Imperative programming

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In computer science, imperative programming is a programming paradigm that uses statements that change a program's state. In much the same way that the imperative mood in natural languages expresses commands, an imperative program consists of commands for the computer to perform. Imperative programming focuses on describing how a program operates.

The term is often used in contrast to declarative programming, which focuses on what the program should accomplish without specifying how the program should achieve the result.

Imperative and procedural programming

Procedural programming is a type of imperative programming in which the program is built from one or more procedures (also known as 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. Heavily-procedural programming, in which state changes 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.[1]

Procedural programming could be considered 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 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.

Declarative programming

Declarative programming contrasts with imperative and procedural programming. Declarative programming is a non-imperative style of programming in which programs describe their desired results without explicitly listing commands or steps that must be performed.

Rationale and foundations of imperative programming

The hardware implementation of almost all computers is imperative;[note 1] 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 variables and more complex statements, but still follow the same paradigm. Recipes and process checklists, while not computer programs, 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 of complex expressions, which may consist of a combination of arithmetic operations and function evaluations, and the assignment of the resulting value to memory. Looping statements (such as in while loops, do while loops and for loops) 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 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" in many languages), switch and the subprogram, or procedure, call (which usually returns to the next statement after the call).

Early in the development of high level languages, 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 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.[2]

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 at 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. 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, and C was created by Dennis Ritchie while he was working at Bell Laboratories. Wirth went on to design Modula-2 and Oberon. For the needs of the United States Department of Defense, Jean Ichbiah and a team at Honeywell began designing 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 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 1960s)—Bjarne Stroustrup designed C++, an object-oriented language based on 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, released by Guido van Rossum in 1990; Visual Basic and Visual C++ (which included MFC 2.0), released by Microsoft in 1991 and 1993 respectively; PHP, released by Rasmus Lerdorf in 1994; Java, first released by Sun Microsystems in 1994, and Ruby released in 1995, by Yukihiro "Matz" Matsumoto. Microsoft's .NET (2002) is imperative at its core, as are its primary target languages, VB.NET and C# that run on it (however Microsoft's F# is a functional language, that also runs on it).

See also

Notes

  1. ↑ Reconfigurable computing is a notable exception.

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  • 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, licensed under the GNU Free Documentation License.

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