Dictionary of Computer and Internet Terms phần 8 pdf

pushdown stack, pushdown store a data structure from which items can only be removed in the opposite of the order in which they were stored. See STACK. pushing the envelope working close to, or at, physical or technological limits. See ENVELOPE. PvE (Player versus Environment) a type of game where players overcome challenges given to them by the game itself rather than by other players. PvP (Player versus Player) a type of game where players compete against each other. pwn comical misspelling of own in the slang sense. See OWN. pyramid scheme (Ponzi scheme) a get-rich-quick scheme in which you receive a message containing a list of names. You’re expected to send money to the first person on the list, cross the first name off, add your name at the bottom, and distribute copies of the message. Pyramid schemes are presently common on the Internet, but they are illegal in all 50 states and in most other parts of the world. They can’t work because there is no way for everyone to receive more money than they send out; money doesn’t come out of thin air. Pyramid schemers often claim the scheme is legal, but that doesn’t make it so. See also COMPUTER LAW. Python a programming language invented by Guido van Rossum for quick, easy construction of relatively small programs, especially those that involve character string operations. Figure 207 shows a simple Python program that forms the plurals of English nouns. Python is quickly replacing Awk and Perl as a scripting language. Like those languages, it is run by an interpreter, not a compiler. That makes it easy to store programs compactly as source code and then run them when needed. It is also easy to embed operating system commands in the program. Python is also popular for teaching programming to non-program- mers, since even a small, partial knowledge of the language enables peo- ple to write useful programs. The syntax of Python resembles C and Java, except that instead of enclosing them in braces, groups of statements, such as the interior of a while loop, are indicated simply by indentation. Powerful data structures are easy to create in Python. These include lists and dictionaries, where a dictionary is an array whose elements are identified by character strings. Operations such as sorting, searching, and data conversion are built in. Free Python interpreters and more information can be obtained from www.python.org. See also AWK; INTERPRETER; PERL; STRING OPERATIONS. 387 Python 7_4105_DO_CompInternetTerms_P 12/29/08 10:35 AM Page 387 # File plural6.py -M. Covington 2002 # Python function to form English plurals def pluralize(s): ”Forms the plural of an English noun” # Exception dictionary. More could be added. e = { ”child” : ”children”, ”corpus” : ”corpora”, ”ox” : ”oxen” } # Look up the plural, or form it regularly if e.has_key(s): return e[s] elif s[-1] == ”s” \ or s[-2:] == ”sh” \ or s[-2:] == ”ch”: return s + ”es” else: return s + ”s” FIGURE 207. Python program Python 388 7_4105_DO_CompInternetTerms_P 12/29/08 10:35 AM Page 388 Q QoS Quality of Service quad-core having four CPU cores. See CORE (definition 1). quantum computing a possible method for creating future computers based on the laws of quantum mechanics. Classical computers rely on physical devices that have two distinct states (1 and 0). In quantum mechanics, a particle is actually a wave function that can exist as a superposition (combination) of different states. A quantum computer would be built with qubits rather than bits. Theoretical progress has been made in designing a quantum computer that could perform computations on many numbers at once, which could make it possible to solve prob- lems now intractable, such as factoring very large numbers. However, there are still practical difficulties that would need to be solved before such a computer could be built. quantum cryptography an experimental method for securely transmitting encryption keys by using individual photons of polarized light. A funda- mental principle of quantum mechanics, the Heisenberg uncertainty principle, makes it impossible for anyone to observe a photon without disturbing it. Therefore, it would be impossible for an eavesdropper to observe the signal without being detected. See ENCRYPTION. qubit a quantum bit. See QUANTUM COMPUTING. query language a language used to express queries to be answered by a database system. For an example, see SQL. queue 1. a data structure from which items are removed in the same order in which they were entered. Contrast STACK. 