Barrons Publishing Dictionary of Computer and Internet Terms 10th

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 people 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.

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 problems 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 fundamental 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 published in 1962. Quicksort is faster than any other sorting algorithm available 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 preceding 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.

(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 rightmost 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 sublists to the left and in right of it. Quicksort is difficult to express in languages, 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 algorithm 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 Windowsbased 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.

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 computer 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 chosen 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.

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 single 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 reconstructed 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 language or document definition. Railroad-like switches are used to indicate possible locations of different elements. Figure 212 shows an example illustrating the syntax of an address label. First name, last

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 important 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.