Architecture (562409), страница 15

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Statements known as data declarations give names and properties to elements of a program called variables. Variables can be assigned different values within the program. The properties variables can have are called types, and they include such things as what possible values might be saved in the variables, how much numerical accuracy is to be used in the values, and how one variable may represent a collection of simpler values in an organized fashion, such as a table or array. In many programming languages, a key data type is a pointer. Variables that are pointers do not themselves have values; instead, they have information that the computer can use to locate some other variable—that is, they point to another variable.

An expression is a piece of a statement that describes a series of computations to be performed on some of the program’s variables, such as X + Y/Z, in which the variables are X, Y, and Z and the computations are addition and division. An assignment statement assigns a variable a value derived from some expression, while conditional statements specify expressions to be tested and then used to select which other statements should be executed next.

Procedure and function statements define certain blocks of code as procedures or functions that can then be returned to later in the program. These statements also define the kinds of variables and parameters the programmer can choose and the type of value that the code will return when an expression accesses the procedure or function. Many programming languages also permit minitranslation programs called macros. Macros translate segments of code that have been written in a language structure defined by the programmer into statements that the programming language understands.

Programming languages date back almost to the invention of the digital computer in the 1940s. The first assembly languages emerged in the late 1950s with the introduction of commercial computers. The first procedural languages were developed in the late 1950s to early 1960s: Fortran (FORmula TRANslation), created by John Backus, and then COBOL (COmmon Business Oriented Language), created by Grace Hopper. The first functional language was LISP (LISt Processing), written by John McCarthy in the late 1950s. Although heavily updated, all three languages are still widely used today.

In the late 1960s, the first object-oriented languages, such as SIMULA, emerged. Logic languages became well known in the mid 1970s with the introduction of PROLOG, a language used to program artificial intelligence software. During the 1970s, procedural languages continued to develop with ALGOL, BASIC, PASCAL, C, and Ada. SMALLTALK was a highly influential object-oriented language that led to the merging of object-oriented and procedural languages in C++ and more recently in JAVA. Although pure logic languages have declined in popularity, variations have become vitally important in the form of relational languages for modern databases, such as SQL (Structured Query Language).

Fortran

Fortran, in computer science, acronym for FORmula TRANslation. The first high-level computer language (developed 1954-1958 by John Backus) and the progenitor of many key high-level concepts, such as variables, expressions, statements, iterative and conditional statements, separately compiled subroutines, and formatted input/output. Fortran is a compiled, structured language. The name indicates its scientific and engineering roots; Fortran is still used heavily in those fields, although the language itself has been expanded and improved vastly over the last 35 years to become a language that is useful in any field.

COBOL

COBOL, in computer science, acronym for COmmon Business-Oriented Language, a verbose, English-like programming language developed between 1959 and 1961. Its establishment as a required language by the U.S. Department of Defense, its emphasis on data structures, and its English-like syntax (compared to those of Fortran and ALGOL) led to its widespread acceptance and usage, especially in business applications. Programs written in COBOL, which is a compiled language, are split into four divisions: Identification, Environment, Data, and Procedure. The Identification division specifies the name of the program and contains any other documentation the programmer wants to add. The Environment division specifies the computer(s) being used and the files used in the program for input and output. The Data division describes the data used in the program. The Procedure division contains the procedures that dictate the actions of the program. See also Computer.

C (computer)

C (computer), in computer science, a programming language developed by Dennis Ritchie at Bell Laboratories in 1972; so named because its immediate predecessor was the B programming language. Although C is considered by many to be more a machine-independent assembly language than a high-level language, its close association with the UNIX operating system, its enormous popularity, and its standardization by the American National Standards Institute (ANSI) have made it perhaps the closest thing to a standard programming language in the microcomputer/workstation marketplace. C is a compiled language that contains a small set of built-in functions that are machine dependent. The rest of the C functions are machine independent and are contained in libraries that can be accessed from C programs. C programs are composed of one or more functions defined by the programmer; thus C is a structured programming language.

Pascal (computer)

Pascal (computer), a concise procedural computer programming language, designed 1967-71 by Niklaus Wirth. Pascal, a compiled, structured language, built upon ALGOL, simplifies syntax while adding data types and structures such as subranges, enumerated data types, files, records, and sets. Acceptance and use of Pascal exploded with Borland International's introduction in 1984 of Turbo Pascal, a high-speed, low-cost Pascal compiler for MS-DOS systems that has sold over a million copies in its various versions. Even so, Pascal appears to be losing ground to C as a standard development language on microcomputers.

BASIC

BASIC, in computer science, acronym for Beginner's All-purpose Symbolic Instruction Code. A high-level programming language developed by John Kemeny and Thomas Kurtz at Dartmouth College in the mid-1960s. BASIC gained its enormous popularity mostly because of two implementations, Tiny BASIC and Microsoft BASIC, which made BASIC the first lingua franca of microcomputers. Other important implementations have been CBASIC (Compiled BASIC), Integer and Applesoft BASIC (for the Apple II), GW-BASIC (for the IBM PC), Turbo BASIC (from Borland), and Microsoft QuickBASIC. The language has changed over the years. Early versions are unstructured and interpreted. Later versions are structured and often compiled. BASIC is often taught to beginning programmers because it is easy to use and understand and because it contains the same major concepts as many other languages thought to be more difficult, such as Pascal and C.

