Programming Language
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I
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INTRODUCTION
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Application of
Programming Languages
Programming
languages allow people to communicate with computers. Once a job has been
identified, the programmer must translate, or code, it into a list of
instructions that the computer will understand. A computer program for a given
task may be written in several different languages. Depending on
the task, a
programmer will generally pick the language that will involve the least
complicated program. It may also be important to the programmer to pick a
language that is flexible and widely compatible if the program will have a
range of applications. The examples shown here are programs written to average
a list of numbers. Both C and BASIC are commonly used programming languages. The
machine interpretation shows how a computer would process and execute the
commands from the programs.
Programming Language, in computer science,
artificial language used to write a sequence of instructions (a computer
program) that can be run by a computer. Similar to natural languages, such as
English, programming languages have a vocabulary, grammar, and syntax. However,
natural languages are not suited for programming computers because they are
ambiguous, meaning that their vocabulary and grammatical structure may be
interpreted in multiple ways. The languages used to program computers must have
simple logical structures, and the rules for their grammar, spelling, and
punctuation must be precise.
Programming languages vary greatly in their sophistication
and in their degree of versatility. Some programming languages are written to
address a particular kind of computing problem or for use on a particular model
of computer system. For instance, programming languages such as Fortran and
COBOL were written to solve certain general types of programming
problems—Fortran for scientific applications, and COBOL for business
applications. Although these languages were designed to address specific
categories of computer problems, they are highly portable, meaning that they
may be used to program many types of computers. Other languages, such as
machine languages, are designed to be used by one specific model of computer
system, or even by one specific computer in certain research applications. The
most commonly used programming languages are highly portable and can be used to
effectively solve diverse types of computing problems. Languages like C,
PASCAL, and BASIC fall into this category.
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II
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LANGUAGE TYPES
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Programming languages can be classified as either low-level
languages or high-level languages. Low-level programming languages, or machine
languages, are the most basic type of programming languages and can be
understood directly by a computer. Machine languages differ depending on the
manufacturer and model of computer. High-level languages are programming
languages that must first be translated into a machine language before they can
be understood and processed by a computer. Examples of high-level languages are
C, C++, PASCAL, and Fortran. Assembly languages are intermediate languages that
are very close to machine language and do not have the level of linguistic
sophistication exhibited by other high-level languages, but must still be
translated into machine language.
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A
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Machine Languages
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In machine languages, instructions are written as
sequences of 1s and 0s, called bits, that a computer can understand directly.
An instruction in machine language generally tells the computer four things:
(1) where to find one or two numbers or simple pieces of data in the main
computer memory (Random Access Memory, or RAM), (2) a simple operation to
perform, such as adding the two numbers together, (3) where in the main memory
to put the result of this simple operation, and (4) where to find the next
instruction to perform. While all executable programs are eventually read by
the computer in machine language, they are not all programmed in machine
language. It is extremely difficult to program directly in machine language
because the instructions are sequences of 1s and 0s. A typical instruction in a
machine language might read 10010 1100 1011 and mean add the contents of
storage register A to the contents of storage register B.
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B
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High-Level Languages
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High-level languages are relatively sophisticated sets
of statements utilizing words and syntax from human language. They are more
similar to normal human languages than assembly or machine languages and are
therefore easier to use for writing complicated programs. These programming
languages allow larger and more complicated programs to be developed faster.
However, high-level languages must be translated into machine language by
another program called a compiler before a computer can understand them. For
this reason, programs written in a high-level language may take longer to execute
and use up more memory than programs written in an assembly language.
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C
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Assembly Language
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Computer programmers use assembly languages to make
machine-language programs easier to write. In an assembly language, each
statement corresponds roughly to one machine language instruction. An assembly
language statement is composed with the aid of easy to remember commands. The
command to add the contents of the storage register A to the contents of
storage register B might be written ADD B,A in a typical assembly
language statement. Assembly languages share certain features with machine
languages. For instance, it is possible to manipulate specific bits in both
assembly and machine languages. Programmers use assembly languages when it is
important to minimize the time it takes to run a program, because the
translation from assembly language to machine language is relatively simple.
Assembly languages are also used when some part of the computer has to be
controlled directly, such as individual dots on a monitor or the flow of
individual characters to a printer.
