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In
computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...
, imperative programming is a
programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
of
software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...
that uses statements that change a program's
state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States * ''Our S ...
. In much the same way that the
imperative mood The imperative mood is a grammatical mood that forms a command or request. The imperative mood is used to demand or require that an action be performed. It is usually found only in the present tense, second person. To form the imperative mood, ...
in natural languages expresses commands, an imperative program consists of
command Command may refer to: Computing * Command (computing), a statement in a computer language * COMMAND.COM, the default operating system shell and command-line interpreter for DOS * Command key, a modifier key on Apple Macintosh computer keyboards * ...
s for the computer to perform. Imperative programming focuses on describing ''how'' a program operates step by step, rather than on high-level descriptions of its expected results. The term is often used in contrast to declarative programming, which focuses on ''what'' the program should accomplish without specifying all the details of ''how'' the program should achieve the result.


Imperative and procedural programming

Procedural programming Procedural programming is a programming paradigm, derived from imperative programming, based on the concept of the '' procedure call''. Procedures (a type of routine or subroutine) simply contain a series of computational steps to be carrie ...
is a type of imperative programming in which the program is built from one or more procedures (also termed subroutines 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. Heavy procedural programming, in which
state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States * ''Our S ...
changes are localized to procedures or restricted to explicit arguments and returns from procedures, is a form of structured programming. From the 1960s onwards, structured programming and
modular programming Modular programming is a software design technique that emphasizes separating the functionality of a Computer program, program into independent, interchangeable modules, such that each contains everything necessary to execute only one aspect of th ...
in general have been promoted as techniques to improve the
maintainability In engineering, maintainability is the ease with which a product can be maintained to: * correct defects or their cause, * Repair or replace faulty or worn-out components without having to replace still working parts, * prevent unexpected working ...
and overall quality of imperative programs. The concepts behind
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
attempt to extend this approach. Procedural programming could be considered a step toward 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 details of how it 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.


Rationale and foundations of imperative programming

The programming paradigm used to build programs for almost all computers typically follows an imperative model.
Reconfigurable computing Reconfigurable computing is a computer architecture combining some of the flexibility of software with the high performance of hardware by processing with very flexible high speed computing fabrics like field-programmable gate arrays (FPGAs). Th ...
is a notable exception.
Digital computer hardware is designed to execute
machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a ve ...
, which is native to the computer and is usually written in the imperative style, although low-level compilers and interpreters using other paradigms exist for some architectures such as
lisp machine Lisp machines are general-purpose computers designed to efficiently run Lisp as their main software and programming language, usually via hardware support. They are an example of a high-level language computer architecture, and in a sense, they ...
s. 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 Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...
s and more complex statements, but still follow the same paradigm. Recipes and process checklists, while not
computer program A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components. A 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 statements, in imperative paradigm, 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 Evaluation is a systematic determination and assessment of a subject's merit, worth and significance, using criteria governed by a set of standards. It can assist an organization, program, design, project or any other intervention or initiative to ...
of complex expressions, which may consist of a combination of
arithmetic operations Arithmetic () is an elementary part of mathematics that consists of the study of the properties of the traditional operations on numbers—addition, subtraction, multiplication, division, exponentiation, and extraction of roots. In the 19th ce ...
and
function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...
evaluations, and the assignment of the resulting value to memory. Looping statements (as in
while loop In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The ''while'' loop can be thought of as a repeating if statement. Overview The ' ...
s,
do while loop In most computer programming languages a do while loop is a control flow statement that executes a block of code and then either repeats the block or exits the loop depending on a given boolean condition. The ''do while'' construct consists o ...
s, and
for loop In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two par ...
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 is met. Conditional 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 an execution sequence to be transferred to another part of a program. These include the jump (called '' goto'' in many languages),
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
, and the subprogram, subroutine, or procedure call (which usually returns to the next statement after the call). Early in the development of
high-level programming language In computer science, a high-level programming language is a programming language with strong abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language ''elements'', be easier to us ...
s, the introduction of the
block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
enabled the construction of programs in which a group of statements and declarations could be treated as if they were one statement. This, alongside the introduction of subroutines, enabled complex structures to be expressed by hierarchical decomposition into simpler procedural structures. Many imperative programming languages (such as Fortran, BASIC, and C) are abstractions of assembly language.


