Encyclopedia of Computer Science and Technology

224 health, personalTo search a hashed database, the hashing formula is first applied tothe search key, yielding a hash value. That value can then be usedin a binary search to quickly zero in on the matching record, if any.the unique relationship between the original message andthe hash maintains a high degree of security.Finally, hashing can be used for error detection. If amessage and its hash are sent together, the recipient canhash the received text. If the hash value generated matchesthe one received, it is highly likely the message was receivedintact (see also error correction).Further ReadingPartow, Arash. “General Purpose Hash Function Algorithms.”Available online. URL: http://www.partow.net/programming/hashfunctions/. Accessed August 6, 2007.Pieprzyk, Josef, and Babak Sadeghiyan. Design of Hashing Algorithms.New York: Springer-Verlag, 1993.health, personal See personal health informationmanagement.heapIn operating systems and certain programming languages(such as LISP), a heap is a pool of memory resources availablefor allocation by programs. The memory segments(sometimes called cells) can be the same size or of variablesize. If the same size, they are linked together by pointers(see list processing). Memory is then allocated for a variableby traversing the list and setting the required numberof cells to be “owned” by that variable. (While some languagessuch as Pascal and C use explicit memory allocationor deallocation functions, other languages such as LISP usea separate runtime module that is not the responsibility ofthe programmer.)Deallocation (the freeing up of memory no longerneeded by a variable so it can be used elsewhere) is morecomplicated. In many languages several different pointerscan be used to refer to the same memory location. It istherefore necessary not only to disconnect a given pointerfrom the cell, but to track the total number of pointers connectedto the cell so that the cell itself is deallocated onlywhen the last pointer to it has been disconnected. Oneway to accomplish this is by setting up an internal variablecalled a reference counter and incrementing or decrementingit as pointers are connected or disconnected. The disadvantagesof this approach include the memory overhead neededto store the counters and the execution overhead of havingto continually check and update the counters.An alternative approach is garbage collection. Here theruntime system simply connects or disconnects pointersas required by the program’s declarations, without makingan attempt to reclaim the disconnected (“dead”) cells. Ifand when the supply of free cells is exhausted, the runtimesystem takes over and begins a three-stage process. First,it provisionally sets the status indicator bit for each cell toshow that it is “garbage.” Each pointer in the program isthen traced (that is, its links are followed) into the heap,and if a valid cell is found that cell’s indicator is reset to“not garbage.” Finally, the garbage cells that remain arelinked back to the pool of free cells available for future allocation.The chief drawback of garbage collection is that themore cells actually being used by the program, the longerthe garbage-collecting process will take (since all of thesecells have to be traced and verified). Yet it is precisely whenmost cells are in use that garbage collection is most likely tobe required.The need for garbage collection has diminished in manyprogramming environments because modern computersnot only have large amounts of memory, most operatingsystems also implement virtual memory, which allows adisk or other storage device to be treated as an extension ofmain memory.Note: the term heap is also used to describe a particulartype of binary tree. (See tree.)Further ReadingLafore, Robert. Data Structures & Algorithms in Java. 2nd ed. Indianapolis:Sams, 2002.Preiss, Bruno R. “Heaps and Priority Queues.” Available online.URL: http://www.brpreiss.com/books/opus4/html/page352.html. Accessed August 6, 2007.Sebesta, Robert W. Concepts of Programming Languages. 8th ed.Boston: Addison-Wesley, 2007.help systemsIn the early days of computing, the programmers of a systemtended to also be its users and were thus intimatelyfamiliar with the program’s operation and command set.