2. a list, maintained by the operating system, of jobs waiting to be printed or processed in some other way. See PRINT SPOOLER. Quicken a popular financial record keeping program produced by INTUIT. Quicksort a sorting algorithm invented by C. A. R. Hoare and first pub- lished in 1962. Quicksort is faster than any other sorting algorithm avail- able unless the items are already in nearly the correct order, in which case it is relatively inefficient (compare MERGE SORT). Quicksort is a recursive procedure (see RECURSION). In each iteration, it rearranges the list of items so that one item (the “pivot”) is in its final position, all the items that should come before it are before it, and all the items that should come after it are after it. Then the lists of items pre- ceding and following the pivot are treated as sublists and sorted in the same way. Figure 208 shows how this works: (a) Choose the last item in the list, 41, as the pivot. It is excluded from the searching and swapping that follow. 389 Quicksort 7_4105_DO_CompInternetTerms_Q 12/29/08 10:35 AM Page 389 (b), (c) Identify the leftmost item greater than 41 and the rightmost item less than 41. Swap them. (d), (e), (f), (g) Repeat steps (b) and (c) until the leftmost and right- most markers meet in the middle. (h), (i) Now that the markers have met and crossed, swap the pivot with the item pointed to by the leftmost marker. (j) Now that the pivot is in its final position, sort each of the two sub- lists to the left and in right of it. Quicksort is difficult to express in lan- guages, such as BASIC, that do not allow recursion. The amount of memory required by Quicksort increases exponentially with the depth of the recursion. One way to limit memory requirements is to switch to another type of sort, such as selection sort, after a certain depth is reached. (See SELECTION SORT.) Figure 209 shows the Quicksort algo- rithm expressed in Java. FIGURE 208. Quicksort in action QuickTime a standard digital video and multimedia framework originally developed for Macintosh computers, but now available for Windows- based systems. The QuickTime Player plays back videos and other multimedia presentations and is available as a free download from www.apple.com/downloads. The premium version of QuickTime provides video editing capability as well as the ability to save QuickTime movies (.mov files). Compare AVI FILE; MOV. QuickTime 390 7_4105_DO_CompInternetTerms_Q 12/29/08 10:35 AM Page 390 391 quit class quicksortprogram { /* This Java program sorts an array using Quicksort. */ static int a[] = {29,18,7,56,64,33,128,70,78,81,12,5}; static int num = 12; /* number of items in array */ static int max = num-1; /* maximum array subscript */ static void swap(int i, int j) { int t=a[i]; a[i]=a[j]; a[j]=t; } static int partition(int first, int last) { /* Partitions a[first] a[last] into 2 sub-arrays using a[first] as pivot. Value returned is position where pivot ends up. */ int pivot = a[first]; int i = first; int j = last+1; do { do { i++; } while ((i<=max) && (a[i]<pivot)); do { j——; } while ((j<=max) && (a[j]>pivot)); if (i<j) { swap(i,j); } } while (j>i); swap(j,first); return j; } static void quicksort(int first, int last) { /* Sorts the sub-array from a[first] to a[last]. */ int p=0; if (first<last) { p=partition(first,last); /* p = position of pivot */ quicksort(first,p-1); quicksort(p+1,last); } } public static void main(String args[]) { quicksort(0,max); for (int i=0; i<=max; i++) { System.out.println(a[i]); } } } FIGURE 209. Quicksort quit to clear an application program from memory; to EXIT. Most software prompts you to save changes to disk before quitting. Read all message boxes carefully. 7_4105_DO_CompInternetTerms_Q 12/29/08 10:35 AM Page 391 R race condition, race hazard in digital circuit design, a situation where two signals are “racing” to the same component from different places, and although intended to be simultaneous, they do not arrive at exactly the same time. Thus, for a brief moment, the component at the destination receives an incorrect combination of inputs. radial fill a way of filling a graphical object with two colors such that one color is at the center, and there is a smooth transition to another color at the edges. See FOUNTAIN FILL. Contrast LINEAR FILL. FIGURE 210. Radial fill radian measure a way of measuring the size of angles in which a complete rotation measures 2π radians. The trigonometric functions in most com- puter languages expect their arguments to be expressed in radians. To convert degrees to radians, multiply by π/180 (approximately 1/57.296). radio buttons small circles in a dialog box, only one of which can be cho- sen at a time. The chosen button is black and the others are white. Choosing any button with the mouse causes all the other buttons in the set to be cleared. Radio buttons acquired their name because they work like the buttons on older car radios. Also called OPTION BUTTONS. FIGURE 211. Radio buttons radix the base of a number system. Binary numbers have a radix of 2, and decimal numbers have a radix of 10. radix sort an algorithm that puts data in order by classifying each item immediately rather than comparing it to other items. For example, you might sort cards with names on them by putting all the A’s in one bin, all the B’s in