1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

<object-oriented> In {object-oriented programming}, an
	instance of the data structure and behaviour defined by the
	object's {class}.  Each object has its own values for the
	{instance variables} of its class and can respond to the
	{methods} defined by its class.

	For example, an object of the "Point" class might have
	instance variables "x" and "y" and might respond to the "plot"
	method by drawing a dot on the screen at those coordinates.

	(2004-01-26)

<programming> An individual {object} of a certain {class}.
	While a class is just the type definition, an actual usage of
	a class is called "instance".  Each instance of a class can
	have different values for its {instance variables}, i.e. its
	{state}.

	(1998-03-06)

<programming> The name given in {Smalltalk} and other
	{object-oriented languages} to a procedure or routine
	associated with one or more {classes}.  An {object} of a
	certain class knows how to perform actions, e.g. printing
	itself or creating a new instance of itself, rather than the
	function (e.g. printing) knowing how to handle different types
	of object.

	Different classes may define methods with the same name
	(i.e. methods may be {polymorphic}).  The term "method" is used
	both for a named operation, e.g. "PRINT" and also for the code
	which a specific class provides to perform tha
	t operation.

	Most methods operate on objects that are instances of a
	certain class.  Some object-oriented languages call these
	"object methods" to distinguish then from "{class methods}".

	In {Smalltalk}, a method is defined by giving its name,
	documentation, temporary local variables and a sequence of
	expressions separated by "."s.

	(2000-03-22)

1. <mathematics> (Or "map", "mapping") If D and C are sets
	(the domain and codomain) then a function f from D to C,
	normally written "f : D -> C" is a subset of D x C such that:

	1. For each d in D there exists some c in C such that (d,c) is
	an element of f.  I.e. the function is defined for every
	element of D.

	2. For each d in D, c1 and c2 in C, if both (d,c1) and (d,c2)
	are elements of f then c1 = c2.  I.e. the function is uniquely
	defined for every element of D.

	See also {image}, {inverse}, {partial function}.

	2. <programming> Computing usage derives from the mathematical
	term but is much less strict.  In programming (except in
	{functional programming}), a function may return different
	values each time it is called with the same argument values
	and may have {side effects}.

	A {procedure} is a function which returns no value but has
	only {side-effects}.  The {C} language, for example, has no
	procedures, only functions.  {ANSI C} even defines a {type},
	{void}, for the result of a function that has no result.

	(1996-09-01)

<programming> A special value used in several languages to
	represent the thing referred to by an uninitialised pointer.

	<database> A special value that may be stored in some database
	columns to represent an unknown, missing, not applicable, or
	undefined value.  Nulls are treated completely differently
	from ordinary values when evaluating SQL expressions and there
	are several SQL constructs for dealing with nulls.

	(2003-06-17)

1. <application, algorithm> To arrange a collection of items
	in some specified order.  The items - {records} in a file or
	data structures in memory - consist of one or more {fields} or
	members.  One of these fields is designated as the "sort key"
	which means the records will be ordered according to the value
	of that field.  Sometimes a sequence of key fields is
	specified such that if all earlier keys are equal then the
	later keys will be compared.  Within each field some ordering
	is imposed, e.g. ascending or descending numerical, {lexical
	ordering}, or date.

	Sorting is the subject of a great deal of study since it is a
	common operation which can consume a lot of computer time.
	There are many well-known sorting {algorithms} with different
	time and space behaviour and programming {complexity}.

	Examples are {quicksort}, {insertion sort}, {bubble sort},
	{heap sort}, and {tree sort}.  These employ many different
	data structures to store sorted data, such as {arrays},
	{linked lists}, and {binary trees}.

	2. <tool> The {Unix} utility program for sorting lines of
	files.

	{Unix manual page}: sort(1).

	(1997-02-12)

See {followup}.

