easier to parse, requiring less lookahead.


No distinguished keywords

Fortran and PL/1 do not have reserved keywords, allowing statements like: * in PL/1: **IF IF = THEN THEN ... (the second IF and the first THEN are variables). * in Fortran: **IF (A) X = 10... conditional statement (with other variants) **IF (A) = 2 assignment to a subscripted variable named IF ::As spaces were optional up to Fortran 95, a typo could completely change the meaning of a statement: :*DO 10 I = 1,5 start of a loop with I running from 1 to 5 :*DO 10 I = 1.5 assignment of the value 1.5 to the variable DO10I


Flagged words

In Algol 60 and Algol 68, special tokens were distinguished explicitly: for publication, in boldface e.g. begin; for programming, with some special marking, e.g., a flag ('begin), quotation marks ('begin'), or underlined (begin 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 COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural and, since 2002, object-oriented language. COBOL is primarily u ...
(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.Trouble spots in Algol 60 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 if and goto. The
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comput ...
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. Expressions always evaluate to a value, which statements do not. However, expressions are often used as part of a larger statement. In most programming languages, a statement can consist little more than an expression, usually by following the expression with a statement terminator (semicolon). In such a case, while the expression evaluates to a value, the complete statement does not (the expression's value is discarded). For instance, in C, C++, C#, and many similar languages, x = y + 1 is an expression that will set x to the value of y plus one, and the whole expression itself will evaluate to the same value that x is set to. However, x = y + 1; (note the semicolon at the end) is a statement that will still set x to the value of y plus one because the expression within the statement is still evaluated, but the result of the expression is discarded, and the statement itself does not evaluate to any value. Expressions can also be contained within other expressions. For instance, the expression x = y + 1 contains the expression y + 1, which in turn contains the values y and 1, which are also technically expressions. Although the previous examples show assignment expressions, some languages do not implement assignment as an expression, but rather as a statement. A notable example of this is Python, where = is not an operator, but rather just a separator in the assignment statement. Although Python allows multiple assignments as each assignment were an expression, this is simply a special case of the assignment statement built into the language grammar rather than a true expression.


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 In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an '' ...
* Expression (contrast)


References


External links


PC ENCYCLOPEDIA: Definition of: program statement
{{DEFAULTSORT:Statement (Programming) Programming language concepts Statements