<programming, algorithm> (Or "hashing") A scheme for providing
rapid access to data items which are distinguished by some
key. Each data item to be stored is associated with a key,
e.g. the name of a person. A hash function is applied to
the item's key and the resulting hash value is used as an
index to select one of a number of "hash buckets" in a hash
table. The table contains pointers to the original items.
If, when adding a new item, the hash table already has an
entry at the indicated location then that entry's key must be
compared with the given key to see if it is the same. If two
items' keys hash to the same value (a "hash collision") then
some alternative location is used (e.g. the next free location
cyclically following the indicated one). For best
performance, the table size and hash function must be
tailored to the number of entries and range of keys to be
used. The hash function usually depends on the table size so
if the table needs to be enlarged it must usually be
completely rebuilt.
When you look up a name in the phone book (for example), you
typically hash it by extracting its first letter; the hash
buckets are the alphabetically ordered letter sections.
See also: btree, checksum, CRC, pseudorandom number,
random, random number, soundex.
(1997-08-03)
Nearby terms:
hash « hash bucket « hash character « hash coding » hash collision » hash function » hashing
