programming-for-dummies

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Storing Fixed Size Data in Random-Access Files
The Portable Document Format (PDF)
Although you can share plain text files among
different computers, you always lose the formatting
of that data. To avoid this problem,
Adobe developed its own “universal” file
format — the Portable Document Format (PDF).
The idea behind PDF files is to allow people to
create and distribute files that display data
exactly the same no matter what computer they
may use. So if you create a flyer or a newsletter,
the appearance of your flyer or newsletter
looks the same on a Macintosh computer as it
does on a computer running a completely different
operating system, such as Windows or
Linux.
PDF files have two drawbacks. First, you can’t
edit them without special software, so PDF files
are meant more for displaying information than
for letting you actually change that information.
That’s why many governments distribute documents
and forms as PDF files so people can see
the information, such as tax forms, but they
can’t change it.
Second, you can’t view the contents of a PDF
file unless you have a special PDF viewing or
reader program, which Adobe gives away for
free. Unfortunately, if Adobe doesn’t make a
PDF reader program for your operating system,
you can’t read PDF files.
Despite these drawbacks, PDF files are popular
for sharing information among different
computers and operating systems while retaining
the original appearance of a file, such as a
word processor document. If you just want to
share information, PDF files are currently the
most popular way to do so.
Many Web sites can store information on your computer in a cookie. A
cookie is nothing more than a text file that stores your Web site settings,
such as your username. That way if you visit that Web site again, the Web
site retrieves the cookie off your computer and uses that information to customize
the Web page that you see.
For programmers, the most common text file is the source code to any program
whether that program is written in C++, Perl, Tcl, or Prolog.
Storing Fixed Size Data in Random-Access Files
One of the biggest problems with text files is that retrieving data requires
reading the entire text file from beginning to end. In a large text file, this
makes retrieving data clumsy and slow.
Whereas text files act more like audiocassette tapes, random-access files are
more like compact discs that allow you to skip right to the data you want to
retrieve (which is why they’re random-access files).
A random-access file organizes data in equal sized chunks called a record. A
record defines what type of data to store, such as a first and last name, age,
and phone number, as shown in Figure 8-1.