01:198:170 Chapter Notes - Chapter 5, 7: Pagerank, Alt Attribute, Anchor Text
16 May 2018
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Search Engines
How a Search Engine Works
●Step 1: crawling
○Visits every website it can fine
○Where does the crawler-- the software that does the crawling-- gets the pages?
■Crawler has a to do list of urls that it is given to begin with
●Step 2: query processing
●Main work of crawler is to build an index
○The index is a list of tokens (such as words) that are associated with that token
○For each token, crawlers create a list of the url associated with that token
●Search engine --> (crawls the web to find new or updated web pages)--> web pages -->
(catalogues and indexes the information on the web pages into a database) --> search engine
database --> (search engine software searches the database for matches to keywords or
concepts you have entered
Multiword searches
●Generally when we make multiword query, we want the pages returned by the query processor
to be appropriate for all words
●When we type :human powered flight: we mean each page returned should be associated with
ALL those words
Descriptive terms
●"hits" on a page means the search term is "associated" with the page
●But words might not be "on" the page
●Descriptive text:
○Title: encloses a short phrase describing the whole page
○Anchor text: highlighted text link, which is inside a <a…/> tage, described the page it
links to
○Meta: web page created can add a <meta name="description"..> tag in the head section,
which can give a several sentence description of the content of the page
■<meta name=”description” content=”travel photos; volcanoes of Hawaii”/>
○Alt attribute: alt tags are a good source of descriptive terms for the image and the page
it's on
Page ranks
●The order in which hits are returned to a query is determined, by a number Google, calls the
PageRank
●Generally the higher the pagerank, the closer to the top of the list a page will be
Logical operators: AND, OR, NOT
●AND
○Require all words given must be associated with the page for it to be a hit
■Human AND powered AND flight
●OR
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