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{{Programming paradigms}}

In [[computer science]], '''imperative programming''' is a [[programming paradigm]] that describes computation in terms of [[statement (programming)|statement]]s that change a program [[state (computer science)|state]].  In much the same way that [[imperative mood]] in [[natural language]]s expresses commands to take action, imperative programs define sequences of commands for the computer to perform.

The term is used in opposition to [[declarative programming]], which expresses ''what'' needs to be done, without prescribing ''how'' to do it in terms of sequences of actions to be taken.  [[Functional programming|Functional]] and [[logical programming]] are examples of a more declarative approach.

== Imperative, procedural, and declarative programming ==

[[Procedural programming]] is imperative programming in which the program is built from one or more procedures (also known as [[subroutine]]s or functions). The terms are often used as synonyms, but the use of procedures has a dramatic effect on how imperative programs appear and how they are constructed. Heavily procedural programming, in which [[state change]]s are localized to procedures or restricted to explicit arguments and returns from procedures, is known as [[structured programming]]. From the 1960s onwards, structured programming and [[modular programming]] in general, have been promoted as techniques to improve the [[maintainability]] and overall quality of imperative programs. [[Object-oriented programming]] extends this approach.{{Citation needed|date=September 2009}}<!-- this does not follow at all? -->

Procedural programming could be considered as a step towards declarative programming. A programmer can often tell, simply by looking at the names, arguments and return types of procedures (and related comments), what a particular procedure is supposed to do - without necessarily looking at the detail of how the procedure achieves its result. At the same time, a complete program is still imperative since it 'fixes' the statements to be executed and their order of execution to a large extent.

[[Declarative programming]] is a non-imperative style of programming in which programs describe the desired results of the program, without explicitly listing command or steps that need to be carried out to achieve the results. [[Functional programming|Functional]] and [[logical programming]] languages are characterized by a declarative programming style.

In a [[pure functional language]], such as [[Haskell (programming language)|Haskell]], all functions are [[pure function|without side effects]], and state changes are only represented as functions that transform the state. Although pure functional languages are non-imperative, they often provide a facility for describing the effect of a function as a series of steps. Other functional languages, such as [[Lisp (programming language)|Lisp]], [[OCaml]] and [[Erlang (programming language)|Erlang]], support a mixture of procedural and functional programming.

In [[logic programming|logical programming languages]], programs consist of logical statements, and the program executes by searching for proofs of the statements. As in functional programming languages, some logical programming languages such as [[Prolog]], and [[database query language]]s such as [[SQL]], while declarative in principle,
also support a procedural style of programming.

==Overview==
The hardware implementation of almost all computers is imperative; nearly all computer hardware is designed to execute [[machine code]], which is native to the computer, written in the imperative style.  From this low-level perspective, the program state is defined by the contents of memory, and the statements are instructions in the native machine language of the computer.  Higher-level imperative languages use [[variable (programming)|variable]]s and more complex statements, but still follow the same paradigm.  [[Recipe]]s and process [[checklist]]s, while not [[computer program]]s, are also familiar concepts that are similar in style to imperative programming; each step is an instruction, and the physical world holds the state.  Since the basic ideas of imperative programming are both conceptually familiar and directly embodied in the hardware, most computer languages are in the imperative style.

[[Assignment (computer science)|Assignment statements]], in general, perform an operation on information located in memory and store the results in memory for later use.  High-level imperative languages, in addition, permit the [[Evaluation (disambiguation)|evaluation]] of complex [[Expression (programming)|expressions]], which may consist of a combination of [[Arithmetic#Arithmetic operation|arithmetic operations]] and [[function (mathematics)|function]] evaluations, and the assignment of the resulting value to memory.  Looping statements (such as in [[while loop]]s, [[do while loop]]s and [[for loop]]s) allow a sequence of statements to be executed multiple times.  Loops can either execute the statements they contain a predefined number of times, or they can execute them repeatedly until some condition changes.  [[Conditional (programming)|Conditional]] [[Branch (computer science)|branching]] statements allow a sequence of statements to be executed only if some condition is met.  Otherwise, the statements are skipped and the execution sequence continues from the statement following them.  Unconditional branching statements allow the execution sequence to be transferred to some other part of the program.  These include the jump (called "[[GOTO|goto]]" in many languages), [[SWITCH]] and the subprogram, or [[procedure]], call (which usually returns to the next statement after the call).
==Block structure==
{{main|block (programming)}}
Early in the development of [[high level language]]s, the introduction of the block enabled the construction of programs in which a group of statements and declarations could be treated as if they were a single statement.  This, alongside the introduction of [[subroutine|subroutine]]s, enabled complex structure to be expressed by hierarchical decomposition into simpler procedural structures.
==History==
The earliest imperative languages were the machine languages of the original computers.  In these languages, instructions were very simple, which made hardware implementation easier, but hindered the creation of complex programs.  [[FORTRAN]], developed by [[John Backus]] at [[International Business Machines|IBM]] starting in 1954, was the first major programming language to remove the obstacles presented by machine code in the creation of complex programs.  FORTRAN was a [[compiled language]] that allowed named variables, complex expressions, subprograms, and many other features now common in imperative languages.  The next two decades saw the development of a number of other major high-level imperative programming languages.  In the late 1950s and 1960s, [[ALGOL]] was developed in order to allow mathematical algorithms to be more easily expressed, and even served as the [[operating system]]'s target language for some computers.  [[COBOL]] (1960) and [[BASIC]] (1964) were both attempts to make programming syntax look more like English.  In the 1970s, [[Pascal programming language|Pascal]] was developed by [[Niklaus Wirth]], and [[C (programming language)|C]] was created by [[Dennis Ritchie]] while he was working at [[Bell Laboratories]].  Wirth went on to design [[Modula-2]], and [[Oberon programming language|Oberon]].  For the needs of the [[United States Department of Defense]], [[Jean Ichbiah]] and a team at [[Honeywell]] began designing [[Ada programming language|Ada]] in 1978, after a 4-year project to define the requirements for the language. The specification was first published in 1983, with revisions in 1995 and 2005/6.