<process> The approach that engineering (and some other)
	disciplines use to specify how to create or do something.  A
	successful design must satisfies a (perhaps informal)
	{functional specification} (do what it was designed to do);
	conforms to the limitations of the target medium (it is
	possible to implement); meets implicit or explicit
	requirements on performance and resource usage (it is
	efficient enough).

	A design may also have to satisfy restrictions on the design
	process itself, such as its length or cost, or the tools
	available for doing the design.

	In the {software life-cycle}, design follows {requirements
	analysis} and is followed by implementation.

	["Object-Oriented Analysis and Design with Applications", 2nd
	ed., Grady Booch].

	(1996-12-08)

<storage> A {byte}, {word} or other small unit of storage
	space in a computer's {main memory} that is identified by its
	starting {address} (and size).

	(1999-04-19)

<graphics> (pixel) The smallest resolvable rectangular area of
	an {image}, either on a screen or stored in memory.  Each
	pixel in a {monochrome} image has its own brightness, from 0
	for black to the maximum value (e.g. 255 for an eight-bit
	pixel) for white.  In a colour image, each pixel has its own
	brightness and colour, usually represented as a triple of red,
	green and blue intensities (see {RGB}).

	Compare {voxel}.

	(1998-05-08)

1. <unit, text> (Sometimes abbreviated "pt") The unit of
	length used in {typography} to specify text character height,
	{rule} width, and other small measurements.

	There are six slightly different definitions: {Truchet point},
	{Didot point}, {ATA point}, {TeX point}, {Postscript point},
	and {IN point}.

	In Europe, the most commonly used is Didot and in the US, the
	formerly standard ATA point has essentially been replaced by
	the PostScript point due to the demise of traditional
	typesetting systems and rise of desktop computer based systems
	running software such as {QuarkXPress}, {Adobe InDesign} and
	{Adobe Pagemaker}.

	There are 20 {twips} in a point and 12 points in a {pica}
	(known as a "Cicero" in the Didot system).

	{Different point systems
	(http://www.vakcer.com/oberon/dtp/fonts/point.htm)}.

	(2004-12-23)

	2. <hardware> To move a {pointing device} so that the
	on-screen pointer is positioned over a certain object on the
	screen such as a {button} in a {graphical user interface}.  In
	most {window systems} it is then necessary to {click} a
	(physical) button on the pointing device to activate or select
	the object.  In some systems, just pointing to an object is
	known as "mouse-over" {event} which may cause some help text
	(called a "tool tip" in {Windows}) to be displayed.

	(2001-05-21)

<operating system> (GUI) The use of pictures rather than just
	words to represent the input and output of a program.  A
	program with a GUI runs under some {windowing system}
	(e.g. The {X Window System}, {MacOS}, {Microsoft Windows},
	{Acorn} {RISC OS}, {NEXTSTEP}).  The program displays certain
	{icons}, {buttons}, {dialogue boxes}, etc. in its {windows} on
	the screen and the user controls it mainly by moving a
	{pointer} on the screen (typically controlled by a {mouse})
	and selecting certain objects by pressing buttons on the mouse
	while the pointer is pointing at them.  This contrasts with a
	{command line interface} where communication is by exchange of
	strings of text.

	Windowing systems started with the first {real}-time graphic
	display systems for computers, namely the {SAGE} Project
	[Dates?] and {Ivan Sutherland}'s {Sketchpad} (1963).  {Douglas
	Engelbart}'s {Augmentation of Human Intellect} project at
	{SRI} in the 1960s developed the {On-Line System}, which
	incorporated a mouse-driven cursor and multiple windows.
	Several people from Engelbart's project went to Xerox PARC in
	the early 1970s, most importantly his senior engineer, {Bill
	English}.  The Xerox PARC team established the {WIMP} concept,
	which appeared commercially in the {Xerox 8010} (Star) system
	in 1981.