another bin, and so on. You could then sort the contents of each bin the same way using the second letter of each name, and so on. race condition, race hazard 392 7_4105_DO_CompInternetTerms_R 12/29/08 10:36 AM Page 392 The radix sort method can be used effectively with binary numbers, since there are only two possible bins for the items to be placed. For other sorting methods, see SORT and references there. ragged margin a margin that has not been evened out by justification and at which the ends of words do not line up. This is an example of flush-left, ragged-right type. See also FLUSH LEFT, FLUSH RIGHT. RAID (redundant array of inexpensive disks) a combination of disk drives that function as a single disk drive with higher speed, reliability, or both. There are several numbered “levels” of RAID. RAID 0 (“striping”) uses a pair of disk drives with alternate sectors written on alternate disks, so that when a large file is read or written, each disk can be transferring data while the other one is moving to the next sector. There is no error protection. RAID 1 (“mirroring”) uses two disks which are copies of each other. If either disk fails, its contents are preserved on the other one. You can even replace the failed disk with a new, blank one, and the data will be copied to it automatically with no interruption in service. RAID 2 (rarely used) performs striping of individual bits rather than blocks, so that, for instance, to write an 8-bit byte, you need 8 disks. Additional disks contain bits for an error-correcting code so that any missing bits can be reconstructed. Reliability is very high, and any sin- gle disk can be replaced at any time with no loss of data. RAID 3 (also uncommon) performs striping at the byte rather than bit or sector level, with error correction. RAID 4 performs striping at the level of sectors (blocks) like RAID 0, but also includes an additional disk for error checking. RAID 5 is like RAID 4 except that the error-checking blocks are not all stored on the same disk; spreading them among different disks helps equalize wear and improve speed. RAID 5 is one of the most popular configurations. If any single disk fails, all its contents can be recon- structed just as in RAID 1 or 2. RAID 6 is like RAID 5 but uses double error checking and can recover from the failure of any two disks, not just one. Caution: RAID systems do not eliminate the need for backups. Even if a RAID system is perfectly reliable, you will still need backups to retrieve data that is accidentally deleted and to recover from machine failures that affect all the disks at once. railroad diagram a diagram illustrating the syntax of a programming lan- guage or document definition. Railroad-like switches are used to indi- cate possible locations of different elements. Figure 212 shows an example illustrating the syntax of an address label. First name, last 393 railroad diagram 7_4105_DO_CompInternetTerms_R 12/29/08 10:36 AM Page 393 name, and City-State-Zip Code are required elements, so all possible routes include those elements. Either Mr. or Ms. is required, so there are two possible tracks there. A middle name is optional, so one track bypasses that element. There may be more than one address line, so there is a return loop track providing for multiple passes through that element. FIGURE 212. Railroad diagram. RAM (Random-Access Memory) a memory device whereby any location in memory can be found as quickly as any other location. A computer’s RAM is its main working memory. The size of the RAM (measured in megabytes or gigabytes) is an important indicator of the capacity of the computer. See DRAM; EDO; MEMORY. random-access device any memory device in which it is possible to find any particular record as quickly, on average, as any other record. The computer’s internal RAM and disk storage devices are examples of ran- dom-access devices. Contrast SEQUENTIAL-ACCESS DEVICE. random-access memory see RAM. random-number generator a computer program that calculates numbers that seem to have been chosen randomly. In reality, a computer cannot generate numbers that are truly random, since it always generates the numbers according to a deterministic rule. However, certain generating rules produce numbers whose behavior is unpredictable enough that they can be treated as random numbers for practical purposes. Random-number generators are useful in writing programs involving games of chance, and they are also used in an impor- tant simulation technique called MONTE CARLO SIMULATION. rapid prototyping the construction of prototype machines quickly with computer aid. Computerized CAD-CAM equipment, such as milling machines and THREE-DIMENSIONAL PRINTERS, can produce machine parts directly from computer-edited designs. raster graphics graphics in which an image is generated by scanning an entire screen or page and marking every point as black, white, or another color, as opposed to VECTOR