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III
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CLASSIFICATION OF
HIGH-LEVEL LANGUAGES
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High-level languages are commonly classified as
procedure-oriented, functional, object-oriented, or logic languages. The most
common high-level languages today are procedure-oriented languages. In these
languages, one or more related blocks of statements that perform some complete
function are grouped together into a program module, or procedure, and given a
name such as “procedure A.” If the same sequence of operations is needed
elsewhere in the program, a simple statement can be used to refer back to the
procedure. In essence, a procedure is just a mini-program. A large program can
be constructed by grouping together procedures that perform different tasks.
Procedural languages allow programs to be shorter and easier for the computer
to read, but they require the programmer to design each procedure to be general
enough to be used in different situations.
Functional languages treat procedures like mathematical
functions and allow them to be processed like any other data in a program. This
allows a much higher and more rigorous level of program construction.
Functional languages also allow variables—symbols for data that can be
specified and changed by the user as the program is running—to be given values
only once. This simplifies programming by reducing the need to be concerned
with the exact order of statement execution, since a variable does not have to
be redeclared, or restated, each time it is used in a program statement. Many
of the ideas from functional languages have become key parts of many modern
procedural languages.
Object-oriented languages are outgrowths of functional
languages. In object-oriented languages, the code used to write the program and
the data processed by the program are grouped together into units called
objects. Objects are further grouped into classes, which define the attributes
objects must have. A simple example of a class is the class Book. Objects
within this class might be Novel and Short Story. Objects also have certain
functions associated with them, called methods. The computer accesses an object
through the use of one of the object’s methods. The method performs some action
to the data in the object and returns this value to the computer. Classes of
objects can also be further grouped into hierarchies, in which objects of one
class can inherit methods from another class. The structure provided in
object-oriented languages makes them very useful for complicated programming tasks.
Logic languages use logic as their mathematical
base. A logic program consists of sets of facts and if-then rules, which
specify how one set of facts may be deduced from others, for example:
If the statement X is true, then the statement
Y is false.
In the execution of such a program, an input
statement can be logically deduced from other statements in the program. Many
artificial intelligence programs are written in such languages.
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IV
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LANGUAGE STRUCTURE AND
COMPONENTS
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Programming languages use specific types of statements, or
instructions, to provide functional structure to the program. A statement in a
program is a basic sentence that expresses a simple idea—its purpose is to give
the computer a basic instruction. Statements define the types of data allowed,
how data are to be manipulated, and the ways that procedures and functions
work. Programmers use statements to manipulate common components of programming
languages, such as variables and macros (mini-programs within a
program).
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.
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V
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HISTORY
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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).
Understanding HTML
Hypertext Markup Language (HTML) in
computer science, the standard text-formatting language since 1989 for
documents on the interconnected computing network known as the World Wide Web.
HTML documents are text files that contain two parts: content that is meant to
be rendered on a computer screen; and markup or tags, encoded
information that directs the text format on the screen and is generally hidden
from the user. HTML is a subset of a broader language called Standard
Generalized Markup Language (SGML), which is a system for encoding and
formatting documents, whether for output to a computer screen or to paper.
Some tags in an HTML document
determine the way certain text, such as titles, will be formatted. Other tags
cue the computer to respond to the user's actions on the keyboard or mouse. For
instance, the user might click on an icon (a picture that represents a
specific command), and that action might call another piece of software to
display a graphic, play a recording, or run a short movie. Another important
tag is a link, which may contain the Uniform Resource Locator (URL) of
another document. The URL can be compared to an address where a particular
document resides. The document may be stored on the same computer as the parent
document or on any computer connected to the World Wide Web. The user can
navigate from document to document simply by clicking on these links. HTML also
includes markups for forms, that let the user fill out information and
electronically send, or e-mail, the data to the document author, initiate
sophisticated searches of information on the Internet, or order goods and
services.
The software that permits the user
to navigate the World Wide Web and view HTML-encoded documents is called a browser.
It interprets the HTML tags in a document and formats the content for screen
display. Since HTML is an accepted standard, anyone can build a browser without
concerning themselves with what form various documents will assume, unlike
documents produced by typical word processors, which must be translated into a
different format if another word processing application is used. Most sites on
the World Wide Web adhere to HTML standards and, because HTML is easy to use,
the World Wide Web has grown rapidly. HTML continues to evolve, however, so
browsers must be upgraded regularly to meet the revised standards.