Encyclopedia of Computer Science and Technology

396 queueIn an empty queue, the head and tail pointers point to the firstcell in memory. To add a value, it is placed at the cell pointed toby the head pointer, and the tail pointer is moved up one cell. If anitem is removed, the head and tail pointers are moved down oneplace. (Note that items must be added at the tail and removed atthe head.)example, chloroform), would represent one qubit, anda large number of molecules would be used, for redundancy.Since each “observation” (that is, setting or readingdata) affects only a few of the many molecules foreach qubit, the stability of the information in the systemis not compromised. However, this approach is limited bythe number of atoms in the chosen molecule—perhaps to30 or 40 qubits.There are many potential applications for quantumcomputing. While the technology could be used to crackconventional cryptographic keys, researchers have suggestedthat it could also be used to generate unbreakablekeys that depend on the “entanglement” of observersand what they observe. The sheer computational powerof a quantum computer might make it possible to developmuch better computer models of complex phenomenasuch as weather, climate, the economy—or of quantumbehavior itself.Further ReadingBurda, Ioan. Introduction to Quantum Computation. Boca Raton,Fla.: Universal Publishers, 2005.Kaye, Phillip, Raymond LaFlamme, and Michele Mosca. An Introductionto Quantum Computing. New York: Oxford UniversityPress, 2007.Quantum Computer News. Science Daily. Available online.URL: http://www.sciencedaily.com/news/computers_math/quantum_computers/. Accessed August 17, 2007.West, Jacob. The Quantum Computer. Available online. URL: http://www.cs.caltech.edu/~westside/quantum-intro.html. AccessedAugust 17, 2007.queueA queue is basically a “line” of items arranged according topriority, much like the customers waiting to check out in asupermarket. Many computer applications involve receiving,tracking, and processing requests. For example, an operatingsystem running on a computer with a single processor mustkeep track of which application should next receive the processor’sattention. A print spooler holds documents waiting tobe printed. A web or file server must keep track of requests forWeb pages, files, or other services. Queues provide an orderlyway to process such requests. Queues can also be used toefficiently store data in memory until it can be processed by arelatively slow device such as a printer (see buffering).As a data structure, a queue is a type of list (see listprocessing). New items are inserted at one end andremoved (deleted) from the other end. This contrasts witha stack, where all insertions and deletions are made at thesame end (see stack). Just as the next person served at thesupermarket is the one at the head of the line, the end of aqueue from which items are removed is called the head orfront. And just as new people arriving at the supermarketline join the end of the line, the part of the queue wherenew items are added is called the tail or rear. Since the firstitem in line is the first to be removed, a queue is called aFIFO (first in, first out) structure.To create a queue, a program first allocates a block ofmemory. It then sets up to pointers (see pointers and indirection).One pointer stores the address of the item at thehead of the queue; the other has the address of the item atthe tail. When the queue starts out, it is empty. This meansthat both the head and tail pointer start out pointing to thesame location.A circular queue works in the same way as a “straight” queue,except that when the last cell in the allotted memory block isreached, the pointer or data “wraps around” to the first cell.