History of imperative and object-oriented languages

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 John Warner Backus (December 3, 1924 – March 17, 2007) was an American computer scientist. He directed the team that invented and implemented FORTRAN, the first widely used high-level programming language, and was the inventor of the Backu ...
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 many other major high-level imperative programming languages. In the late 1950s and 1960s,
ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
was developed in order to allow mathematical algorithms to be more easily expressed and even served as the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
's target language for some computers. MUMPS (1966) carried the imperative paradigm to a logical extreme, by not having any statements at all, relying purely on commands, even to the extent of making the IF and ELSE commands independent of each other, connected only by an intrinsic variable named $TEST. COBOL (1960) and BASIC (1964) were both attempts to make programming syntax look more like English. In the 1970s, Pascal was developed by
Niklaus Wirth Niklaus Emil Wirth (born 15 February 1934) is a Swiss computer scientist. He has designed several programming languages, including Pascal, and pioneered several classic topics in software engineering. In 1984, he won the Turing Award, generally ...
, and C was created by Dennis Ritchie while he was working at
Bell Laboratories Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...
. Wirth went on to design Modula-2 and
Oberon Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fairi ...
. For the needs of the
United States Department of Defense The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national sec ...
,
Jean Ichbiah Jean David Ichbiah (25 March 1940 – 26 January 2007) was a French computer scientist and the initial chief designer (1977–1983) of Ada, a general-purpose, strongly typed programming language with certified validated compilers. Ea ...
and a team at
Honeywell Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...
began designing
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, ...
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, 2005, and 2012. The 1980s saw a rapid growth in interest in
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
. These languages were imperative in style, but added features to support
objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
. The last two decades of the 20th century saw the development of many such languages. Smalltalk-80, originally conceived by
Alan Kay Alan Curtis Kay (born May 17, 1940) published by the Association for Computing Machinery 2012 is an American computer scientist best known for his pioneering work on object-oriented programming and windowing graphical user interface (GUI) d ...
in 1969, was released in 1980, by the Xerox Palo Alto Research Center ( PARC). Drawing from concepts in another object-oriented language— Simula (which is considered the world's first
object-oriented programming language Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
, developed in the 1960s)—
Bjarne Stroustrup Bjarne Stroustrup (; ; born 30 December 1950) is a Danish computer scientist, most notable for the invention and development of the C++ programming language. As of July 2022, Stroustrup is a professor of Computer Science at Columbia University ...
designed
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
, an object-oriented language based on C. Design of
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
began in 1979 and the first implementation was completed in 1983. In the late 1980s and 1990s, the notable imperative languages drawing on object-oriented concepts were
Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
, released by
Larry Wall Larry Arnold Wall (born September 27, 1954) is an American computer programmer and author. He created the Perl programming language. Personal life Wall grew up in Los Angeles and then Bremerton, Washington, before starting higher education at ...
in 1987;
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
, released by
Guido van Rossum Guido van Rossum (; born 31 January 1956) is a Dutch programmer best known as the creator of the Python programming language, for which he was the "benevolent dictator for life" (BDFL) until he stepped down from the position on 12 July 2018 ...
in 1990;
Visual Basic Visual Basic is a name for a family of programming languages from Microsoft. It may refer to: * Visual Basic .NET (now simply referred to as "Visual Basic"), the current version of Visual Basic launched in 2002 which runs on .NET * Visual Basic ( ...
and
Visual C++ Microsoft Visual C++ (MSVC) is a compiler for the C, C++ and C++/CX programming languages by Microsoft. MSVC is proprietary software; it was originally a standalone product but later became a part of Visual Studio and made available in both tri ...
(which included
Microsoft Foundation Class Library Microsoft Foundation Class Library (MFC) is a C++ object-oriented library for developing desktop applications for Windows. MFC was introduced by Microsoft in 1992 and quickly gained widespread use. While Microsoft has introduced alternative app ...
(MFC) 2.0), released by
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washin ...
in 1991 and 1993 respectively;
PHP PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group. ...
, released by
Rasmus Lerdorf Rasmus Lerdorf (born 22 November 1968) is a Danish-Canadian programmer. He co-authored and inspired the PHP scripting language, authoring the first two versions of the language and participating in the development of later versions led by a gro ...
in 1994;
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 mos ...
, by
James Gosling James Gosling (born May 19, 1955) is a Canadian computer scientist, best known as the founder and lead designer behind the Java programming language. Gosling was elected a member of the National Academy of Engineering in 2004 for the conception ...
( Sun Microsystems) in 1995,
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
, by
Brendan Eich Brendan Eich (; born July 4, 1961) is an American computer programmer and technology executive. He created the JavaScript programming language and co-founded the Mozilla project, the Mozilla Foundation, and the Mozilla Corporation. He served ...
( Netscape), and
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
, by Yukihiro "Matz" Matsumoto, both released in 1995. Microsoft's
.NET Framework The .NET Framework (pronounced as "''dot net"'') is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until bein ...
(2002) is imperative at its core, as are its main target languages,
VB.NET Visual Basic, originally called Visual Basic .NET (VB.NET), is a multi-paradigm, object-oriented programming language, implemented on .NET, Mono, and the .NET Framework. Microsoft launched VB.NET in 2002 as the successor to its original Visua ...
and C# that run on it; however Microsoft's F#, a functional language, also runs on it.


