Encyclopedia of Computer Science and Technology

queue 397To add an item, the tail pointer is moved back one locationand the item is stored there. To remove an item, thehead pointer is simply moved back one location. (The datathat had been pointed to by the head pointer can be eitherretrieved or discarded, depending on the application.)In actuality it’s not quite so simple. As items are addedto the queue, the tail pointer keeps moving back in memorywith the head pointer trailing behind as items are deleted. Ifthe queue is sufficiently active (many items are being addedand removed), the queue will end up “crawling” throughmemory somewhat like a worm until all the memory isconsumed.In a real line at the supermarket, as a customer leavesthe checkout stand, each of the persons in line movesup one space. In a computer queue this could be accomplishedby moving each item up one location wheneveran item is removed at the head. However, having to moveall the data items each time one is changed would be veryinefficient. Instead, one could allow the head of the queueto move only up to some specified location. At that point,the head is moved back to the beginning of the memoryblock, and thus the space that had been vacated by thetail as it moved up is reutilized. In effect this wraps thememory around into a circle, so this is called a circularqueue.Further ReadingBrookshear, J. Glenn. Computer Science: An Overview. 6th ed. Reading,Mass.: Addison-Wesley, 2000.Skiena, Steven S. “Priority Queues.” Available online. URL: http://www2.toki.or.id/book/AlgDesignManual/BOOK/BOOK3/NODE130.HTM. Accessed August 17, 2007.Suh, Eric. “The Queue Data Structure.” Available online. URL:http://www.cprogramming.com/tutorial/computersciencetheory/queue.html. Accessed August 17, 2007.