C++

C++, in computer science, an object-oriented version of the C programming language, developed by Bjarne Stroustrup in the early 1980s at Bell Laboratories and adopted by a number of vendors, including Apple Computer, Sun Microsystems, Borland International, and Microsoft Corporation.

Assembly Language

Assembly Language, in computer science, a type of low-level computer programming language in which each statement corresponds directly to a single machine instruction. Assembly languages are thus specific to a given processor. After writing an assembly language program, the programmer must use the assembler specific to the microprocessor to translate the assembly language into machine code. Assembly language provides precise control of the computer, but assembly language programs written for one type of computer must be rewritten to operate on another type. Assembly language might be used instead of a high-level language for any of three major reasons: speed, control, and preference. Programs written in assembly language usually run faster than those generated by a compiler; use of assembly language lets a programmer interact directly with the hardware (processor, memory, display, and input/output ports).

LISP

LISP, in computer science, acronym for List Processing. A list-oriented computerprogramming language developed in 1959-1960 by John McCarthy and used primarily to manipulate lists of data. LISP was a radical departure from the procedural languages (Fortran, ALGOL) then being developed; it is an interpreted language in which every expression is a list of calls to functions. LISP continues to be heavily used in research and academic circles and has long been considered the “standard” language for artificial-intelligence (AI) research, although Prolog has made inroads into that claim in recent years.

PROLOG

PROLOG, in computer science, an acronym for programming in logic, a computer programming language important in the development of artificial intelligence software during the 1970s and 1980s. Unlike traditional programming languages, which process only numerical data and instructions, PROLOG processes symbols and relationships. It is designed to perform search functions that establish relationships within a program. This combination of symbolic processing and logic searching made PROLOG the preferred language during the mid-1980s for creating programs that mimic human behavior.

PROLOG was developed in 1970 at the University of Marseille in France by Alain Colmerauer. Colmerauer believed that traditional computer-programming languages, such as Fortran and COBOL, were inappropriate for representing human logic in a machine. Colmerauer's primary goal was to communicate with computers using conversational language instead of programmer's jargon. He concluded that strict symbolic logic was the appropriate bridge between human and machine.

A PROLOG program is made up of facts and rules that are usually limited to a single domain, such as marine life, accounting, or aircraft maintenance. Once a database is built for that domain, PROLOG searches the database and forms relationships between facts. PROLOG's functions are designed to prove that a proposition is either valid or invalid. This is done by applying logic to the available facts, such as “A hammerhead is a shark” and “Madeline likes all sharks.” Rules are built by combining facts: “Madeline likes X, and X is a shark.” If the program's database identifies certain symbolic entities—such as hammerheads, makos, and great whites—as sharks, then PROLOG can use the rule to determine that Madeline likes hammerheads, makos, and great whites, even though that information was not specifically programmed into the database.

This form of artificial intelligence is valuable in situations in which a fault or malfunction can be specifically identified—as in equipment maintenance and repair—and in cases in which the answers are of the YES/NO or TRUE/FALSE variety. Due to the rigidity of the applied logic, however, PROLOG has difficulty with imprecise data or fuzzy sets.

In the United States the artificial-intelligence community of the late 1970s ignored PROLOG in favor of a competing artificial-intelligence language, LISP, which was developed by John McCarthy at the Massachusetts Institute of Technology in Cambridge, Massachusetts. In Europe, however, PROLOG captured the interest of researchers and by the mid-1980s it became the preferred language for building expert systems. In 1981, when the Japanese government initiated a national project to develop commercial artificial intelligence, it adopted PROLOG as its standard programming language.

Many of the features that were once unique to PROLOG are now used in modern object-oriented programming, a programming technique that is becoming the standard for software development.

Object-Oriented Programming

Object-Oriented Programming (OOP), in computer science, type of high-level computer language that uses self-contained, modular instruction sets for defining and manipulating aspects of a computer program. These discrete, pre-defined instruction sets are called objects and may be used to define variables, data structures, and procedures for executing data operations. In OOP, objects have built-in rules for communicating with one another. They can also be manipulated or combined in various ways to modify existing programs and to create entirely new ones from pieces of other programs. See also Computer Program, Programming Language.

One especially powerful feature of OOP languages is a property known as inheritance. Inheritance allows an object to take on the characteristics and functions of other objects to which it is functionally connected. Programmers connect objects by grouping them together in different classes and by grouping the classes into hierarchies. These classes and hierarchies allow programmers to define the characteristics and functions of objects without needing to repeat source code, the coded instructions in a program. Thus, using OOP languages can greatly reduce the time it takes for a programmer to write an application, and also reduce the size of the program. OOP languages are flexible and adaptable, so programs or parts of programs can be used for more than one task. Programs written with OOP languages are generally shorter in length and contain fewer bugs, or mistakes, than those written with non-OOP languages.

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