Imperative programming

In computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state. In much the same way as the imperative mood in natural languages 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 and logical programming are examples of a more declarative approach.

Imperative, procedural, and declarative programming

Procedural programming is imperative programming in which the statements are structured into procedures (also known as subroutines or functions); the terms are often used as synonyms, but the use of procedures has a dramatic impact on what imperative programs look like 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, it has been propagated as a technique to improve the maintainability and overall quality of imperative programs. Object-oriented programming extends this approach.

Procedural programming is a step towards declarative specification: by just looking at the names, arguments and return types of procedures, and comments added by the programmer, we can often understand what a procedure is supposed to do without understanding how. At the same time, a complete program is imperative: it fixes the statements to be executed and their order of execution to a large extent. A more radical application of this approach leads to functional or logical programming.

In a pure functional language, such as Haskell, all functions are without side effects, and no explicit state or state changes exist at all. This leaves much room for the implementation to perform smart optimizations, but also more room to do things in a suboptimal way; it becomes harder for the programmer to control the performance of programs. In addition, some tasks are difficult to specify in a purely functional way.Therefore, most functional languages, e.g. Lisp, OCaml and Erlang, still allow explicit assignment, and thereby, support arbitrary mixes of procedural and functional programming.

A similar situation exists for logical programming languages such as Prolog, and database query languages such as SQL: while declarative in principle, they still support a procedural style of programming, which is often employed in practice.

In summary: imperative and declarative programming are the extreme ends on a continuum of programming styles; in practice, a middle ground is often taken or a combination of styles is used.

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 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 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 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 block 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 the block. 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, and the subprogram, or procedure, call.

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 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 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 late 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; PHP, released by Rasmus Lerdorf in 1994; Java, first released by Sun Microsystems in 1994 and Ruby, released in 1995 by Yukihiro “matz” Matsumoto.

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.

ee also

*Declarative programming (contrast)
*Programming paradigms
*Object-oriented programming

"Originally based on the article 'Imperative programming' by Stan Seibert, from Nupedia, licensed under the GNU Free Documentation License."