GRAPHICS. A video screen and a laser printer are raster graphics devices; a pen plotter is a vector graphics device because it marks only at specified points on the page. raster image processor (RIP) a device that handles computer output as a grid of dots. Dot-matrix, inkjet, and laser printers are all raster image processors. RAM 394 7_4105_DO_CompInternetTerms_R 12/29/08 10:36 AM Page 394 395 real estate rasterize to convert an image into a bitmap of the right size and shape to match a raster graphics output device. See BITMAP; RASTER GRAPHICS; VECTOR GRAPHICS. RAW in digital photography, unprocessed; the actual binary data from the camera, with, at most, only the processing that is unavoidably done by the camera itself. Although commonly written uppercase (RAW), this is simply the familiar word raw, meaning “uncooked.” Raw image files contain more detail than JPEG compressed images but are much larger and can only be read by special software. ray tracing the computation of the paths of rays of light reflected and/or bent by various substances. Ray-tracing effects define lighting, shadows, reflections, and trans- parency. Such computations often are very lengthy and may require sev- eral hours of computer time to produce a RENDERING (realistic drawing of three-dimensional objects by computer). RCA plug (also called PHONO PLUG) an inexpensive shielded plug some- times used for audio and composite video signals (see Figure 213); it plugs straight in without twisting or locking. Contrast BNC CONNECTOR. FIGURE 213. RCA plug RDRAM (Rambus dynamic random access memory) a type of high-speed RAM commonly used with the Pentium IV, providing a bus speed on the order of 500 MHz. Contrast SDRAM. read to transfer information from an external medium (e.g., a keyboard or diskette) into a computer. read-only pre-recorded and unable to be changed. See ATTRIBUTES; CD- ROM; LOCK; WRITE-PROTECT. read-only memory computer memory that is permanently recorded and cannot be changed. See ROM. readme (from read me) the name given to files that the user of a piece of software is supposed to read before using it. The readme file contains the latest information from the manufacturer; it often contains major corrections to the instruction manual. real estate (informal) space on a flat surface of limited size, such as a motherboard (on which different components consume different amounts of real estate) or even a computer screen. Compare SCREEN ESTATE. 7_4105_DO_CompInternetTerms_R 12/29/08 10:36 AM Page 395 real number any number that can be represented either as an integer or a decimal fraction with any finite or infinite number of digits. Real num- bers correspond to points on a number line. Examples are 0, 2.5, 345, –2134, 0.00003, , , and π. However, is not a real number (it does not exist anywhere among the positive or negative numbers). Contrast COMPLEX NUMBER. On computers, real numbers are represented with a finite number of digits, thus limiting their accuracy. See ROUNDING ERROR. In many programming languages, “real number” means “floating- point number.” See DOUBLE; FLOATING-POINT NUMBER. real-time programming programming in which the proper functioning of the program depends on the amount of time consumed. For instance, computers that control automatic machinery must often both detect and introduce time delays of accurately determined lengths. RealAudio a communication protocol developed by Real Networks (www.realaudio.com) that allows audio signals to be broadcast over the Internet. The user hears the signal in real time, rather than waiting for an audio file to be downloaded and then played. RealAudio is used to dis- tribute radio broadcasts. See INTERNET RADIO; PROTOCOL. RealPlayer a widely used program for playing RealAudio files, distributed by Real Networks. See REALAUDIO. ream 500 sheets of paper. reboot to restart a computer (i.e., turn it off and then on again). Many oper- ating systems, including UNIX and Windows, have to be shut down properly before power to the computer is turned off; otherwise, data will be lost. See BOOT. record a collection of related data items. For example, a company may store information about each employee in a single record. Each record consists of several fields—a field for the name, a field for a Social Security number, and so on. The Pascal keyword record corresponds to struct in C. See STRUCT. recovering erased files retrieval of deleted files whose space has not yet been overwritten by other data. In Windows and on the Macintosh, deleted files usually go into a TRASH can or RECYCLE BIN from which they can be retrieved. The disk space is not actually freed until the user empties the trash. Until then, the files can be restored to their original locations. Even after the trash can or recycle bin has been emptied, the physical disk space that the file occupied is marked as free, but it is not actually overwritten until the space is needed for something else. If you erase a file accidentally, you can often get it back by using special software. As soon as you realize you want to recover a file, do everything you can to −1 2 1 3 real number 396 7_4105_DO_CompInternetTerms_R 12/29/08 10:36 AM Page 396 [...]