	Beginning in 1980(?), led by {Jef Raskin}, the {Macintosh}
	team at {Apple Computer} (which included former members of the
	Xerox PARC group) continued to develop such ideas in the first
	commercially successful product to use a GUI, the Apple
	Macintosh, released in January 1984.  In 2001 Apple introduced
	{Mac OS X}.

	{Microsoft} modeled the first version of {Windows}, released
	in 1985, on Mac OS.  Windows was a GUI for {MS-DOS} that had
	been shipped with {IBM PC} and compatible computers since
	1981.  Apple sued Microsoft over infringement of the
	look-and-feel of the MacOS.  The court case ran for many
	years.

	[Wikipedia].

	(2002-03-25)

1. <hardware> the maximum number of {pixels} that can be
	displayed on a {monitor}, expressed as (number of horizontal
	pixels) x (number of vertical pixels), i.e., 1024x768.  The
	ratio of horizontal to vertical resolution is usually 4:3, the
	same as that of conventional television sets.

	2. <logic> A mechanical method for proving statements of
	{first order logic}, introduced by J. A. Robinson in 1965.
	Resolution is applied to two {clauses} in a {sentence}.  It
	eliminates, by {unification}, a {literal} that occurs
	"positive" in one and "negative" in the other to produce a new
	clause, the {resolvent}.

	For example, given the sentence:

		(man(X) => mortal(X))  AND  man(socrates).

	The literal "man(X)" is "negative".  The literal
	"man(socrates)" could be considered to be on the right hand
	side of the degenerate implication

		True => man(socrates)

	and is therefore "positive".  The two literals can be unified
	by the binding X = socrates.

	The {truth table} for the implication function is

		A | B | A => B
		--+---+-------
		F | F |   T
		F | T |   T
		T | F |   F
		T | T |   T

	(The implication only fails if its premise is true but its
	conclusion is false).  From this we can see that

		A => B   ==   (NOT A) OR B

	Which is why the left hand side of the implication is said to
	be negative and the right positive.  The sentence above could
	thus be written

		((NOT man(socrates)) OR mortal(socrates))
		AND
		man(socrates)

	Distributing the AND over the OR gives

		((NOT man(socrates)) AND man(socrates))
		OR
		mortal(socrates) AND man(socrates)

	And since (NOT A) AND A == False, and False OR A == A we can
	simplify to just

		mortal(socrates) AND man(socrates)

	So we have proved the new literal, mortal(socrates).

	Resolution with {backtracking} is the basic control mechanism
	of {Prolog}.

	See also {modus ponens}, {SLD Resolution}.

	3. <networking> {address resolution}.

	(1996-02-09)

<text> A set of {glyphs} ({images}) representing the
	{characters} from some particular {character set} in a
	particular size and {typeface}.  The image of each character
	may be encoded either as a {bitmap} (in a {bitmap font}) or by
	a higher-level description in terms of lines and areas (an
	{outline font}).

	There are several different computer representations for
	fonts, the most widely known are {Adobe Systems, Inc.}'s
	{PostScript} font definitions and {Apple}'s {TrueType}.
	{Window systems} can display different fonts on the screen and
	print them.

	[Other types of font?]

	(2001-04-27)

1. <language, programming> {programming language}.

	2. <human language> {natural language}.

	(1998-09-07)

1. <philosophy, mathematics> A branch of philosophy and
	mathematics that deals with the formal principles, methods and
	criteria of validity of {inference}, reasoning and
	{knowledge}.

	Logic is concerned with what is true and how we can know
	whether something is true.  This involves the formalisation of
	logical arguments and {proofs} in terms of symbols
	representing {propositions} and {logical connectives}.  The
	meanings of these logical connectives are expressed by a set
	of rules which are assumed to be self-evident.

	{Boolean algebra} deals with the basic operations of truth
	values: AND, OR, NOT and combinations thereof.  {Predicate
	logic} extends this with existential and universal
	{quantifiers} and symbols standing for {predicates} which may
	depend on variables.  The rules of {natural deduction}
	describe how we may proceed from valid premises to valid
	conclusions, where the premises and conclusions are
	expressions in {predicate logic}.

