Barrons Publishing Dictionary of Computer and Internet Terms 10th
.pdf
linked list |
286 |
the physical locations in which they are stored. Each data item consists of two parts: the data itself, and a number giving the location of the next item. Figure 155 shows how this is usually diagrammed. To read the items in order, you need only know which item is in the beginning (the head) of the list; having located it, you can go next to the item whose address was stored with it; and so on.
FIGURE 156. Linked list: inserting elements
Linked lists allow items to be added or removed without requiring that other items be moved to make room. For instance, the list A–D–E of Figure 155 can be changed into A–B–C–D–E by adding two items. As Figure 156 shows, the newly added items B and C can be placed in the unused area after the E, and inserted into the list by changing the address associated with item A.
Figure 157 shows that an item can be deleted by changing the addresses so that there is no longer a path to that item. In either case, using linked lists can eliminate the need to move hundreds or thousands of data items whenever an insertion or deletion takes place.
FIGURE 157. Linked list: deleting an element
Figure 158 shows a way to construct a linked list in an ordinary twodimensional array; this can be done in practically any programming language. Each row of the array contains a data item and an integer indicating which row the next item is on (or zero, to indicate that there are more items). In the example, it is assumed that the first item in the list will always be in row 1; if you wish to be able to delete the first item, you can use a separate integer, outside the array, to keep track of where the list starts. See also DATA STRUCTURES.
Item |
Data |
Address of |
No. |
item |
next item |
|
|
|
1 |
A |
4 |
2 |
D |
3 |
3 |
E |
0 |
4 |
B |
5 |
5 |
C |
2 |
FIGURE 158. Linked list stored in an array
287 |
Lisp |
LinkedIn a social networking site (www.linkedin.com) designed for business professionals to use for work contacts and communication.
linker a program that puts separately compiled routines together to make a complete, working program. See LINK (definition 5).
Linspire a commercial distribution of Linux, based on Debian and later Ubuntu, that was marketed by Linspire, Inc. (www.linspire.com). In 2008, the company’s name changed to Digital Cornerstone. See DEBIAN;
LINUX; UBUNTU.
Linux (usually understood as “Linus’ UNIX”) a freely distributed UNIXcompatible operating system for PCs and a number of other processors.
Linux was developed by Linus Torvalds and others and is distributed under terms similar to those of Gnu’s “copyleft” (see GNU). Copies can be given away free provided they are complete and intact, but most users prefer to purchase commercially produced CD-ROMs containing Linux together with application software.
Linux is quite reliable and highly compatible with UNIX; as a result, it is very popular with universities, Internet service providers, and small businesses that need multi-user computing at minimum cost. More information can be found on the World Wide Web at www.linux.org. See also
DEBIAN; LINSPIRE; RED HAT; UBUNTU.
Linux box (slang) a small computer running Linux.
Lion common misspelling of LI-ION.
liquid crystal display see LCD.
Lisp (List Processor) a programming language developed in the late 1950s at MIT under the direction of John McCarthy. Because of the ease with which it can handle complex data structures, Lisp is used for artificial intelligence research and for writing programs whose complexity would render them unmanageable in other languages.
A Lisp program is easy to recognize because of the accumulation of closing parentheses at the end of the program. All Lisp statements and most Lisp data structures are linked lists, written as lists of elements in parentheses (see LINKED LIST). Programmers build a complete program by defining their own statements (actually functions) in terms of those previously defined. For example, a function that computes the factorial of X is:
(DEFUN FACTORIAL (X) (IF (= X0)
1
(* FACTORIAL(-X 1))))
Translating into English: “This is the definition of a function called FACTORIAL whose parameter is X. If X is zero, its factorial is 1; otherwise, its factorial is equal to X times the factorial of X – 1.” The IF keyword works like a Pascal if-then-else statement. This function calls itself recursively; recursion is a normal way of expressing repetition in Lisp.
list |
288 |
list
1.a set of data items that are to be accessed in a particular order; for instance, a list of the students in a class might be accessed in alphabetical order. Lists are stored in arrays or linked lists. See ARRAY; LINKED LIST.
2.to display a program line by line (especially in BASIC).
3.a MAILING LIST to which messages are distributed by e-mail.
list administrator the person responsible for maintaining a MAILING LIST.
list box an area in a dialog box where the user can choose among a list of items, such as files, directories, printers, or the like. For an illustration, see DIALOG BOX.
list processing
1.the manipulation of linked lists. See LINKED LIST; LISP.
2.the processing of mailing lists and similar data. See DATABASE MAN-
AGEMENT.
