1.What are search engines?
• In all aspects of life, if we have to deal with something or make the extreme usage of it we have to be aware of every minute detail in it.
• So if we need to search about any information in any field and we don't know where to find it, for example if you are interested in sports and you don’t know the sites where you can find every thing related to sports and your favorite team and their news as well, you can simply use the search engines to lead you to those sites.
• Search engines are a system or a set of programs.
• So, We can detect that search engines are huge databases of web page files, works by human inputs and algorithm, depends on keywords to find a specific documents where those keywords exist, they present the results in a list called" hits" while these results can be images, files, web pages or videos.
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2.Types of search engines:
Search engines contain two major types:
1-individual search engines:
Depend mainly on humans compile their own search-able databases on the web, they are classified into:
I)Crawl-based search engines:
• Lists are created automatically and categories web pages.
• Their spiders crawl over the web then people search through what they have found.
• They are affected by any changes that occur in web pages and always updating their information.
•They are composed of:
a) A spider: it's named also as "Web Crawler" as it can crawl over web and goes to every web page that supposed to be search-able. While large search engines have number of spiders working together.
b) An indexer: it's named also as "catalog" it reads documents then creates an index based on the words contained in each document to facilitate fast and accurate information recuperation.
c)The program that receives your search requests and returns the results to you.
II)Human powered directories:
• Lists depend on human editors.
• A directory gets its information from the site's editors or from the description of the site.
• They aren't affected by any changes occur in Web Pages.
III)Hybrid search engines:
• It is the extremely common type of search engines nowadays.
• We can say that it is a mix of the two previous types.
• It favors one type of listing over another.
2- Meta search engines:
We can consider them as a collecting channels that combine the information they get from individual search engines into one large list.
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3.Mechanisms of search engines:
I think we become familiar with search engines and get some knowledge about them. So let's now see how they work in a series of steps.
Different search engines have different ways to work but they are all operating on three main steps:
a) Web crawling: it means searching web pages that contain the keywords you type in the search bar. When the spider builds its list and look at a lot of pages we can say that we are in the web crawling stage. This spider starts at the very popular web sites and follow their links too. In huge sites there is more than one spider to work together or a system of robots to obtain fast and accuracy in the search process.
b) Index creation: this stage begins when the spider almost complete its list. It can be built by different ways such as:
- Storing the word and the URL
- Or by storing the number of the time that the word appears on the page in addition to the word and URL.
The most effective way to build the index is by building a hash table in which each word is attached to a numerical value (numerical distribution) instead of alphabet distribution which is less effective.
c) Building a search: the search depends on the user query that can be a single word only but when it becomes a complex we need (Boolean operators).
- Boolean operators are:
And: Which means that the terms joined by (and) will appear in the pages.
Or: Which means that at least one of those terms will appear in the pages.
Not: This means that the terms following (not) won't appear in the pages.
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4.Examples of search engines:
We will take some examples of each type of search engines:
Crawl-based search engines
Human powered directories
Hybrid search engines
Meta search engines