Examples


Fortran

FORTRAN (1958) was unveiled as "The IBM Mathematical FORmula TRANslating system." It was designed for scientific calculations, without string handling facilities. Along with declarations, expressions, and
statements Statement or statements may refer to: Common uses *Statement (computer science), the smallest standalone element of an imperative programming language *Statement (logic), declarative sentence that is either true or false *Statement, a declarative ...
, it supported: *
arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
* subroutines * "do" loops It succeeded because: * programming and debugging costs were below computer running costs * it was supported by IBM * applications at the time were scientific. However, non IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler. The American National Standards Institute (ANSI) developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports: *
records A record, recording or records may refer to: An item or collection of data Computing * Record (computer science), a data structure ** Record, or row (database), a set of fields in a database related to one entity ** Boot sector or boot record, r ...
*
pointers Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a lis ...
to arrays


COBOL

COBOL (1959) stands for "COmmon Business Oriented Language." Fortran manipulated symbols. It was soon realized that symbols didn't need to be numbers, so strings were introduced. The
US Department of Defense The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national sec ...
influenced COBOL's development, with
Grace Hopper Grace Brewster Hopper (; December 9, 1906 – January 1, 1992) was an American computer scientist, mathematician, and United States Navy rear admiral. One of the first programmers of the Harvard Mark I computer, she was a pioneer of compu ...
being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal. COBOL's development was tightly controlled, so dialects didn't emerge to require ANSI standards. As a consequence, it wasn't changed for 25 years until 1974. The 1990s version did make consequential changes, like
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
.


Algol

ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
(1960) stands for "ALGOrithmic Language." It had a profound influence on programming language design. Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable structured design. Algol was first to define its syntax using the Backus–Naur form. This led to syntax-directed compilers. It added features like: * block structure, where variables were local to their block * arrays with variable bounds * "for" loops * functions *
recursion Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathemati ...
Algol's direct descendants include Pascal, Modula-2,
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, ...
, Delphi and
Oberon Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fairi ...
on one branch. On another branch there's C,
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
and
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 mos ...
.


Basic

BASIC (1964) stands for "Beginner's All Purpose Symbolic Instruction Code." It was developed at
Dartmouth College Dartmouth College (; ) is a private research university in Hanover, New Hampshire. Established in 1769 by Eleazar Wheelock, it is one of the nine colonial colleges chartered before the American Revolution. Although founded to educate Native ...
for all of their students to learn. If a student didn't go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the
microcomputers A microcomputer is a small, relatively inexpensive computer having a central processing unit (CPU) made out of a microprocessor. The computer also includes memory and input/output (I/O) circuitry together mounted on a printed circuit board (PC ...
manufactured in the late 1970s. As the microcomputer industry grew, so did the language. Basic pioneered the interactive session. It offered
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
commands within its environment: * The 'new' command created an empty slate * Statements evaluated immediately * Statements could be programmed by preceding them with a line number * The 'list' command displayed the program * The 'run' command executed the program However, the Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions.
Microsoft's Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, personal computers, and related services headquartered at th ...
Visual Basic Visual Basic is a name for a family of programming languages from Microsoft. It may refer to: * Visual Basic .NET (now simply referred to as "Visual Basic"), the current version of Visual Basic launched in 2002 which runs on .NET * Visual Basic ( ...
is still widely used and produces a
graphical user interface The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows users to interact with electronic devices through graphical icons and audio indicator such as primary notation, inst ...
.