LISTSERV a commercial software package for operating e-mail mailing lists and discussion groups, produced by L-Soft International (www.lsoft.com). LISTSERV runs on a server, which can be a mainframe or microcomputer. The first version of LISTSERV was implemented by Eric Thomas on BITNET in 1986. The current version includes the ability to filter out spam and viruses. Compare MAJORDOMO.
Usage note: “LISTSERV” does not mean “e-mail list.” Not all e-mail mailing lists use LISTSERV software.
literal in a programming language, a written representation that always represents the same value. For example, the literal 2.5 always stands for the number 2.5, and the literal “abc” always stands for the character string abc. Names defined by the programmer, such as variable and function names, are not literals.
little-endian a system of memory addressing in which numbers that occupy more than one byte in memory are stored “little-end-first,” with the lowest 8 bits at the lowest address.
For example, the 16-digit binary number 1010111010110110 occupies two 8-bit bytes in memory. On a little-endian computer such as the IBM PC, the lower byte, 10110110, is stored at the first address and the upper byte, 10101110, is stored at the next higher address. On a bigendian machine, the order is reversed. Contrast BIG-ENDIAN.
The terms “big-endian” and “little-endian” are from Gulliver’s Travels; they originally referred to the parties in a dispute over which end of a boiled egg should be broken first.
Live Microsoft’s collection of online services (e-mail, photo sharing, etc.; web address: www.windowslive.com/Home).
Live Messenger an Instant Messaging application provided by Microsoft. A competing service is AOL’s AIM (AOL Instant Messenger). A key fea-
289 |
local variable |
ture of IM programs is their ability to display your status, whether “Online” or “Away,” to your regular contacts. IM programs also allow you to designate “Friends” and provide the ability to block communication with unwanted persons. IM messages are typically brief and heavily abbreviated. Icons are sometimes used to express emotions.
Messaging programs are also becoming popular with businesses, especially when members of a tight-knit work group are traveling.
LiveJournal a web site (www.livejournal.com) providing a popular web log service (see BLOG) that also provides basic SOCIAL NETWORKING features. Users can control whether their posts are public or only visible to defined FRIENDs. Compare BLOGGER; WORDPRESS; XANGA.
ln the function, in several programming languages, that calculates the natural (base e) logarithm of its argument. For example, ln(X) finds the natural logarithm of X. See LOGARITHM; E.
LN abbreviation for “like new” (describing items for sale).
LN– abbreviation for “like new minus” (i.e., almost new, almost unused, showing only slight wear). Contrast EX+.
LNIB abbreviation for “like new, in box” (i.e., slightly used but supplied with original packaging). Compare NOS (definition 2).
load to transfer information from a disk or other outside device into the memory of a computer. Contrast SAVE. See also LOADER.
loader a computer program whose function is to load another program into memory and transfer control to it. All operating systems include loaders. For example, in Windows, if you have a program named myfile.exe and you type the command
C:\> myfile
you are telling the loader to find myfile.exe and load it.
local located at the user’s computer or site. Contrast REMOTE.
local-area network (LAN) a network that connects several computers that are located nearby (in the same room or building), allowing them to share files and devices such as printers. See ETHERNET. Contrast WIDEAREA NETWORK.
local bus a separate bus in a computer, designed to provide extra-fast access to the CPU for specific devices, such as video cards. It contrasts with the main bus, which connects to most other parts of the computer. For examples see PCI, AGP.
local variable a variable that has meaning only within a particular function, subroutine, or other program unit. The name of a local variable can be used in another subroutine elsewhere in the program, where it will refer
localization |
290 |
to an entirely different variable. Local variables contrast with global variables, which are recognized throughout the program.
The advantage of using local variables is not obvious in short programs. However, it is a good idea when writing a long program to make as many variables as possible local, because then there will be no problem if you wish to use the same name to mean something else elsewhere in the program. This rule is even more important if several different people are writing subroutines that will be combined into one main program.
See also SIDE EFFECT.
localization the process of adapting software to run in a particular part of the world. Localization might involve translating screen displays into French or German, adapting to a foreign-language keyboard, printing the date differently (e.g., 2009 Oct 21 in Japan vs. 21 Oct 2009 in Britain and Oct. 21, 2009 in the United States), setting the clock for daylight saving time on different dates, or even writing numbers differently (3,000.95 vs. 3 000.95 or even 3.000,95).
lock
1.(on the Macintosh) to mark a file or disk as “Do not change” by clicking on the “Locked” box in the “Get Info” window.
2.(in various operating systems) to mark a file as in use so that other programs running concurrently will not change it.
log the function, in many programming languages, that calculates the natural (base-e) logarithm of its argument. However, in some languages and spreadsheets, log(x) is the common (base-10) logarithm and ln(x) is the natural logarithm. See LOGARITHM.
log in see LOG ON.
log on, log in to identify yourself as an authorized user of a computer or a network at the beginning of a work session.
logarithm the power to which a number must be raised in order to give another number.