The 1980s saw a rapid growth in interest in [[object-oriented programming]].  These languages were imperative in style, but added features to support [[object (computing)|objects]].  The last two decades of the 20th century saw the development of a considerable number of such programming languages. [[Smalltalk-80]], originally conceived by [[Alan Kay]] in 1969, was released in 1980 by the [[Xerox Palo Alto Research Center]].  Drawing from concepts in another object-oriented language—[[Simula]] (which is considered to be the world's first [[object-oriented programming language]], developed in the late 1960s)—[[Bjarne Stroustrup]] designed [[C++]], an object-oriented language based on [[C (programming language)|C]]. [[C++]] was first implemented in 1985.  In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were [[Perl]], released by [[Larry Wall]] in 1987; [[Python (programming language)|Python]], released by [[Guido van Rossum]] in 1990; [[PHP]], released by [[Rasmus Lerdorf]] in 1994; [[Java (programming language)|Java]], first released by [[Sun Microsystems]] in 1994 and [[Ruby (programming language)|Ruby]], released in 1995 by Yukihiro “matz” Matsumoto. Microsoft's .NET platform (2002) is at core imperative as are its primary target languages, VB.NET and C#. 

[[:Category:Procedural programming languages]] lists additional imperative programming languages.

==References==
*Pratt, Terrence W. and Marvin V. Zelkowitz. ''Programming Languages: Design and Implementation'', 3rd ed. Englewood Cliffs, N.J.: Prentice Hall, 1996.
*Sebesta, Robert W.  ''Concepts of Programming Languages'', 3rd ed. Reading, Mass.: Addison-Wesley Publishing Company, 1996.

==See also== 
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*[[Declarative programming]] (contrast)
*[[Programming paradigm]]s
*[[Object-oriented programming]]

<!--- reduced license notice by special permission of Stan Seibert, see Talk --->
''Originally based on the article 'Imperative programming' by Stan Seibert, from [[Nupedia]], licensed under the [[GNU Free Documentation License]].''

{{Programming language}}

[[Category:Programming paradigms]]
[[Category:Programming language classification]]

[[ast:Llinguaxe de programación imperativu]]
[[be-x-old:Імпэратыўнае праграмаваньне]]
[[bs:Imperativno programiranje]]
[[cs:Imperativní programování]]
[[de:Imperative Programmierung]]
[[el:Προστακτικός προγραμματισμός]]
[[es:Programación imperativa]]
[[fa:زبان‌های دستوری]]
[[fr:Programmation impérative]]
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[[ko:명령형 프로그래밍]]
[[nl:Imperatief programmeren]]
[[ja:命令型プログラミング]]
[[pl:Programowanie imperatywne]]
[[pt:Programação imperativa]]
[[ro:Programare imperativă]]
[[ru:Императивное программирование]]
[[sr:Императивно програмирање]]
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[[fi:Imperatiivinen ohjelmointi]]
[[sv:Imperativ programmering]]
[[uk:Імперативне програмування]]
[[zh:指令式編程]]

====References====
* Adapted from the [[Wikipedia]] article, "Imperative_programming" [http://en.wikipedia.org/wiki/Imperative_programming http://en.wikipedia.org/wiki/Imperative_programming], used under the [[GNU Free Documentation License]]