C

C programming language (1973) got its name because the language BCPL was replaced with B, and
AT&T Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...
called the next version "C." Its purpose was to write the
UNIX Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
. C is a relatively small language -- making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language, but uses a high-level syntax. It added advanced features like: *
inline assembler In computer programming, an inline assembler is a feature of some compilers that allows low-level code written in assembly language to be embedded within a program, among code that otherwise has been compiled from a higher-level language such as C ...
* arithmetic on pointers * pointers to functions * bit operations * freely combining complex
operators Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...
''C'' allows the programmer to control which region of memory data is to be stored. ''Global variables'' and ''static variables'' require the fewest
clock cycles In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits. A clock signa ...
to store. The stack is automatically used for the standard variable declarations. Heap memory is returned to a pointer variable from the malloc() function. * The ''global and static data'' region is located just above the ''program'' region. (The program region is technically called the ''text'' region. It's where machine instructions are stored.) :* The global and static data region is technically two regions. One region is called the ''initialized
data segment In computing, a data segment (often denoted .data) is a portion of an object file or the corresponding address space of a program that contains initialized static variables, that is, global variables and static local variables. The size of thi ...
'', where variables declared with default values are stored. The other region is called the '' block started by segment'', where variables declared without default values are stored. :* Variables stored in the ''global and static data'' region have their addresses set at compile-time. They retain their values throughout the life of the process. :* The global and static region stores the ''global variables'' that are declared on top of (outside) the main() function. Global variables are visible to main() and every other function in the source code. : On the other hand, variable declarations inside of main(), other functions, or within block delimiters are ''local variables''. Local variables also include '' formal parameter variables''. Parameter variables are enclosed within the parenthesis of function definitions. They provide an
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
to the function. :* ''Local variables'' declared using the static prefix are also stored in the ''global and static data'' region. Unlike global variables, static variables are only visible within the function or block. Static variables always retain their value. An example usage would be the function int increment_counter() * The stack region is a contiguous block of memory located near the top memory address. Variables placed in the stack, ironically, are populated from top to bottom. A
stack pointer In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or mach ...
is a special-purpose
register Register or registration may refer to: Arts entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), th ...
that keeps track of the last memory address populated. Variables are placed into the stack via the ''assembly language'' PUSH instruction. Therefore, the addresses of these variables are set during runtime. The method for stack variables to lose their
scope Scope or scopes may refer to: People with the surname * Jamie Scope (born 1986), English footballer * John T. Scopes (1900–1970), central figure in the Scopes Trial regarding the teaching of evolution Arts, media, and entertainment * Cinem ...
is via the POP instruction. :* ''Local variables'' declared without the static prefix, including formal parameter variables, are called ''automatic variables'' and are stored in the stack. They are visible inside the function or block and lose their scope upon exiting the function or block. * The heap region is located below the stack. It is populated from the bottom to the top. The
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
manages the heap using a ''heap pointer'' and a list of allocated memory blocks. Like the stack, the addresses of heap variables are set during runtime. An
out of memory Out of memory (OOM) is an often undesired state of computer operation where no additional memory can be allocated for use by programs or the operating system. Such a system will be unable to load any additional programs, and since many programs ...
error occurs when the heap pointer and the stack pointer meet. :* ''C'' provides the malloc() library function to allocate heap memory. Populating the heap with data is an additional copy function. Variables stored in the heap are economically passed to functions using pointers. Without pointers, the entire block of data would have to be passed to the function via the stack.