If y = ax, then x is the logarithm of y to the base a (written as x = loga y). The most commonly used bases for logarithm functions are 10 and e (approximately 2.718). Base-10 logarithms are called common logarithms; base-e logarithms, natural logarithms (because integrals and derivatives are simpler with base e than with any other base). For example, the common logarithm of 10,000 is 4 (log10 10,000 = 4) because 104 = 10,000.
If no base is specified in the expression log a, then usually base 10 is meant; the natural logarithm of a is written loge a or ln a.
logged drive see CURRENT DRIVE.
logic circuits electronic circuits that accept binary digits (bits) as inputs and produce an output bit according to a specified rule. For examples see
291 |
logic circuits |
AND GATE; OR GATE; NAND GATE; NOR GATE; NOT GATE; FLIP-FLOP. For information on how logic circuits are used, see BINARY ADDITION; COMPUTER
ARCHITECTURE; DECODER; XOR GATE.
A typical computer represents 1 (logic “true”) as +5 volts and 0 as 0 volts. More precisely, 1 is represented by a connection to the +5-volt power supply (directly or through a resistance), and 0 is represented by a connection to ground. Note that 0 is not merely the absence of a voltage; logic circuits differ as to how they handle an unconnected input.
Basically, logic circuits are switching circuits. Figure 159(A) shows a NOT gate implemented as a switch. The output is +5 volts (binary 1, logic “true”) whenever the switch is not closed. (When the switch is closed, the resistor dissipates the voltage and the output is connected to ground.) That is, the output is the negation of the state of the switch.
FIGURE 159. NOT gate built with a switch (A) and a transistor (B)
For this to be usable in a computer, the switching has to be controlled by an electrical signal. Figure 159(B) shows what happens when the switch is replaced by a switching transistor. The transistor conducts when its base is at least 0.6 volts above ground (i.e., when its input is binary 1). When the transistor is conducting, the effect is the same as the closed switch, and the output is 0. Thus, the output is the negation of the input, and the NOT gate works correctly.
Figure 160 shows how to build a NAND gate out of two diodes, two resistors, and a transistor. This circuit is very similar to what is used inside TTL integrated circuits. The output is 0 (“false”) if and only if both of the inputs are binary 1 (+5 volts). In that situation, the diodes do not conduct, the base of the transistor receives current through the resistor, and the transistor conducts. But if even one of the inputs is binary 0 (connected to ground), the base of the transistor is held low and the transistor does not conduct, so the output is binary 1. To understand this circuit, it is very important to remember that binary 0 is represented by a
logic circuits |
292 |
connection to ground, not merely the absence of a voltage. Like real TTL ICs, this circuit happens to treat disconnected inputs as binary 1.
NAND gates are important because all the other gates can be built from them (Figure 161). A NOT gate is simply a NAND gate with only one input, or with all its inputs tied together; an AND gate is a NAND gate followed by a NOT gate; and so on. In a similar way, all the types of gates can be built from NOR gates.
FIGURE 160. NAND gate built with transistors and diodes
FIGURE 161. Logic circuits made from NAND gates
293 |
LOGO |
Instead of TTL circuits, newer ICs use CMOS (complementary metaloxide semiconductor) switching transistors, which come in pairs that respond to opposite polarities, so that one switches off whenever the other switches on. This makes it easy to connect the output either to +5 volts or to ground depending on the input. However, the circuits inside practical CMOS gates are too complicated to diagram here.
logic diagram an electronic circuit diagram that shows gates and other components that affect logic signals, but does not show the power supply or other non-digital electronic subsystems. See
DIAGRAM SYMBOLS.
logic programming a method of writing computer programs based on the mathematical study of logical reasoning. Logic programming is used in the computer modeling of human thinking. For examples, see PROLOG.
logical
1.possessing or pertaining to logic (in any of various senses).
2.described from the viewpoint of software. For example, if a single disk drive is divided into two partitions which the computer handles separately, it can be said to comprise two logical disk drives.
logical design
1.the design of an electronic circuit using logic gates. See GATE and cross-references there.
2.the design of the logic of a computer program (as opposed to its user interface or data files).
3.the practice of designing a document by using tags to indicate the function rather than the appearance of each element. For example, chapters are labeled as such rather than just being indicated by words typed in a particular arrangement on the page.
Logical design is the approach followed by LATEX, SGML, and XML; it is not followed by WYSIWYG word processors. Logical design is generally superior for complicated documents because decisions about the appearance of elements of the document can be made independently of the text. If you want to change the appearance of chapter headings, for instance, you need to make the change in only one place because all chapter headings are recognized as instances of the same unit. In a WYSIWYG system, you would need to change each heading individually because the computer does not know that they are alike. Documents with tags specifying the logical design are also easier to handle effectively in computer databases.
logical drive one of several divisions of a single partition on a hard disk. Logical drives are treated as separate disk drives.
logo a trademark or printed emblem; short for logotype.
