Information Technology Reference
In-Depth Information
even by computers—requires considerable time and cannot be ac
complished fast enough to give you a response in just a few sec
onds. Instead, search engines maintain various indexes to help
guide their processing. Although the technical details for particu
lar search engines are trade secrets, I can describe the basic idea
with a familiar example.
Suppose you want to learn about “virtual memory” in this
book. How might you proceed? If you have a particularly good
memory, you might remember that this was a topic in Chapter 3,
and turn directly to that material. Alternatively, you might start on
page 1 of the topic and scan each page until you found the relevant
passage. As a third technique, you might consult a glossary to find a
basic definition. None of these approaches, however, is very gen
eral. The first requires that you already know approximately where
to look, the second assumes you have sufficient time and patience,
and the third gives only a short definition.
Instead of any of these approaches, a more effective and general
strategy would be to take advantage of the index. An index contains
a listing of terms and the page numbers where they are discussed. In
this case, you could look up “virtual memory” and (hopefully) find
appropriate page references to read. As a variation, you might look
up the word “virtual” to get one listing of passages, and you might
look up “memory” to obtain a second listing. Comparing common
references on both lists would provide you with information related
to both topics. By looking up each word separately, you might not
find the term “virtual memory” itself on a specific page, but you
would expect both terms to be used in some way—perhaps in dif
ferent paragraphs—on the page.
Search engines use this same idea of an index to speed up their
searches. When a search engine company stores a Web page, it also
updates its indexes that keep track of where to find various terms.
Thus, if you conduct a Web search for the word “virtual,” the
search engine will consult an appropriate index for this word and
identify Web pages that contain this word. Similarly, if you search
for “memory,” you will get a corresponding list of Web pages.
A search for “virtual memory” could proceed in either of two
ways. Often, if you do not place these words in double quotes, the
search engine will consult the indexes for the words “virtual” and
