Building hash tables is one of the best ways that a search engine can index all of the information that its spiders have found. To build a hash table, a formula is used to give each word a numerical value so that the words are evenly distributed over a predetermined amount of divisions. This system of distribution is different than if the words were distributed according to the actual letters they contain. There are many more words that begin with the letter "S," for example, than words that begin with the letter "Z." Hashing evens the grouping of words so that a search engine can find a word faster and more efficiently.
They're not just for search engine use, however. Database interaction would be another area where they're used. A specific example may help illustrate just what a hash table is. Among the many programming languages that use them is Perl, an interpreted language that counts hashes among its available data structures. In very simple terms, a "hash" is comprised of key/value pairs. The key is used in Perl to "point to" the value. The value can be a single thing, a list of things or even a reference to another hash (which would be outside the scope of our discussion). For example, if a database about dogs was being accessed, a Perl program written to access the database might end up using a key "breeds" that points to a list "shepherds, beagles, huskies." The key -- breeds -- when called in the program is how the list of "shepherds, beagles, huskies" is accessed.
Hash tables are used in other programming languages as well, including the very popular PHP scripting language. In such languages, hash tables are just one of many different types of data structures that can be employed.
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