LOGO a programming language developed by Seymour Papert of MIT for use in teaching programming to children. Papert’s fundamental insight
lol, LOL |
294 |
was that computer-aided instruction is of little use unless the pupil can control the computer, rather than the other way around. To experiment with this idea, he designed a language that is markedly easier to use than BASIC and does not share BASIC’s preoccupation with numerical calculation.
Although LOGO offers a full range of computer functions, most elementary LOGO exercises revolve around the “turtle,” originally a robot that rolled around on a sheet of paper making marks with a pen. (The present-day turtle is a triangle that moves around the screen, drawing a line if told to do so.) Drawing shapes with the turtle appeals to children who would not be attracted to mathematical calculation or verbal inputoutput; at the same time, it serves as a good medium for teaching geometry and logical problem solving.
LOGO is an extensible language; that is, programs are constructed by defining statements in terms of previously defined statements. For example, the following procedure draws a square:
TO SQUARE
CLEARSCREEN
FORWARD 50
RIGHT 90
FORWARD 50
RIGHT 90
FORWARD 50
RIGHT 90
FORWARD 50
END
That is: “Clear the screen (and put the turtle in the center), go forward (up) 50 units, do a 90-degree right turn, go forward 50 units, do a 90degree right turn,” and so forth. Since LOGO procedures can call themselves recursively, complicated snowflake-like patterns are relatively easy to generate. See also KOCH SNOWFLAKE.
lol, LOL online abbreviation for “laugh out loud.”
long an integer with more bits stored than the normal-length integer. For example, in Java, a variable of type int fills 32 bits; a variable of type
long fills 64 bits (allowing 264 different values, ranging from –263 to 263 – 1 (approximately ± 9 × 1018).
long cross the character †, a symbol used to mark footnotes. See also FOOTNOTE. Also called a DAGGER or OBELISK.
Longhorn internal code name used for Windows Vista (Windows 6.0) before its release. See WINDOWS (MICROSOFT). Compare BLACKCOMB; CAIRO; CHICAGO; MEMPHIS; WHISTLER.
look and feel the overall visual appearance and USER INTERFACE of a computer program. See COPYRIGHT.
295 |
lurk |
loop
1.a series of statements in a computer program that are to be executed repeatedly. For examples see FOR and WHILE.
2.anything that receives electrical energy from a POWER SUPPLY.
3.(more fully, feedback loop) a control system in which one thing affects another, and its effect is sensed in order to make control decisions. For example, a heater, the air temperature, and a thermostat form a feedback loop. See also FEEDBACK; IN THE LOOP.
loose letterspacing that has been adjusted to increase the space between the letters. Contrast TIGHT. See LETTERSPACING for an illustration.
lost cluster a group of disk sectors that are not marked as free but are not allocated to a file. Lost clusters result when the operation of creating a file is interrupted. They waste space and should be cleaned up periodically; under Windows this is done with the SCANDISK tool.
Lotus 1-2-3 a popular SPREADSHEET program, widely used on IBM PCs since its introduction in 1983 by Lotus Development Corporation. Lotus is now part of IBM (web address: www-306.ibm.com/software/lotus). The original Lotus 1-2-3 contained significant innovations in graphing and data handling ability.
lowercase the “small” letters a, b, c, d, and so on, as opposed to uppercase or capital letters, A, B, C, D, and so on. The term lowercase goes back to the early days of letterpress printing. The metal type was kept in divided drawers called cases; the capital letters were traditionally kept in the upper case, and the small letters in the lower.
LPI (lines per inch) a measure of the resolution of a halftone screen (see HALFTONE). Most newspaper screens are 85 LPI; good quality magazines use 150 LPI. 300-DPI screened output is roughly equivalent to a 50-LPI screen (draft quality). 600 DPI on a plain paper typesetter should be acceptable for most work; it can produce the equivalent of a 100-LPI halftone. When higher resolutions are needed, the file should be output to a 2400-DPI imagesetter.
LPT1 the filename by which Windows refers to the first parallel printer port. Additional parallel ports are known as LPT2 and LPT3.
luminosity brightness; the property of glowing with light. Some 3D programs can render objects that seem to be emitting light by setting a high luminosity level.
lurk (slang) to read an online forum regularly without contributing any messages of your own. It’s advisable to lurk for a while before posting any messages in order to make sure you understand the purpose and nature of the discussion. Most forums have more lurkers than the participants realize.