... order of execution Now Now Now Now Now The The The The The 24 looking for factorial looking for factorial looking for factorial looking for factorial looking for factorial factorial of 0 is 1 factorial of 1 is 1 factorial of 2 is 2 factorial of 3 is 6 factorial of 4 is 24 of of of of of 4 3 2 1 0 Any iterative (repetitive) program can be expressed recursively, and recursion is the normal way of expressing... your own computer remote located on a computer far away from the user Contrast LOCAL Remote Desktop a feature of some versions of Microsoft Windows that allows one computer to serve as the screen, keyboard, and mouse of another; thus, any computer can be operated remotely This is particularly handy for administering servers that may be located in a different room To enable remote access to a computer, ... each of which can be executed very quickly The Sun Sparcstation and the PowerPC are examples of RISC computers The opposite of RISC is CISC RISC architecture was developed for speed A RISC computer can execute each instruction faster because there are fewer instructions to choose between, and thus less time is taken up identifying each instruction RISC and CISC computers can run the same kinds of software;... or T568B at both ends A crossover cable is T568A at one end and T568B at the other RL abbreviation for “real life” in e-mail and online games rlogin (remote login) the UNIX command that allows you to use your computer as a terminal on another computer Unlike telnet, rlogin does more than just establish a communication path: it also tells the other computer what kind of terminal you are using and sends... computer; if you add the computer s representation of 0.1 to 0 ten times, you will not get exactly 1 To avoid rounding error, some computer programs represent numbers as decimal digits See BINARY-CODED DECIMAL Route 1 28 a highway that skirts the west side of Boston, Massachusetts, passing through the cities of Needham and Waltham Route 1 28 has been the home of a number of computer companies, including... 10,000 RPM RPN see POLISH NOTATION RS-232 an Electronics Industries Association (EIA) recommended standard for transmitting serial data by wire This standard is now officially known as EIA-232D Computer serial ports follow the RS-232 standard 7_4105_DO_CompInternetTerms_R 12/29/ 08 10:36 AM Page 4 18 4 18 RS-422, RS-423A TABLE 14 RS-232 PIN CONNECTIONS (25-PIN) (Pin numbers are embossed on the connector.)... business and education; only those specific to computers are covered here registrar an organization authorized to register TLD For example, the domain covingtoninnovations.com belongs to three of the authors of this book because they have registered it with a registrar 7_4105_DO_CompInternetTerms_R 401 12/29/ 08 10:36 AM Page 401 relational database registration 1 the act of informing the manufacturer of. .. high resolution (e.g., 280 0 dots per inch), which means they control the position of the ink sprayer to a precision of 1/ 280 0 inch The actual dots of colored ink are much larger than 1/ 280 0 inch in size However, halftoning is not needed; each dot can be any color or shade of gray 7_4105_DO_CompInternetTerms_R 12/29/ 08 10:36 AM Page 406 406 resource The human eye normally resolves about 150 lines per... complete elimination of lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers in electronic equipment sold in Europe Similar restrictions are being adopted elsewhere The main effect of RoHS is to mandate the use of lead-free solder and to eliminate nickel-cadmium batteries See NICD; SOLDER 7_4105_DO_CompInternetTerms_R 12/29/ 08 10:36 AM 413 Page 413... x; means “exit, returning the value of x as the value of the function,” and return; means “exit, returning no value.” Return key the key on a computer keyboard that tells the computer that the end of a line has been reached On most keyboards the Return key is marked Enter On IBM 3270 terminals, the Return and Enter keys are separate reusable components pieces of software that can be used in other programs . ”es” else: return s + ”s” FIGURE 207. Python program Python 388 7_4105_DO_CompInternetTerms_P 12/29/ 08 10:35 AM Page 388 Q QoS Quality of Service quad-core having four CPU cores. See CORE (definition. excluded from the searching and swapping that follow. 389 Quicksort 7_4105_DO_CompInternetTerms_Q 12/29/ 08 10:35 AM Page 389 (b), (c) Identify the leftmost item greater than 41 and the rightmost item. 390 7_4105_DO_CompInternetTerms_Q 12/29/ 08 10:35 AM Page 390 391 quit class quicksortprogram { /* This Java program sorts an array using Quicksort. */ static int a[] = {29, 18, 7,56,64,33,1 28, 70, 78, 81,12,5}; static