Commodore 1581 - Chapter 5: Relative Data Files; Files, Records, and Fields

CHAPTERS
RELATIVE DATA FILES
Sequential files are very useful when you're just working with a
continuous stream of data — i.e., information that can be fead or
written all at once. However, sequential files are not useful in some
situations. For example, after writing a large list of mail labels, you
wouldn't want to have to reread the entire list each time you need a
person's record. Instead, you need some kind of random access, a way
to get to a particular label in your file without having to read through
all those preceding it.
As an example, compare a record turntable with a cassette re
corder. You have to listen to a cassette from beginning to end, but a
turntable needle can be picked up at any time, and instantly moved to
any spot on the record. Your disk drive works like a turntable in that
respect. In this chapter you will learn about a type of file that reflects
this flexibility.
Actually, two different types of random access files may be used
on Commodore disk drives: relative files and random files. Relative
files are much more convenient for most data handling operations, but
true random access file commands are also available to advanced
users, and will be discussed in the next chapter.
FILES, RECORDS, AND FIELDS
When learning about sequential files, you did not worry about the
organization of data within a file, so long as the variables used to write
the file matched up properly with those which read it back into the
computer. But in order for relative access to work, you need a more
structured and predictable environment for our data.
The structure you will use is similar to that used in the traditional
filing cabinet. In a traditional office, all customer records might be
kept in a single file cabinet. Within this file, each customer has a
personal record in a file folder with their name on it, that contains
everything the office knows about that person. Likewise, within each
file folder, there may be many small slips of paper, each containing
one bit of information about that customer, such as a home phone
number or the date of the most recent purchase.
In a computerized office, the file cabinet is gone, but the concept
of a file containing all the information about a group or topic remains.
The file folders are gone too, but the notion of subdividing the file into
individual records remains. The slips of paper within the personal
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