C++

In the 1970s,
software engineers Software engineering is a systematic engineering approach to software development. A software engineer is a person who applies the principles of software engineering to design, develop, maintain, test, and evaluate computer software. The term '' ...
needed language support to break large projects down into
modules Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a s ...
. One obvious feature was to decompose large projects ''physically'' into separate files. A less obvious feature was to decompose large projects ''logically'' into abstract
datatypes In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most progra ...
. At the time, languages supported concrete ( scalar) datatypes like
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
numbers, floating-point numbers, and strings of
characters Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
. Concrete datatypes have their representation as part of their name. Abstract datatypes are
structures A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
of concrete datatypes — with a new name assigned. For example, a
list A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby unio ...
of integers could be called integer_list. In object-oriented jargon, abstract datatypes are called classes. However, a ''class'' is only a definition; no memory is allocated. When memory is allocated to a class, it's called an
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
. '' Object-oriented imperative languages'' developed by combining the need for classes and the need for safe
functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...
. A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a
method Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
,
member function A method in object-oriented programming (OOP) is a procedure associated with a message and an object. An object consists of ''state data'' and ''behavior''; these compose an ''interface'', which specifies how the object may be utilized by any of ...
, or ''
operation Operation or Operations may refer to: Arts, entertainment and media * ''Operation'' (game), a battery-operated board game that challenges dexterity * Operation (music), a term used in musical set theory * ''Operations'' (magazine), Multi-Ma ...
''. ''Object-oriented programming'' is executing ''operations'' on ''objects''. ''Object-oriented languages'' support a syntax to model subset/superset relationships. In
set theory Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly conce ...
, an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other persons don't have. ''Object-oriented languages'' model ''subset/superset'' relationships using
inheritance Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Officia ...
. ''Object-oriented programming'' became the dominant language paradigm by the late 1990s.
C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
(1985) was originally called "C with Classes." It was designed to expand
C's C's may refer to: * The Boston Celtics, American basketball franchise * Citizens (Spanish political party) Citizens ( es, link=no, Ciudadanos ; ca, link=no, Ciutadans ; eu, link=no, Hiritarrak; gl, link=no, Cidadáns; shortened as Cs—C's un ...
capabilities by adding the object-oriented facilities of the language Simula. An object-oriented module is composed of two files. The definitions file is called the
header file Many programming languages and other computer files have a directive, often called include (sometimes copy or import), that causes the contents of the specified file to be inserted into the original file. These included files are called copybooks ...
. Here is a C++ ''header file'' for the ''GRADE class'' in a simple school application: // grade.h // ------- // Used to allow multiple source files to include // this header file without duplication errors. // ---------------------------------------------- #ifndef GRADE_H #define GRADE_H class GRADE ; #endif A constructor operation is a function with the same name as the class name. It is executed when the calling operation executes the new statement. A module's other file is the ''
source file In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the wo ...
''. Here is a C++ source file for the ''GRADE class'' in a simple school application: // grade.cpp // --------- #include "grade.h" GRADE::GRADE( const char letter ) int GRADE::grade_numeric( const char letter ) Here is a C++ ''header file'' for the ''PERSON class'' in a simple school application: // person.h // -------- #ifndef PERSON_H #define PERSON_H class PERSON ; #endif Here is a C++ ''source file'' for the ''PERSON class'' in a simple school application: // person.cpp // ---------- #include "person.h" PERSON::PERSON ( const char *name ) Here is a C++ ''header file'' for the ''STUDENT class'' in a simple school application: // student.h // --------- #ifndef STUDENT_H #define STUDENT_H #include "person.h" #include "grade.h" // A STUDENT is a subset of PERSON. // -------------------------------- class STUDENT : public PERSON; #endif Here is a C++ ''source file'' for the ''STUDENT class'' in a simple school application: // student.cpp // ----------- #include "student.h" #include "person.h" STUDENT::STUDENT ( const char *name ): // Execute the constructor of the PERSON superclass. // ------------------------------------------------- PERSON( name ) Here is a driver program for demonstration: // student_dvr.cpp // --------------- #include #include "student.h" int main( void ) Here is a
makefile In software development, Make is a build automation tool that automatically builds executable programs and libraries from source code by reading files called ''Makefiles'' which specify how to derive the target program. Though integrated devel ...
to compile everything: # makefile # -------- all: student_dvr clean: rm student_dvr *.o student_dvr: student_dvr.cpp grade.o student.o person.o c++ student_dvr.cpp grade.o student.o person.o -o student_dvr grade.o: grade.cpp grade.h c++ -c grade.cpp student.o: student.cpp student.h c++ -c student.cpp person.o: person.cpp person.h c++ -c person.cpp


See also

*
Functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...
*
Comparison of programming paradigms This article attempts to set out the various similarities and differences between the various programming paradigms as a summary in both graphical and tabular format with links to the separate discussions concerning these similarities and differ ...
*
Reactive programming In computing, reactive programming is a declarative programming paradigm concerned with data streams and the propagation of change. With this paradigm, it's possible to express static (e.g., arrays) or dynamic (e.g., event emitters) data streams ...
*
History of programming languages The history of programming languages spans from documentation of early mechanical computers to modern tools for software development. Early programming languages were highly specialized, relying on mathematical notation and similarly obscure ...
* List of imperative programming languages


Notes


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. : ''Originally based on the article 'Imperative programming' by Stan Seibert, from
Nupedia Nupedia was an English-language, online encyclopedia whose articles were written by volunteer contributors with appropriate subject matter expertise, reviewed by expert editors before publication, and licensed as free content. It was founded by ...
, licensed under the GNU Free Documentation License.'' {{DEFAULTSORT:Imperative Programming Programming paradigms