	Symbolic logic uses a {meta-language} concerned with truth,
	which may or may not have a corresponding expression in the
	world of objects called existance.  In symbolic logic,
	arguments and {proofs} are made in terms of symbols
	representing {propositions} and {logical connectives}.  The
	meanings of these begin with a set of rules or {primitives}
	which are assumed to be self-evident.  Fortunately, even from
	vague primitives, functions can be defined with precise
	meaning.

	{Boolean logic} deals with the basic operations of {truth
	values}: AND, OR, NOT and combinations thereof.  {Predicate
	logic} extends this with {existential quantifiers} and
	{universal quantifiers} which introduce {bound variables}
	ranging over {finite} sets; the {predicate} itself takes on
	only the values true and false.  Deduction describes how we
	may proceed from valid {premises} to valid conclusions, where
	these are expressions in {predicate logic}.

	Carnap used the phrase "rational reconstruction" to describe
	the logical analysis of thought.  Thus logic is less concerned
	with how thought does proceed, which is considered the realm
	of psychology, and more with how it should proceed to discover
	truth.  It is the touchstone of the results of thinking, but
	neither its regulator nor a motive for its practice.

	See also fuzzy logic, logic programming, arithmetic and logic unit,
	first-order logic,

	See also {Boolean logic}, {fuzzy logic}, {logic programming},
	{first-order logic}, {logic bomb}, {combinatory logic},
	{higher-order logic}, {intuitionistic logic}, {equational
	logic}, {modal logic}, {linear logic}, {paradox}.

	2. <electronics> {Boolean} logic circuits.

	See also {arithmetic and logic unit}, {asynchronous logic},
	{TTL}.

	(1995-03-17)

<programming> (Or "remark") Explanatory text embedded in
	program {source} (or less often data) intended to help human
	readers understand it.

	Code completely without comments is often hard to read, but
	too heavily commented code isn't much better, especially if
	the comments are not kept up-to-date with changes to the code.
	Too much commenting may mean that the code is
	over-complicated.  A good rule is to comment everything that
	needs it but write code that doesn't need much of it.

	A particularly irksome form of over-commenting explains
	exactly what each statement does, even when it is obvious to
	any reasonably competant programmer, e.g.

		/* Open the input file */
		infd = open(input_file, O_RDONLY);

	(1998-04-28)

<programming> The first stage of software development which
	defines what the potential users want the system to do.  In
	modern methods these requirements should be testable, and will
	usually be traceable in later development stages.  A common
	feature of nearly all software is that the requirements change
	during its lifetime.

	See {software life-cycle}.

	(1995-11-11)

<language, tool> A {Unix} tool written in {Tcl} and a {script
	language} for automating the operation of {interactive}
	applications such as {telnet}, {FTP}, {passwd}, {fsck},
	{rlogin}, {tip}, etc..  Expect can feed input to other
	programs and perform {pattern matching} on their output.  It
	is also useful for testing these applications.  By adding
	{Tk}, you can also wrap interactive applications in {X11}
	{GUIs}.

	{Home (http://expect.nist.gov/)}.

	["expect: Scripts for Controlling Interactive Tasks", Don
	Libes, Comp Sys 4(2), U Cal Press Journals, Nov 1991].

	(1997-06-09)

Not working properly (of programs).

1. Executable code, especially a "pure code" portion shared
	between multiple instances of a program running in a
	{multitasking} {operating system}.

	Compare {English}.

	2. Textual material in the mainstream sense; data in ordinary
	{ASCII} or {EBCDIC} representation (see {flat ASCII}).  "Those
	are text files; you can review them using the editor."

	These two contradictory senses confuse hackers too.

	[{Jargon File}]

	(1995-03-16)