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Veddferg
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星期二 十二月 25, 2007 9:15 am |
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html sitemap html sitemap html sitemap html sitemap html sitemap html sitemap html sitemap html sitemap html sitemap html sitemap
Search engines provide html sitemap an interface to a group of items that enables users to specify criteria about an item of interest and have the engine find the matching items. The criteria are referred to as a search query. In the case of text search engines, the search query is typically expressed as a set of words that identify the desired concept that one or more documents may contain There are several styles of search query syntax that vary in strictness. Where as some text search engines require users to enter two or three words separated by white space, other search engines may enable users to specify entire documents, pictures, sounds, and various forms of natural language. Some search engines apply improvements to search queries to increase the likelihood of providing a quality set of items through a process known as query expansion.
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The list of items that meet the criteria specified by the query is typically sorted, or ranked, in some regard so as to place the most relevant items first. Ranking items by relevance (from highest to lowest) reduces the time required to find the desired information. Probabilistic search engines rank items based on measures of similarity and sometimes popularity or authority. Boolean search engines typically only return items which match exactly without regard to order.To provide a set of matching items quickly, a search engine will typically collect metadata about the group of items under consideration beforehand through a process referred to as indexing.
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The search engine may store of copy of each item in a cache so that users can see the state of the item at the time it was indexed or for archive purposes or to make repetitive processes work more efficiently and quickly. Notably, some search engines do not store an index. Crawler, or spider type search engines may collect and assess items at the time of the search query. Meta search engines simply reuse the index or results of one or more other search engines. The very first tool used for searching on the Internet was Archie.[1] The name stands for "archive" without the "vee". It was created in 1990 by Alan Emtage, a student at McGill University in Montreal. The program downloaded the directory listings of all the files located on public anonymous FTP (File Transfer Protocol) sites, creating a searchable database of file names; however, Archie did not index the contents of these files.
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The rise of Gopher (created in 1991 by Mark McCahill at the University of Minnesota) led to two new search programs, Veronica and Jughead. Like Archie, they searched the file names and titles stored in Gopher index systems. Veronica (Very Easy Rodent-Oriented Net-wide Index to Computerized Archives) provided a keyword search of most Gopher menu titles in the entire Gopher listings. Jughead (Jonzy's Universal Gopher Hierarchy Excavation And Display) was a tool for obtaining menu information from specific Gopher servers. While the name of the search engine "Archie" was not a reference to the Archie comic book series, "Veronica" and "Jughead" are characters in the series, thus referencing their predecessor.
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The first Web search engine was Wandex, a now-defunct index collected by the World Wide Web Wanderer, a web crawler developed by Matthew Gray at MIT in 1993. Another very early search engine, Aliweb, also appeared in 1993, and still runs today. |
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